106 - Final Design Consideration - Superstructure

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106.1 Introduction

The purpose of this section is to establish Department policies and procedures for the final design and detailing of superstructure elements for new, typical Delaware bridges, as well as for their replacement and rehabilitation.

106.2 Terms

AASHTO LRFD
AASHTO LRFD Bridge Design Specifications.
HLMR Bearings
High load multi-rotational bearings
NEPCOAT
Northeast Protective Coating Committee.
NSTM
Non-redundant steel tension member.
PTFE
Polytetrafluoroethylene—a synthetic fluoropolymer of tetrafluoroethylene that has numerous applications for bridge construction, but mainly in providing a low-friction sliding surface. The best-known brand name of PTFE-based formulas is Teflon®.
SIP Forms
Stay-in-place forms.
SRM
System-redundant member.

106.3 Design Loads

106.3.1 Dead Loads

The Department follows AASHTO LRFD for estimation of dead loads, including values for material unit weights.

Non-composite dead loads shall include the weight of the beams, diaphragms/cross-frames, deck slab, SIP forms, haunches, and additional deck-overhang concrete, as applicable. Depending on when utilities are installed, such as waterline and scupper drain pipes, these loads may also need to be included. A note should be added to the camber and deflection tables to alert the Contractor regarding which loads are included during each stage of construction.

Composite dead loads shall include the weight of the bridge barriers, and/or railings and sidewalks, as applicable. Miscellaneous dead loads on bridges (including, but not limited to, utilities railings, protective fencing, and bridge lighting) will preferably be composite dead loads, but this is dependent on when (composite or non-composite condition) miscellaneous dead loads are installed. Consideration for loading from future utilities is not required, because new utilities are not permitted on bridges in Delaware.

Unless required or otherwise specified by design, the following non-composite dead loads shall be used:

  1. Integral Wearing Surface:  The top 0.5 inch of concrete bridge deck shall be considered an integral wearing surface, accounted for in dead load; but it is not to be considered in the structural design of the deck slab or as part of the composite section.
  2. SIP forms: 15 pounds per square foot (includes concrete-in-form corrugations). Refer to Section 106.4.2 – Concrete Decks for criteria for the use of reduced loading to account for the weight of SIP forms.

Unless required or otherwise specified by design, the following composite dead loads shall be used:

  1. Future Wearing Surface:  25 pounds per square foot.

Unless required or specified by design, the following dead-load unit weights shall be used:

  1. Lightweight concrete: The permissible range for unit weight of lightweight concrete shall be 110 to 130 pounds per cubic foot. The design unit weight value shall be provided on the Plans, and be in accordance with the specified lightweight mix design, also to be included in contract documents.
  2. Fill soil: 120 pounds per cubic foot.

Temporary construction loads on overhang formwork, such as Bidwell wheel loads and walkway live load, shall be verified as part of the design of exterior beams. Refer to Section 106.4.2.7 – Deck Overhangs for further description of this temporary construction loading condition.

106.3.1.1 Considerations for Deck Haunch

For the non-composite condition, the designer may conservatively estimate the haunch thickness for dead load estimation as part of the analysis and design of the beam, in lieu of providing accurate haunch dead loads throughout the length of the beam. For the composite condition, the typical approach shall be to account for the haunch in terms of its weight, but not in terms of increased capacity, due to the additional offset that the haunch provides in relation to the centroid of the deck and the centroid of the steel or concrete beam. When taking that approach, however, the designer shall consider the significance of such assumptions with regard to the overall composite member stiffness (deflections), and in the determination of the location of the centroid of the composite section.

106.3.1.2 Distribution of Dead Loads

Unless advanced analysis (2-D grillage or 3-D finite element analysis) techniques are employed, or justification for alternate distribution provided, the following distribution of dead loads shall be used for line girder analysis of typical multi-beam bridges:

  1. Simple distribution for non-composite dead loads;
  2. Composite dead loads shall be equally distributed among all beams in the bridge cross section, except for the following:
    1. Bridge barriers on deck overhangs should be distributed 75 percent to the exterior and 25 percent to the first interior beam in the cross section.
    2. Exterior sidewalks should be distributed by simple distribution to the girders below the sidewalk.
    3. Staged construction distribution of dead load may depend on the sequence of the bridge construction.
    4. For bridge widths greater than 40 feet, the designer shall consider not distributing to all beams, but applying rationale for limiting loads to adjacent two to three beams. This recognizes that in wide bridges, it is less likely for beams a significant distance from partial-width loads to feel the effect of the load.

Overhang widths and beam spacing should be adjusted to minimize differences in deflection between beams due to non-composite dead loads. When differential deflection is proposed, additional consideration should be given to the deflection behavior of the beams with cross-frames attached during the deck pour.

The designer shall consider applicability of above methodologies for load distribution when 2‑D grillage or 3-D finite element analysis methods are used.

106.3.2 Live Loads

Live loads and lane loads used for design shall comply with AASHTO LRFD. Application of live loads, including vehicles and pedestrians, will be in accordance with the AASHTO LRFD and as modified in Section 203 – Loads and Load Factors.

The designer shall recognize that new bridges and reconstructed bridge elements shall be verified for load-rating factors to be greater than or equal to 1.0 at Strength I (inventory level) for all Delaware legal loads. Refer to Section 108 – Bridge Load Rating and Figure 108‑3 (1), (2), (3), (4) for a listing and description of the Delaware legal loads.

The designer shall also recognize that new bridges and reconstructed bridge elements shall be verified for load-rating factors to be greater than or equal to 1.0 at the Strength II (operating level) for Delaware permit vehicle(s). Refer to Section 108 – Bridge Load Rating and Figure 108‑4 for a listing and description of the Delaware permit vehicle(s).

When travel lanes are striped less than 12 feet over the bridge, the design shall recognize the number of striped lanes on the bridge.

For bridges with mountable (8 inches or less) curbed sidewalks, the designer shall consider the cases of pedestrian loading on the sidewalk or vehicular loading (one truck only, no uniform load) on the sidewalk. This addresses instances when trucks overcome the curb. The designer shall also consider the case of the bridge being converted to full width for full vehicular traffic (sidewalk removed). Refer to Section 203.6.1.6 – Pedestrian Loads.

Consideration may be given to designing to the AASHTO Strength II load case for short-term staged conditions; and future re-decking conditions for all live-load vehicles.

106.4 Bridge Decks

106.4.1 Deck Type Considerations

The preferred bridge deck type is a reinforced-concrete deck using normal-weight concrete. Lightweight concrete and open, filled, or partially filled steel-grid decks shall only be considered for bridge rehabilitation projects as necessary, and/or as approved by the Bridge Design Engineer. Note that lightweight concrete bridge decks are intended to provide an equivalent compressive strength to normal-weight concrete bridge decks; however, the modulus of elasticity of lightweight concrete will be less than normal-weight concrete, which affects properties for stiffness provided by the composite section.

The Department recommends the use of decks that are designed to be composite with the superstructure. Composite action decks are typically designed so that both the deck and beam or girder respond to live loads and superimposed dead loads as a unit. For a breakdown of non-composite and superimposed composite dead loads, see Section 106.3.1 – Dead Loads. For steel bridges, the interconnection of the beams to the concrete deck is accomplished using welded shear studs attached to the top flange. For concrete beams, the interconnection is accomplished using steel reinforcing bars embedded in the beam, extending into the deck. Typically, the stirrups are extended above the top of the beam to serve as the interconnection between the beam and the deck.

106.4.2 Concrete Decks

Refer to Section 205.4.2.1 – Compressive Strength for deck concrete material properties.

The use of galvanized S.I.P. deck forms for the construction of cast-in-place concrete bridge decks is preferred. No beneficial structural contributions from the S.I.P. form and the concrete in the valleys of the form shall be taken into consideration in the deck design.

Refer to Section 325.01 – Concrete Deck Details for the typical concrete deck section formed with SIP forms.

The welding of deck forms to structural steel components is not permitted in areas where the top flange can be subject to tension under Strength I Limit State. For SIP form connection details in compression zones and tension zones, refer to Section 335.01 – Steel Beam Bridge Details.

For dead load calculations and the establishment of deck form connection details, the type of deck form and additional dead load from the forms must be provided on the design plans. Refer to Section 106.3.1– Dead Loads for typical weight of SIP forms to be considered in the design. The use of removable forms, placement of preformed cellular polystyrene in the valleys of the deck forms, or the use of soffitted forms must be specified on the Plans, but only when required by design.

106.4.2.1 Concrete Deck Design Considerations

For new bridges, and when within the AASHTO criteria for its use, the concrete bridge deck shall be designed by the Empirical Method, in accordance with Section A9.7.2 – Empirical Design.

The design of deck edges or edge beams and the design of transverse reinforcement in the deck overhangs shall be designed in accordance with the AASHTO Traditional Design method, per A9.7.3 – Traditional Design. Refer to Section 109.3.4.4 – Widening and Partial-Width Re-decking for bridge widening and partial re-decking projects. For full re-decking projects, refer to Section 109.3.5 – Concrete Deck Replacement.

The deck overhangs shall not only resist vertical effects of dead and live loads, but also the traffic barrier collisions loads, in accordance with AASHTO LRFD.

For staged sequence of deck construction, the designer shall consider the potential for interim deck conditions; particularly temporary barrier loadings, and the temporary overhang conditions between stages of construction.

106.4.2.2 Deck Thickness

When using the Empirical Method of design, use an 8½-inch deck thickness for beam spacings, ranging from 4 feet to 12 feet. Note that the 8½-inch-thick deck shall be considered effectively an 8-inch-deck, accounting for the ½-inch integral sacrificial wearing surface.

When using the Traditional Method of design, use the minimum 8½-inch deck thickness (8‑inch effective thickness). The designer shall increase the deck thickness by ½-inch increments only as required to maintain a minimum 6-inch rebar spacing and maximum bar size. Note the maximum-size deck reinforcing in Section 106.4.2.3 – Deck-Reinforcing Steel.

Note that the deck thicknesses listed above refer to the thickness between beams. The thickness of the deck in the overhang shall be a minimum of 1 inch thicker than the thickness between the beams, and is a function of the exterior beam haunch thickness and standard detailing of the deck overhang (refer to Section 325.01 – Concrete Deck Details). The deck thickness in the overhang may vary along the length of the bridge, and may exceed 10 inches.

If a concrete deck is proposed for superstructures with adjacent beam configurations, such as NEXT beam and adjacent concrete box beam structures, the deck thickness shall be a minimum of 5 inches.

The deck thickness includes a ½-inch integral wearing surface. The integral wearing surface is not considered a part of the design thickness. Therefore, as an example, the minimum design thickness is 8 inches for an 8½-inch-thick deck.

Where corrugated metal SIP forms are used, the thickness should be measured to the top of the corrugation, as shown in Section 325.01 – Concrete Deck Details.

106.4.2.3 Deck-Reinforcing Steel

Reinforcing steel meeting the requirements for AASHTO M31, Grade 60, should be specified.

Epoxy coating conforming to ASTM A775 should be specified. All deck-reinforcing steel should be protected with fusion-bonded epoxy, except for new deck construction adjacent to existing concrete with black reinforcing steel. For new deck construction adjacent to existing concrete, the new deck-reinforcing steel should match that in the existing deck section.

In consideration of crack control, as a general rule, the use of smaller reinforcing bar sizes at closer spacing is preferable to larger bars at increased spacing. The minimum size of reinforcing in bridge decks shall be a #4 bar; and the maximum size of reinforcing in bridge decks shall be a #6 bar, as required by design. Although anticipated to be the exception, larger bars may be required by design for transverse bars in deck overhangs and longitudinal bars over interior supports.

Lap splices and mechanical splices, when needed, shall be staggered every other bar, when practical; however, it is understood that staged construction may limit the designer’s ability to stagger splices.

106.4.2.3.1 Deck Reinforcing for Spread Beam Bridges
106.4.2.3.1.1 Transverse Reinforcement

For multi-beam bridges, the transverse deck reinforcing bars shall be placed as the top and bottom bar in the top and bottom mat of reinforcement, respectively.

Effect of Bridge Skew:

  1. For bridges with support skews equal to or less than 25 degrees, the transverse reinforcing shall be placed parallel to the abutments. The deck span length shall be determined along the direction of the transverse reinforcement. Bar spacing shall be specified parallel to the girders on the design plans. When two abutments are skewed at different angles, set the transverse reinforcement in the direction of the milder skew; and at the more sharply skewed end, detail the bars to be fabricated shorter to fit into the acute corner of the deck. When any abutment skew is more severe than 25 degrees, the transverse reinforcement shall be placed perpendicular to the girders, with the bars detailed to be fabricated shorter to fit into the acute corner of the deck.
  2. Bridges with skews greater than 25 degrees, or where the transverse reinforcing will interfere with the shear studs (or stirrup reinforcing for prestressed beams), the transverse reinforcement shall be placed perpendicular to the centerline of the bridge. Refer to Section A9.7.2.5 – Reinforcement Requirements for additional reinforcement required along the skewed edge of the deck at deck joints. Also refer to Section 325.01 – Concrete Deck Details for guidance on detailing of transverse-deck reinforcement at skewed edges of bridge decks.

For curved girder bridges, transverse-deck reinforcement should be placed radially. The bar spacing shall be measured along the centerline of the exterior girder at the outside of the curve.

106.4.2.3.1.2 Longitudinal Reinforcement
  1. Typically, the primary deck reinforcement is transverse, or perpendicular to traffic. In these cases, the longitudinal reinforcement is considered secondary reinforcement, or distribution reinforcement. Refer to Section A9.7.3.2 Distribution Reinforcement for amount of distribution reinforcement required. Secondary (distribution) bars should be small bars at close spacing. Therefore, the required secondary bar size should be a #4, unless the bar spacing becomes less than 6 inches.
  2. In the negative moment regions of superstructures continuous over piers, additional reinforcement shall be added in the longitudinal direction to control deck cracking due to tension in the deck, in accordance with Section A5.7.3.4 – Control of Cracking by Distribution of Reinforcement and Section A6.10.1.7 – Minimum Negative Flexure Concrete Deck Reinforcement. The additional longitudinal reinforcement in the negative moment region should extend the entire length of the dead load negative moment region, plus the development length at each end, into the positive moment region. When feasible, the bar reinforcement shall be continuous throughout the entire length of the negative moment region. When the longitudinal bars need to be lap-spliced in the dead load moment region, the lap splices shall be staggered.
106.4.2.3.2 Deck Reinforcing for Adjacent Beam Bridges

When deck slabs are specified for adjacent beam bridges, the deck slabs shall be a minimum of 5 inches thick. A single mat of #4 bars spaced at 6 inches in each direction shall be used in the deck, maintaining a clear cover of 2½ inches to the top of the deck. The use of welded-wire fabric is not permitted.

Refer to Section 106.9.8.1 – Grade and Cross-Slope Effects for setting of adjacent box beams with the cross-slope of the bridge to minimize haunch thickness. When cross-slope transitions increase the deck-slab thickness above 6 inches, the use of spread box beams in lieu of adjacent box beams should be considered. If an adjacent box beam superstructure is required with cross-slope transitions that increase the deck-slab thickness above 6 inches; a second, bottom mat of #4 bars shall be provided, spaced at 6 inches in each direction. The bottom mat should maintain a minimum cover of 1½ inches above the top of the beams. The designer will need to adjust the spacing of the bottom mat to avoid the composite bars extending from the beams.

In the dead-load negative-moment regions of superstructures continuous over piers, additional reinforcement shall be added in the longitudinal direction to prohibit deck cracking, in accordance with Section A5.7.3.4 – Control of Cracking by Distribution of Reinforcement and Section A6.10.1.7 – Minimum Negative Flexure Concrete Deck Reinforcement. The bar reinforcement shall be continuous throughout the entire length of the dead-load negative‑moment region, plus the development length on each end beyond the dead-load contraflexure point.

For adjacent box-beam decks, the transverse reinforcing steel should be placed parallel to the abutments regardless of magnitude of skew. If the abutments are not parallel, the transverse reinforcement shall be placed parallel to the abutment with the milder skew. At the more sharply skewed end, detail the bars to be fabricated shorter to fit into the acute corner of the deck.

106.4.2.4 Deck Haunch

For steel superstructures, the deck haunch is defined as the vertical distance from the bottom of deck to the top of the top flange. For prestressed concrete superstructures, the deck haunch is defined as the vertical distance from the bottom of deck to the top of prestressed beam.

For steel beams and girders, the haunch is typically detailed on the Plans as a minimum depth; however, in the field, the haunch depth will vary based on steel camber tolerances. For prestressed beams, the haunch will typically vary to accommodate the difference in the profile to the cambered shape of the prestressed beam. The haunch depth will also vary based on the difference between actual and predicted camber in prestressed concrete beams.

The haunch affords the flexibility in construction to adapt the field conditions to achieve the final top-of-deck elevation and required thickness of the deck. The designer shall consider the haunch as a method to accommodate fabrication and construction tolerances, and unknown or unanticipated conditions in the field for various bridge types and span lengths. Advantages and disadvantages of a deeper haunch shall be considered in the design and detailing.

Bridge decks for spread multi-beam superstructures shall be detailed to have a minimum haunch thickness of 2 inches over the steel- or concrete-beam top flange, as measured from any point along the width of the top flange to the bottom of the deck slab. The haunch shall be no less than 1 inch over splice plates on steel girders, as applicable. The haunch dimensions should be determined at the locations corresponding to the deck elevations over the girders as specified in Section 106.4.3 – Finished Deck Elevations. When determining haunch dimensions, it should be assumed that the cross-frames will eliminate any differential deflection between beams at deck placement.

The deck haunch should accommodate construction tolerances and variations due to beam camber, cross-slope, and/or longitudinal profile. Haunch reinforcement is required for the following cases:

  • For prestressed concrete beams with top flanges greater than or equal to 3 feet, using stay-in-place forms, and when haunch thickness is 5” or greater.
  • For all other beam types, when haunch thickness is 3” or greater.

Refer to Section 325.01 – Concrete Deck Details for haunch reinforcement details.

106.4.2.5 Concrete Cover

See Section 205 – Concrete Structures for concrete cover requirements.

106.4.2.6 Deck Placement Sequence

For multi-span continuous structures that require multiple concrete placements, the assessment of the five items listed below requires that sequential structural analysis (deck placement sequence analysis, a typical feature in most steel design programs) be performed. Deck placement sequence analysis is required for bridges that require that concrete be placed in multiple segments (i.e., cannot be placed in one continuous operation) and where placement can cause negative moment to occur in previously placed concrete deck sections.

An assessment shall be performed to determine an acceptable concrete slab placement sequence. The assessment shall address (but is not limited to) the following items:

  1. The change in stiffness of the composite girder section as different segments of the slab are placed, and as it affects both the temporary stresses and the potential for "locked-in" erection stresses.
  2. Bracing (or lack thereof) of the compression flange of girders, and its effect on the stability and strength of steel girders during slab placement.
  3. Temporary loading conditions induced by overhang deck forms (Section 106.4.2.7.1 – Overhang Forming and Temporary Support Conditions) for steel bridges,
  4. Uplift at bearings.
  5. Tension/cracking in previously placed segments of the deck.

In comparison with the assumption that all non-composite dead load is placed simultaneously over the entire structure, deck placement sequence analysis more accurately represents how dead load stresses are induced into the structure, bracing conditions, deflections sequentially throughout deck placement, and dead load camber. Proper deck placement sequence shall also assure against excessive deck cracking due to tension in previously placed sections of the deck. The analysis of slab placement shall be done in an incremental fashion. The analysis should consider the sensitivity and/or potential for reduced concrete modulus of elasticity, given that previously placed concrete may not have reached its projected concrete modulus of elasticity at 28 days, a function of f’c, at the time of subsequent deck placements. The strength gain of concrete and its corresponding modulus of elasticity (E) correspond to the concrete’s maturity. Before placement, a minimum concrete strength of 0.5f’c, shall be achieved in the previously placed section of deck subject to tension, as specified in the Standard Specifications.

For continuous-span steel structures, a deck placement sequence plan shall be provided in the design plans, matching that of the deck placement sequence calculations performed as part of girder design calculations. Each step in the deck placement sequence shall represent a section of deck that can practically be placed in 1 day. Although the general principle of concrete deck placement sequencing is to place all the positive moment regions first, and then place the negative moment regions, it is generally more cost-effective for contractors to work from one end of the structure to the other. Therefore, the designer shall determine the feasibility of the following sequence:

  1. Place the end span positive moment (Span 1) region.
  2. Place the adjacent positive moment region in the first interior span (Span 2).
  3. Place the adjacent negative moment region over the first interior support
  4. Alternate positive moment region in the next span and then back to the adjacent negative moment region until deck placement is complete.

The concrete placement shall typically begin at the lower end of the segment to be placed, and proceed uphill. Therefore, on the deck pouring sequence shown on the Plans, the designer should show both the numeric sequence of placement and the direction of placement. The design should be cognizant that changing the direction of placement in the sequence will require the Contractor to pick and rotate the deck finishing machine, which generally should be kept to a minimum.

Although it is generally preferable for concrete placement to proceed uphill, for symmetric continuous span configurations, it is recommended that the placement sequence allow the Contractor to start from either end of the structure. If the structure has an asymmetrical continuous span configuration, the designer shall consider performing the analysis from either end to provide the Contractor with either alternative; however, differential effects on dead load camber between the two alternatives would need to be assessed, or provided in the Plans.

In addition to the requirements of the Standard Specifications for continuous steel superstructures, the following note is recommended to be included with the Deck Placement Sequence Plan in the design plans:

  1. Changes to the placement sequence or alternative deck placement sequences proposed by the Contractor during construction must be submitted for approval. The submittal shall be signed and sealed by a Professional Engineer licensed in the State of Delaware. The submittal shall include calculations for the revised deck placement sequence analysis determining the effects on dead load stresses, bracing, and camber.

Similar deck placement procedures are required for prestressed concrete beams; however, the designer shall consider the sequence of placing the concrete intermediate, end diaphragm, and pier diaphragms along with the deck placement.

See AC6.10.3.4 for further commentary regarding deck placement sequence and associated design recommendations and per Section A6.10.1.7– Minimum Negative Flexure Concrete Deck Reinforcement requirements. Provide minimum negative flexure slab reinforcement, as per Section A6.10.1.7 – Minimum Negative Flexure Concrete Deck Reinforcement and Section 106.4.2.3 – Deck-Reinforcing Steel as applicable.

106.4.2.7 Deck Overhangs

Refer to Section 106.4.2.1 – Concrete Deck Design Considerations for deck overhang design methodology.

Concrete deck overhangs shall meet the requirements of A3.6.1.3.4 – Deck Overhang Load. In no case shall the deck overhang be greater than the lesser of half the beam spacing or the beam depth.

Refer to Section 325.01 – Concrete Deck Details for deck overhang details. The overhang should be formed to a minimum thickness of 1 inch greater than the interior deck span thickness. The minimum overhang thickness shall be measured at the exterior edge of the deck, or as required for proper detailing of overhang and barrier anchorage reinforcing bars into the deck. The overhang should be detailed to meet flush with the underside of the top flange of steel girders and to the top side of the top flange of concrete beams.

The exterior termination of the top main flexure reinforcement shall be checked in the overhangs to ensure proper design and development of the reinforcement for both gravity and vehicular collision loads. Vehicular collision forces may require bundling bars in the overhang. A 180 degree hook is required for the exterior termination of the top main flexure reinforcement to ensure its development. The designer shall check the hooked bar in the overhang to ensure that the hook can physically fit while maintaining the necessary clear cover requirements. Vertically oriented hooks are preferred, but the reinforcement can be rotated out of vertical plane to assist with clearance; the allowance to rotate the hooked bar should be noted on the Plans, as applicable.

106.4.2.7.1 Overhang Forming and Temporary Support Conditions

The designer must verify the constructability of the overhang. The designer shall consider the effect of the temporary horizontal construction loading from overhang brackets on exterior beams during deck placement. Refer to Figure 106‑1 for a detail illustrating this temporary construction loading condition on exterior beams due to overhang bracket support systems for deck placement. The effect of this horizontal construction loading is torsion on the exterior beam. For further reference on this condition, the designers may refer to Torsional Analysis for Exterior Girders – TAEG 2.1 (Roddis et al., 2005).

This temporary loading condition due to placement of the concrete overhang shall be checked as part of the design of exterior steel beams. Note also that these temporary overhang loading conditions may also apply to interior beams as part of staged construction conditions.

For the erection condition with the overhang form support system, the bridge designers shall verify the strength and stability of the exterior girder by applying the load of the overhang concrete and construction equipment loading to the girder as follows:

The standard form support system, shown in Figure 106‑1, may be used without additional exterior girder design checks only where:

  1. Girder web depth is less than 8 feet
  2. Deck-slab overhang is less than 4 feet 9 inches
  3. Overhang slab thickness is equal to or less than 10 inches
  4. Transverse stiffener spacing does not exceed the depth of the girder

Note that the details shown in Figure 106‑1 are for guidance for exterior beam design, and are not meant as a construction detail to be shown on the design plans.

FIGURE 106-1. TEMPORARY DECK OVERHANG FORM SUPPORT LOADING

When the four conditions listed above are not met, exterior girders are to be designed for a temporary horizontal construction load of   **   kip per foot, as taken/interpolated from the table provided in Figure 106‑1. The construction load approximates the horizontal reaction of a deck overhang support bracket as depicted in Figure 106‑1. The load accounts for the weight of the concrete, forms, incidental loads, and the deck-finishing machine. Where transverse stiffener spacing is needed to satisfy constructability in amounts less than required for the design shear, the stiffeners needed for the design shear may be used if the overhang forms are supported from the bottom flange of the fascia girder, or if the girder web is adequately braced to prevent buckling due to loads from web-bearing form support brackets.

106.4.2.7.2 Scupper Detailing

Refer to Section 103.3.2.1 – Shoulder Width Requirements for Deck Drainage for the design requirements for sizing and spacing of scuppers on bridge decks. It is preferable to provide scuppers within deck overhangs that can accommodate simplified scupper detailing. If not geometrically feasible to fit within the overhang, the designer shall consider modifications to the scupper details. If necessary, the scupper can be recessed up to 6 inches under the bridge barrier; and the barrier, including barrier reinforcing, shall be modified to accommodate.

Scuppers should be designed with properly sized inlet openings to minimize clogging from siltation or debris. The installation of a drainage inlet upslope of the bridge can minimize the need for bridge deck scuppers and the clogging problem. The downslope drainage inlet beyond the bridge should be designed assuming 50 percent of bridge deck scuppers are clogged. Scuppers should be located at the desirable 2 percent minimum slope, both transversely and longitudinally, to achieve self-cleansing velocity. When a scupper is recessed into the barrier, the minimum height of opening should be 4 inches. This consists of a 3‑inch curb opening and a 1-inch deck depression with proper transition. The bottom of the opening should be adequately sloped.

106.4.2.8 Concrete Deck Finishing

Concrete bridge decks in Delaware are to be textured in accordance with requirements of the Standard Specifications, which outline texturing by mechanical grooving. For the purpose of the structural design of the bridge deck, any reduction to the deck thickness as a result of texturing are to be assumed in the design as part of the ½-inch integral wearing surface.

106.4.2.8.1 Protective Sealers for Concrete Decks

For new concrete bridge decks, the deck should be treated with a high molecular weight methacrylate concrete sealer in accordance with Section 613 of Standard Specifications.

106.4.2.9 Future Wearing Surface/Overlays

Except when specified as part of an adjacent beam superstructure without a concrete deck, overlays are not typically specified as part of new deck construction. Overlays may also be considered for bridge preservation. Refer to Section 109.3.4.3 – Low-Permeability Overlays for design procedures and considerations for bridge deck overlays. New bridges are to be designed for the potential of overlays being added in the future (future wearing surface). Concrete re-decking projects should account for future wearing surface, unless otherwise directed or approved by the Bridge Design Engineer. Bridge preservation projects, where deck overlays are proposed, shall only account for the as-proposed weight of the overlay. In other words, future wearing surface beyond that which is proposed as part of the bridge preservation work need not be accounted for in the bridge loadings. Refer to Section 106.3.1 – Dead Loads for future wearing surface dead loads.

When a bituminous overlay is recommended as part of the original bridge construction of prestressed-concrete adjacent-beam superstructures, the minimum wearing surface thickness shall be 2 inches, but may be greater if used to accommodate roadway cross-slope, or adjust for uneven surface of the adjacent beams. When a bituminous wearing surface is applied to adjacent box beams, a membrane shall be placed between the wearing surface and the top of the beams detailed to prevent penetration of water into the concrete structure. The dead load of the wearing surface shall be assumed to be 25 pounds per square foot (the same as Future Wearing Surface), unless the actual calculated weight of the proposed wearing surface is greater. In that case, the designer shall design for the actual weight of the wearing surface.

106.4.2.10 Concrete Deck Construction Joints

Construction joints, either transverse or longitudinal, are permitted in the bridge deck only at locations shown on the Plans. A construction joint must be used at the break between concrete placements, such as those required by the concrete placement sequence, per Section 106.4.2.6 – Deck Placement Sequence. Normally, construction joints are to be detailed as keyed, cold joints. See Section 325.01 – Concrete Deck Details for typical construction joint details in bridge decks. The number of construction joints in the bridge deck joint should be minimized, to the extent practical.

For transverse joints adjacent to negative moments where additional reinforcement has been provided, it is suggested that the transverse joint be located 6 inches beyond the termination of the additional reinforcement bars. This will simplify the construction of the bulkhead, with less bars interfacing with it.

106.4.2.11 Temporary Protective Shield

A temporary protective shield is required under the superstructure for the construction of the concrete deck and barriers over active roadways and pedestrian facilities to protect vehicles and pedestrians from falling debris, equipment, and materials. The protective shield shall be constructed in accordance with Section 604 of DelDOT’s Standard Specifications.

106.4.3 Finished Deck Elevations

Finished deck elevations are to be shown in the Plans, at the centerline of bearing over each abutment, at the centerline of the pier(s), and at 1/10th points along the span—but at no greater than 10-foot intervals. It is preferred that the deck elevation correspond to beam camber points. The finished deck elevations should be provided:

  1. Longitudinally over each beam;
  2. Longitudinally along the span at the break points in the cross slope of the deck, and
  3. Longitudinally along curb lines at bridge barriers, sidewalks, etc.

106.5 Bridge Barriers and Railings

All bridge railings that are constructed on DelDOT maintained roadways shall be structurally and geometrically crashworthy for the application in which they are applied. In addition, the bridge barrier system must either be transitioned to an acceptable highway approach traffic barrier or continued to a point where it can be safely terminated off the structure.

Bridge railings must accommodate the various bridge users and their unique requirements and characteristics. In general, roadway user groups can be broken into three categories: vehicle traffic, pedestrian traffic, and bicycle traffic. Accordingly, bridge barriers and railings can be divided into four unique applications:

  • Traffic Railing – A traffic railing is a bridge railing that has been designed and crash tested to withstand vehicular impact loading. This bridge railing type is to be used adjacent to vehicular traffic on bridges. The traffic rail that is selected shall be crashworthy for the design traffic which will utilize the facility.
  • Pedestrian Railing – A pedestrian railing must be used adjacent to all pedestrian walkways on bridge structures. See A13.8 – Pedestrian Railings for all pedestrian rail design requirements.
  • Bicycle Railing – A bicycle railing must be used on all bridges that are specifically designed to carry bicycle traffic and at locations where a significant portion of the traffic using the facility will be bicycle traffic. See A13.9 – Bicycle Railings for all bicycle rail requirements.
  • Combination Railing – A combination railing is defined as a bridge railing which meets the requirements of both a traffic railing and either a pedestrian or bicycle railing. Combination rails can either be installed adjacent to traffic or may be installed behind a raised curb and sidewalk. Combination rails installed behind a raised curb are only used on low-speed roadways which are defined as a roadway with either the higher of the design speed or the posted speed being 45 mph or below.

Figures 106-2 to 106-5 visually define the various configurations of bridge rail applications:

FIGURE 106-2. TRAFFIC RAILING

Figure 106-2 depicts traffic railing installed adjacent to vehicle traffic on a roadway where there are no pedestrian accommodations present.

FIGURE 106-3. COMBINATION RAILING

Figure 106-3 depicts a combination rail installed behind a vertical curb. This application should only be used on low-speed roadways which are defined as a roadway with either the higher of the design speed or the posted speed being 45 mph or below.

FIGURE 106-4. SEPARATOR RAIL BETWEEN TRAVEL WAY AND PEDESTRIAN FACILITY

Figure 106-4 depicts a traffic rail installed between vehicle traffic and a sidewalk or shared-user path facility. This application should be used on facilities where the lower of the design speed or the posted speed exceeds 45 mph.

FIGURE 106-5. PEDESTRIAN OR BICYCLE ONLY FACILITY

Figure 106-5 depicts a pedestrian or bicycle railing installed on a bridge where vehicular traffic is prohibited and therefore, traffic railing is not required.

106.5.1 DelDOT Standard Bridge Barrier and Railing Applications

Unless otherwise approved by the Bridge Design Engineer, provide bridge barriers and railings from Section 325.02 –Bridge Railing Details, as listed below meeting the TL rating required by design. For determination of the TL rating required see A13.7.2 – Test Level Selection Criteria. Refer to Section 325.02 – Bridge Railing Details for barrier reinforcement, contraction joint details, end post details and designer notes. Table 106-1 summarizes the characteristics and applications of the barriers included in Section 325.02 – Bridge Railing Details.

  1. Vertical Face Barrier – The 3 feet tall vertical-faced barrier is the preferred bridge barrier for highway vehicular use requiring a minimum of TL-3 on local roads utilizing box culverts, rigid frames, and adjacent box beams. The 3 feet 6 inches tall vertical-faced barriers with form-liners may be used as a TL-4 application in lieu of F-shape barriers for aesthetics reasons.
  2. F-Shape Barrier – The 3 feet F-shape barrier is the preferred bridge barrier for highway vehicular use on arterial and collector roadways and is considered a TL-4 application.
  3. Single Slope Barrier - The 3 feet 6 inches single slope barrier is the preferred barrier for highway vehicular use on freeways and expressways and is considered a TL-5 application. The 4 feet 2 inches tall double-sided single slope median barrier is a TL-5 application and is to be used at median locations as a glare screen and where gap protection is not required. Similarly, the 4 feet 6 inches tall single slope barrier is a TL-5 application to be used adjacent to median gaps ranging from 6 inches to 15 feet wide. Refer to Section 103.3.3 – Protection for Median Gap of Parallel Structures for further description and background information regarding median gap protection.
  4. Parapet with Sidewalk – A 3 feet 6 inches barrier may be placed behind vertical curb and sidewalk to act as a combination rail. This application is considered a TL-2 application and shall only be used at low-speed locations which are defined as roadways with either the higher of the design speed or the posted speed being 45 mph or below. The barrier must meet the applicable requirements of Section A13.8 – Pedestrian Railing or A13.9 – Bicycle Railings.
  5. Three Strand Tube Rail Parapet – The 3 feet 6 inches Three Strand Tube Rail Parapet are utilized on roadways where accelerated bridge techniques preclude the use of a concrete barrier. The bridge rail can also be used at locations where an open rail is desired for aesthetic purposes. The bridge rail is considered a TL-4 application.
  6. Two Strand Tube Rail Parapet – The 3 feet 6 inches Two Strand Tube Rail Parapet is the preferred outside railing for bridges with sidewalks or shared-user paths.
  7. Aluminum Pedestrian Rail – The 4 feet DelDOT pedestrian rail is an alternative type of exterior protection for bridges with sidewalks or shared-user paths. This system is typically used on bridges over waterway and can only be used if there is a crashworthy barrier between the travel way and the pedestrian rail.
  8. Other Barriers – Barriers or railings not contained in Section 325.02 – Bridge Railing Details may be utilized with the approval of the Bridge Design Engineer. In all cases, the proposed barrier must be considered MASH compliant unless a crashworthy barrier is placed between the traffic lanes and the proposed railing.
Table 106-1. DelDOT Bridge Barrier Classifications
DelDOT Bridge Barrier Traffic Rail Pedestrian Rail Bicycle Rail Combination Rail Combination Rail Behind Curb
3’-0” Vertical Face Barrier TL-3 No No No No
3’-6” Vertical Face Barrier TL-4 Yes Yes Yes TL-2
3’-0” F-Shape Barrier TL-4 No No No No
Single Slope Barrier TL-5 Yes Yes Yes No
3 Strand Tube Rail Parapet TL-4 Yes Yes Yes TL-2
2 Strand Tube Rail Parapet TL-4 Yes Yes Yes TL-2
DelDOT Pedestrian Rail NA Yes Yes No No

106.5.2 Bridge Barrier Design Considerations

New concrete bridge barriers shall be detailed with contraction/deflection joints and therefore the barrier will not act compositely with the girder. If the existing barriers are not detailed with contraction joints, concrete barriers shall not be relied on for AASHTO LRFD Strength Limit States. Stiffness contribution of concrete barriers for calculations of deflections may only be used with approval of the Bridge Design Engineer.   

As specified in the Standard Specifications, it is not permissible to slip-form concrete bridge barriers unless allowed by a special provision included in the contract documents.

The designer may consider slip-forming bridge barriers:

  1. When compared to traditional cast-in-place methods, slip-forming provides significant savings in construction cost or construction time.
  2. For bridges longer than 200’ out-to-out length of bridge barrier, including associated barrier on approach slabs and moment slabs. Also consider for projects where shorter bridges occur in conjunction with slip-formed roadway barrier.
  3. When the proposed barrier has a minimum of 2’ horizontal clearance from any obstruction such as an adjacent bridge.

Additional slip-forming considerations:

  1. The designer should review the special provision for item 610510 – Portland Cement Concrete Bridge Parapet Slip Form to determine if it is appropriate for the project.
  2. When to saw control joints is discussed in the specification, but not the spacing between joints. This information must be included in the construction plans.
  3. Form Liners and Aesthetic Treatments are possible with slip-forming methods, but should verified as possible to construct before being specified in the construction plans.

Material properties to be used in the design and detailing of bridge barriers are listed below:

  1. Bridge Barrier Concrete – Refer to Section 205.4.2.1 – Compressive Strength for concrete material properties.
  2. Barrier Protective Coating – Silicone sealer is to be applied to all exposed barrier faces, in accordance with Standard Specifications.
  3. Concrete Barrier Reinforcing Steel – Reinforcing steel meeting the requirements for AASHTO M31, Grade 60 should be specified. The minimum size of reinforcing in concrete bridge barriers should be a #4 bar. All barrier reinforcing steel shall be protected with fusion-bonded epoxy. Epoxy-coating conforming to ASTM A775 shall be specified.
  4. Bridge Barrier Concrete Cover – Provide 2 inches minimum concrete cover for all concrete bridge barrier types.

The end of a bridge rail adjacent to the traveled way presents a roadside hazard which must be made crashworthy in accordance the guidance contained in the AASHTO Roadside Design Guide. The most common and effective way to protect the end of a bridge rail is to connect the bridge rail to the adjacent barrier leading up to the structure. When the bridge rail is connected to an adjacent W-beam guardrail, it is essential to install a proper stiffness transition between the two systems as well as to transition the shape and size of the bridge rail to ensure crashworthiness of the system.

A stiffness transition is required to produce a gradual stiffening of the system to eliminate vehicular pocketing, snagging, or penetration. The bridge rail shape and size must also be transitioned to reduce any potential snag points or any abrupt changes in geometry which could affect the rail’s crashworthiness. Due to the sensitivity of transition systems, the transitions must follow the requirements of Section 325.02 – Bridge Railing Details as well as the DelDOT Standard Construction Details unless approved otherwise by the Bridge Design Engineer.

106.5.3 Protective Screening, Shielding, and Fencing

Protective screening is to be provided to prevent throwing debris onto vehicles passing beneath the bridge at locations over interstates, freeways, arterials, collectors, or as directed by the Bridge Design Engineer. Install protective screening on both sides of the bridge. Refer to the DelDOT Standard Construction Details for screening details. Fencing can affect a roadside system’s performance and crashworthiness and should only be placed where absolutely needed.

Shields are typically required on railroad overpasses to prevent the impact of glare from train headlights. Similarly, protective barriers are required on railroad overpasses, including electrified railroads. The designer shall coordinate the shielding and/or protective fencing requirements with the associated railroad(s).

Limits of protective screening, shielding and fencing shall be identified, presented, and approved as part of the TS&L or preliminary design submission. Refer to Section 103.10 - Requirements for the Design of Highway Bridges over Railroads for limits of protective screening on bridges over railroads.

The designer shall consider the type of shielding to be used not only for weight purposes, but also for lateral-force effects on the bridge. The protective screening/shielding/fencing acts as a partial or full wind block.

106.5.4 Bridge Lighting

It is preferred to locate light poles on bridges at or near support points to minimize vibrations and fatigue in the light pole induced from bridge movement. Lighting should be placed outside of the design vehicle’s zone of intrusion in order to prevent snagging during an impact.

106.6 Deck Joints

For new structures, minimize the number of deck joints on the bridge.

Deck joints shall be considered to be oriented parallel to the centerline of bearings at substructure units unless noted otherwise. Refer to Section 340.01 – Strip Seal Expansion Joint Details, Section 340.02 – Finger Joint Expansion Joint Details, and Section 340.03 – Compression Seal Joint for typical deck joint details.

Deck joints shall be constructed with the use of block-outs in the bridge deck, installed after the placement of the bridge deck, unless specified otherwise by the design. Installing the deck joints after placement of the entire length of bridge deck eliminates the need for the joint to accommodate movements due to non-composite dead load translations and/or rotations, and to allow for the setting of the joint opening for the temperature at the time of placement.

Deck joints at fixed bearings are designed to accommodate movements at the deck level due to beam-end rotation caused by composite dead loads and live loads, including dynamic load allowance for appropriate AASHTO Limit States. For deck joints at fixed bearings, include an additional ¼ inch of movement allowance for construction tolerance. Typical joint types at fixed bearings shall be strip seals. Compression seals are not recommended, particularly at joint locations where failure of the seal will cause leakage to substructure beams seats/bearing areas.

Deck joints at expansion bearings shall be designed to accommodate the combination of expansion and contraction movements of the span caused by temperature change and beam rotations from composite dead loads (typically insignificant) and live loads, including dynamic load allowance. For deck joints at expansion bearings, include an additional 1/2 inch of movement allowance for construction tolerance, in addition to the calculated movements from the above effects. For new construction, the two types of joints used at expansion bearings are strip seals and finger joints, with a preference towards strip seals when movement allowance is less than or equal to 5 inches.

Simplified methods of determining thermal movements are typically appropriate for straight bridges with one fixed bearing line. Refer to Figure 106‑6, for an example calculation using the simplified method of determining deck joint movements.

For bridges with multiple fixed piers, the stiffness of the fixed piers should be considered in the determination of thermal movements.  

Expansion of skewed and/or horizontally curved bridges may not follow the longitudinal direction of the bridge. The seal size and direction of movement must be designed to accommodate joint movement as affected by bridge skew, horizontal curvature, and/or bridge width. It is therefore recommended that thermal analysis also be performed as part of the superstructure analysis for horizontally curved bridges where advanced (2-D or 3-D) superstructure analysis is performed. This advanced thermal analysis, in lieu of simplified methods of determining the magnitude and direction of thermal movement, is recommended for proper design and detailing of bearings and deck joints for horizontally curved superstructures. Refer to Section 106.8.8 – Steel-Plate Girder and Rolled Beam Bridges for guidance of when advanced superstructure, and therefore advanced thermal analysis, is required.

Consideration for the accommodation of transverse thermal movement shall also be provided for bridges over 50 feet wide.

106.6.1 Jointless Bridges

Consistent with the goal of minimizing the number of deck joints on the bridge, the use of continuous superstructures and integral, semi-integral abutments, or bridge deck extensions shall be used when appropriate. Refer to Section 103.6.2 – Abutments and Wingwalls for guidelines for the use of integral and semi-integral abutments, respectively.

106.6.2 Strip Seal Joints

Strip seals are to be considered for new construction, bridge re-decking projects and bridge rehabilitation projects. In selecting strip seals, the designer must consider the relationship of total movement, minimum and maximum joint widths, and installation temperature. Refer to Section 340.01 – Strip Seal Expansion Joint Details for strip seal joint details.

The application of strip seals is limited to a maximum allowable movement of 5 inches, also referred to as 5-inch maximum movement classification. For movements of 3 inches or less, specify the minimum strip seal movement capability (or movement classification) of 3 inches.

FIGURE 106-6. EXAMPLE OF SIMPLIFIED METHOD FOR CALCULATION FOR DECK JOINT MOVEMENTS

106.6.3 Steel Finger Joints

Finger joints are used where movements in excess of 5 inches must be accommodated. A finger joint is an expansion joint with the opening spanned by meshing steel plates formed as fingers or teeth. Finger joints are open, so a trough must be installed to control runoff through the joint. The trough is to be detailed to a minimum gradient of 1/12 (8 percent minimum slope) to ensure drainage and flushing. Refer to Section 340.02 – Finger Joint Expansion Joint Details for steel finger joint details.

106.6.4 Longitudinal Joints

Longitudinal joints in bridge decks may be required for wide bridges, widened bridges, or staged construction bridges. A wide bridge is defined as being over 90 feet wide, or having a span-to-width ratio less than 1 (i.e., the width is greater than the span). Longitudinal joints are always placed between beams or girders. Place them in the median or next to the median, if possible. Avoid placing longitudinal joints in the traveled way because of the hazard to motorcycles. Compression seals are not to be used for longitudinal joints. The designer must determine the amount of joint movement (transverse, vertical, and longitudinal, as applicable) when designing longitudinal strip seals. Unless determined to be insufficient to meet joint design requirements, strip seal joints with 3-inch-movement classification shall be specified for longitudinal joints, when required.

106.7 Approach Slab Design

Approach slab concrete strength shall match that of the bridge deck. Refer to Section 205.4.2.1 – Compressive Strength for concrete material properties.

Except in the area of super elevation transition, the cross slope of the bridge and the roadway should be the same. The designer should lay out grades at corners and center point of the approach slab, including the beginning and the end, along every lane and shoulder line, or as an alternate along beam lines.

Use of elastomeric joint seal for the joint between the concrete pavement and the approach slab is recommended.

Requirements for reinforcing steel, epoxy-coating, and concrete cover shall match that of the bridge deck.

106.7.1 Approach Slab Geometry and Design Requirements

Refer to Section 325.03 – Approach Slab Details for standard approach slab detailing. The minimum length of approach slab shall be 18 feet, and the maximum length shall be 30 feet, providing enough length to span beyond the backfill limits behind the abutment. The thickness of the approach slab shall be 16 inches, unless additional thickness is determined to be needed for the design. The clear concrete cover to the top and bottom mat of reinforcement shall be 2½ inches and 3 inches, respectively.

The approach slab shall be designed as a one-way slab continuously supported at the abutment end and at the roadway end. The concrete slab design shall be in accordance with the methodology outlined in A5.14.4.1 – Cast-in-Place Solid Slab Superstructures; however, longitudinal edge beams need not be provided. The design of the approach slab shall be checked for both bending and shear. The design shall assume no support from the abutment backfill or the base material below the approach slab.

The support at the abutment end shall be provided by one of two details, as shown in Section 325.03 – Approach Slab Details:

  1. Approach slab support notch at the rear face of the abutment backwall, or
  2. A support notch at the top of a concrete end-diaphragm.

When feasible, option 2 above is preferable to option 1.

The support at the roadway end of the approach slab shall be provided by a sleeper slab or deepened end, as detailed in Section 325.03 – Approach Slab Details. The sleeper slab or deepened end-section shall be designed as a beam on elastic foundation (BOEF). A subgrade modulus “kv” between 300 and 500 pound per cubic inch may be used for well-graded, compacted gravel backfill unless otherwise provided in the geotechnical report.

It is preferable that the approach slab be designed and detailed so that the deck joint is provided at the roadway end of the approach slab. When located at the expansion end of the bridge and at integral abutments, the approach slab must be detailed to translate with the superstructure. When this is the intention of the design, the following details shall typically be incorporated into the approach slab detailing, as indicated in Section 325.03 – Approach Slab Details:

  1. The deck joint shall be provided at the roadway end of the approach slab.
  2. A controlled joint in the concrete, with a diagonal bent bar through the joint shall be provided at the bridge end of the approach slab. This detail will allow for rotation of the superstructure relative to the approach slab, while maintaining the ability of the approach slab to translate with the superstructure.
  3. A sliding surface between the underside of the approach slab and the sleeper slab and fill shall be provided.
  4. When the approach slab is detailed to span over the backwall, 1-inch-thick preformed cellular polystyrene joint material or 1-inch-thick neoprene pad surface should be provided below the sliding surface provided to allow the approach slab to translate independently from the backwall.
  5. When the approach slab extends over the approach wingwalls (U-wings), a 1-inch-thick preformed cellular polystyrene filler should be provided to allow the approach slab to move independently from the wingwalls.
  6. If the roadway end of the approach slab or sleeper slab is to be tied to the p.c.c. pavement, consider providing a zero skew for the roadway end of the approach slab or sleeper slab to assist with constructability of the p.c.c. pavement.

It is preferred that the approach slab be detailed as full-width, matching the full width of the bridge deck. As such, the approach slab shall support the bridge barriers as they extend off of the bridge.

106.8 Steel Superstructure Design Considerations

Refer to Section 103.4.1.1 – Structural Steel for recommended types of steel bridges in Delaware.

New steel multi-girder bridges in Delaware shall be designed and constructed as composite with the concrete deck. Refer to Section 106.3.1.1 – Considerations for Deck Haunch for design factors associated with haunch thickness in relation to composite beam design. Longitudinal reinforcing steel in the concrete deck is not to be accounted for in the positive and negative moment regions as part of composite section properties.

106.8.1 Structural Steel – Material Requirements

106.8.1.1 Grade 50 Steel

For new steel bridges in Delaware, AASHTO M270, Grade 50 structural steel is to be used unless otherwise approved by the Bridge Design Engineer. Painting is required on this steel type. For painting requirements, refer to Section 106.8.7 – Protective Coatings.

106.8.1.2 Weathering Steel

AASHTO M270 grade 50W structural steels weather to preclude the need for painting. Weathering steel may be considered for structures over high traffic volume roadways or railroads where access for painting or repainting is limited or dangerous. The use of weathering steel is subject to approval by the Bridge Design Engineer.

Weathering steel should not be used in corrosive environments where there is high humidity or high concentrations of chloride. Refer to the FHWA Technical Advisory T5140.22, Uncoated Weathering Steel in Structures (1989), for further information.

The use of weathering steel must NOT be considered for the following conditions:

  1. If the atmosphere contains concentrated corrosive industrial or chemical fumes.
  2. If the steel is subject to heavy salt-water spray or salt-laden fog.
  3. If the steel is in direct contact with timber decking; timber retains moisture and may be treated with corrosive preservatives.
  4. If the steel is used for a low urban-area bridge or overpass that creates a tunnel-like configuration over a road on which de-icing salt is used. In this situation, road spray from traffic under the bridge causes salt to accumulate on the steel.
  5. If the structure provides low clearance (less than 10 feet) over stagnant or slow-moving water.
  6. Regions where there is constant dampness without drying of the steel.

Provide drip plates (also called drip tabs or drip bars) on outside face of exterior girder bottom flanges of weathering steel girders to divert water runoff from abutments and piers to protect from staining concrete. The drip plates should be located on the high side of piers and abutments, typically 5 feet away from the face of concrete substructures. The designer may increase the distance from the face of tall piers or abutments to limit the potential for wind-blown water to splash on the concrete surfaces.

Ensure that the edges of transverse stiffeners at the corners adjacent to the web and bottom flange are clipped to allow for proper ventilation and drainage. Stiffener details designed and fabricated in accordance with Section 335.01 – Steel Beam Bridge Details will provide for proper ventilation and drainage. Do not detail deck drains to discharge water onto the steel. Therefore, the bottom of drainage pipes should preferably be at an elevation no higher than 1 foot below the bottom of the adjacent girders, unless not possible due to limited under-clearance. In the latter case, the bottom of drainage pipes should not be higher than the bottom of the adjacent girders.

Avoid the use of weathering steel on structures with open-grid decks.

Refer to Section 106.8.7.1.1 – Painting of Weathering Steel regarding the limits of zone painting and requirements for painting of weathering steel.

Refer to Section 106.8.6 – Bolted Connections for requirements associated with the use of bolted connections for weathering steel.

106.8.1.3 High-Performance Steels

AASHTO M270 high-performance steels, Grades HPS 70W and HPS 100W steel, are typically not recommended for use in conventional multi-girder steel bridge construction for spans less than 250 feet. As a general rule, if flange thickness remains in the range of 3 inches or less using Grade 50, the use of higher-strength steels is not recommended. Girder designs using higher-strength steel should NOT significantly increase deflections (flexibility of superstructure) in comparison to the same girder designed with Grade 50 steel.

One potential application of high-strength steel would be the use of HPS 70W steel as part of a hybrid girder. As an example, the use of HPS 70W steel for the top flange over the interior support of a two-span continuous-plate girder may be justified for improved strength, while having minimal change on girder deflections. Generally, however, the use of the same steel type throughout the bridge is preferred, unless cost savings can be justified.

For improved resistance to corrosion, resistance to fracture, and/or to achieve a higher factor-of-safety in design, high-performance steels (grades HPS 50W, HPS 70W, and HPS 100W) are to be used for steel members or elements designated as NSTM or SRM. Refer to Section 106.8.2.1 – Redundancy Requirements for the description and design requirements for NSTMs and SRMs.

106.8.2 Fatigue and Fracture Considerations

The material for all main load-carrying members of steel bridges subject to tensile stresses shall meet AASHTO requirements for notch toughness. Refer to Section A6.6.2 – Fracture and the Standard Specifications. Temperature Zone 2 shall be used to determine the minimum service temperature range in Delaware.

The Department does not permit the use of welded cover plates in the design of new beams, or for beam strengthening as a part of bridge rehabilitation. The Department allows the use of welded with end-bolted cover plates for beam strengthening as a part of bridge rehabilitation, but not for new construction.

Lateral gusset plates with intersecting welds at transverse stiffeners are prohibited. Such details are known to provide tri-axial constraint, which is a fracture-prone detail. Intersecting plates should not be used in new bridge design, but if required, the detail should provide clips in the lateral gusset plate at the girder web and transverse stiffener. The detail shall show a minimum of a ½-inch gap between the vertical and horizontal weld toes at the intersection of the lateral gusset plate, girder web, and transverse stiffener.

Refer to Section 109.9.3.4 – Fatigue Evaluation and Repair for the evaluation and retrofit design for fatigue details on existing bridges.

106.8.2.1 Redundancy Requirements

Whenever practical, new multi-girder steel superstructures shall have a minimum of four longitudinal girders, unless approved by the Bridge Design Engineer.

The reduction or elimination of NSTM shall be a goal of bridge designs. Refer to FHWA Memorandum, [https://www.fhwa.dot.gov/bridge/pubs/MEMO-ATTACHMENT_Inspection-Interval-Implementation-FINAL_508v2.pdf Inspection of Nonredundant Steel Tension Members (2022)]; and FHWA-IF-12-052, Steel Bridge Design Handbook: Redundancy (2012). Redundancy may be classified in one of three ways:

  1. Load Path
  2. Structural (or System)
  3. Internal

Whenever possible, steel superstructures shall incorporate members that meet the requirements for load path redundancy; for example, a minimum of four main longitudinal members, as part of a multi-beam or multi-girder system. All members that do not meet the requirements for load path redundancy shall be classified as either NSTM, or as SRM. An SRM is a member that has demonstrated — through refined analysis — that the structure has adequate strength and stability if the member were removed, or if its primary load path were interrupted. Both NSTMs and SRMs must be designed and fabricated to meet current AASHTO fracture control plan requirements.

SRMs must meet the requirements for structural redundancy proven through refined analysis, per NCHRP Report 406, Redundancy in Highway Bridge Superstructures (1998). If refined analysis is not performed, the members shall be classified as NSTM and load modifiers “h” used per Section A1.3.4 – Redundancy for design.

The Bridge Design Engineer shall approve the designation of SRM instead of NSTM, and retain all necessary documentation for future inspections. Members designed as internally redundant, while good practice, are still recognized as NSTM.

Although no difference will be permitted between NSTM and SRM in fabrication, the SRM designation permits the exemption from NSTM inspection requirements.

All tension elements on NSTM shall be designated as “NSTM” on the Plans. All tension elements on SRMs shall be designated as “SRM” on the Plans. SRMs should have a note included on the Plans to fabricate them in accordance with AWS D1.5 Chapter 12. Materials used for both NSTMs and SRMs shall be as specified in Section 106.8.1.3 – High-Performance Steels.

106.8.2.2 Welding and Weld Procedures

Except for welding shear studs and bearing-sole plates to girder bottom flanges, field bolting should be designed and specified in lieu of field welding. Bolted connections shall be used for field splicing beams and girders. Welded field splices are not permitted.

When practical and feasible, fillet welds are preferred over groove welds. They are typically more cost-effective using manual or semi-automated welding equipment and joint preparation of the steel is eliminated.

When groove welds are required by design, weld symbols placed on the drawings should indicate “CJP” (complete joint penetration) in the tail to allow the fabricator choice of welding equipment, joint type (distortion control), plate preparation, and cost. However, when specific design requirements for weld inspection, finish, contour, limits, backing or field welding are necessary, the designer shall indicate such on the weld symbol. Refer to AWS A2.4, Standard Symbols for Welding, Brazing, and Nondestructive Examination (2020).

Additional inspection or nondestructive testing beyond the requirements of AWS D1.5 shall be specified by the designer.

Review and approval of all Welding Procedure Specifications (WPS) and Procedure Qualification Records (PQR) shall be done by DelDOT Materials and Research.

106.8.3 Steel-Rolled Beams and Plate Girders

Minimize the number of field splices. Field splices shall be at dead-load contraflexure points for continuous spans, where practical. Cost-effective design can often be realized with the use of flange or web transitions at field splice locations. For simple-span steel superstructures over 150 feet, field splices may be needed for shipping or erection. For such span configurations, a field splice should be made at one of the optimal locations for a flange and/or web transition.

The designer shall consider welded material transitions in plate girders (at locations other than at field splices), comparing labor and welding costs against potential material savings.

106.8.3.1 Minimum Plate Thicknesses

The minimum plate and member thickness to be used for primary and secondary permanent elements shall be 3/8 inch.

For plate girders, use a minimum flange plate thickness of 3/4 inch. For minimum thickness of plate girder webs, refer to Section 106.8.3.2.1 – Plate Girder Webs.

106.8.3.2 Plate Girder Geometric Proportionality – General Practice

106.8.3.2.1 Plate Girder Webs

For web depth requirements and recommendations, refer to Section 103.4.1.1 – Structural Steel. A minimum plate thickness of 7/16 inch is recommended for plate girder webs. Web thickness shall vary in 1/16-inch increments.

Plate girder web depths shall vary in whole-inch increments.

106.8.3.2.2 Plate Girder Flange Width

For straight girders, a flange width of approximately one-fourth of the web depth is typically recommended. Generally, the use of flanges less than 15 inches wide is not recommended, to reduce warping during fabrication and improve stability during transportation and erection. Although not common, the use of 12-inch-wide flanges may be acceptable for short spans; and if means of ensuring stability during fabrication, transportation, and erection are specified.

For horizontally curved girders, flange width should be proportioned to approximately one-third the web depth. The extra flange width for curved girders enhances handling stability and helps keep lateral bending stresses within reason.

For deeper girder sections, the recommended flange width requirements may be too conservative. In these cases the designer may consider meeting less conservative requirements as specified in AASHTO LRFD Bridge Specifications.

It is generally best practice to maintain a constant flange width for each girder field section, and make flange width transitions only at field splices. All girders should have the same flange width increase at the same field splice location. Adjacent girders should have the same flange width dimension to simplify slab formwork, and to prevent variation in diaphragm or cross-frame geometry at interior bearings.

Flange-width increments should be in whole inches.

Top and bottom flange widths may be different for composite beams/girders.

106.8.4 Shear Connectors

Welded stud-type shear connectors are to be used for both positive and negative moment regions. Studs with a 0.875-inch diameter are recommended in the typical configuration illustrated in Section 335.01 – Steel Beam Bridge Details. The maximum stud spacing is 2 feet, including the negative-moment regions. Shear studs shall be placed in a minimum of two rows.

106.8.5 Stiffeners, Diaphragms, and Bracing

Refer to Section 335.01 – Steel Beam Bridge Details for standard detailing for stiffeners, diaphragms, and cross-frames.

Transverse stiffeners are provided for one—or a combination of—the following purposes: bearing stiffener, jacking stiffener, intermediate stiffener, and connection plate for diaphragm or cross-frame connections.

For girders with webs less than or equal to 4 feet 6 inches in depth, it is preferable not to use intermediate stiffeners. For girders with webs deeper than 4 feet 6 inches, the web thickness may be increased to limit the transverse stiffeners to only one or two locations near supports beyond those provided for diaphragm or cross-frame connections. Transverse stiffeners must be a minimum of 3/8-inch thick. Stiffeners shall be welded to the web with a minimum 5/16-inch continuous-fillet weld on each side of the stiffener. Intermediate stiffeners shall be welded to the compression flange, and tight-fit to the tension flange.

Transverse stiffeners that are used as connection plates for diaphragms, floorbeams, or cross-frames are to be tight-fit and fillet-welded to both the top and bottom flanges.

Bearing stiffeners shall be mill-to-bear and fillet-welded to bottom flanges and tight-fit and fillet-welded to top flanges; or connected to the top and bottom flanges with full-penetration groove welds. The mill-to-bear/tight-fit and fillet-welded connection is preferred.

Longitudinal stiffeners are typically not economical for modern design of multi-girder spans of less than 250 feet, and are generally not recommended unless proven to be economically justifiable compared to thickening the web and/or providing additional transverse stiffeners. When longitudinal stiffeners are necessary, it is preferable for longitudinal stiffeners to be placed on the opposite side of the girder web from the transverse stiffeners. When a longitudinal stiffener must be on the same side of the web as transverse stiffener(s), the longitudinal stiffener shall be continuous through the transverse stiffener. The transverse stiffener should be made discontinuous at the joint, with a tight fit provided on the top and bottom of the longitudinal stiffener. Longitudinal stiffeners are unnecessary in tension zones, and should be avoided in stress reversal zones. Refer to Section 335.01 – Steel Beam Bridge Details.

Transverse stiffeners shall be chamfered at intersections with flanges and longitudinal stiffeners to prevent intersecting welds.

Refer to Section 106.8.9.1.2 – Construction Loading Conditions for lateral bracing considerations.

106.8.6 Bolted Connections

High-strength 7/8-inch-diameter ASTM F3125 Grade 325 bolts shall be used for the design of bolted connections, unless detailing, constructability, or cost justification is given for the use of alternative size or strength ASTM F3125 Grade 490 bolts. It is preferable that all bolts on a bridge be of the same diameter and strength. Avoid the use of bolts over 1 inch in diameter for structural steel member connections, which require large-installation torques.

Unless approved otherwise by the Bridge Design Engineer, all bolted connections shall be designed as slip-critical connections. Class B faying surface shall typically be used in design. It is important to note that when Class B faying surface is used in design, requirements for testing the paint system must be stated on the contract drawings or construction specifications. Class B faying surface is to be used for unpainted weathering steel and Class C for galvanized steel faying surface, consistent with AASHTO LRFD recommendations. The faying surface classification used in the design shall be provided in the general notes of the bridge design plans.

ASTM F3125 Grade 325 and ASTM F3125 Grade 490 bolts are to be mechanically galvanized in accordance with ASTM B695 Class 50, Type 1, and painted after installation. Mechanical fasteners made of ASTM F3125 Grade 325 and ASTM F3125 Grade 490 weathering steels are suitable for weathering steel bridges. Do not use galvanized carbon-steel bolts for weathering steel bridges. Load indicator washers are not recommended for use with weathering steel bolts.

Twist-off bolt assemblies (ASTM F3125 Grade F1852) are permitted by the Department.

106.8.7 Protective Coatings

The designer is responsible for proposing the appropriate protective coating for steel, specific to each project. As part of the selection of the protective system, the designer shall consider design, construction, and future maintenance implications associated with the protection system, such as requirements for surface preparation, application, permissible shop and/or field applications, time allowances between coats, and containment systems.

Paint systems and painting requirements as provided in this Manual, shall be provided in the general notes on the bridge design plans.

106.8.7.1 Paint Systems

Unless the specifics of the project warrant otherwise, the paint systems specified for use on steel bridges shall match those specified in the Standard Specifications. The types of paint systems, with their associated general applications for use, as presented in the Standard Specifications are provided below:

  1. Type 1 (New Structural Steel): Use a paint system from NEPCOAT Qualified Products List A for shop-painted new structural steel.
  2. Type 2 (Re-painting of Existing Structural Steel): Use a paint system from NEPCOAT Qualified Products List B for field-painting structural steel.
  3. Type 3 (Overcoating of Existing Painted Structural Steel): Use a paint system from NEPCOAT Qualified Products List M for over-coating existing painted structural steel.
  4. Type 4 (Painting of Galvanized Steel): Use an MIO aluminum moisture-cured urethane paint system from NEPCOAT Qualified Products List M for painting galvanized steel surfaces.

Only the primer coat (Type 1 primer coat) shall be applied to the steel within the limits of the steel-to-steel faying surface, except for weathering steel. Where concrete is to be in contact with steel, such as the top of a girder top flange, only the primer coat shall be applied to the steel within the limits of the steel-to-concrete mating surface, except for weathering steel.

All crevices where pack rust could form or which could exhibit pack rust shall be treated with a 100 percent solids penetrating sealer, and sealed using a paintable caulk. At a minimum, the caulk must be painted with one coat of topcoat color.

106.8.7.1.1 Painting of Weathering Steel

To minimize deterioration due to salt spray, it is required to paint the exterior face and top and bottom surfaces of bottom flange (each side of web) of weathering steel fascia beams for spans over highway traffic. Refer to fascia girder painting limits detail in Section 335.01 – Steel Beam Bridge Details. To minimize staining of concrete abutments and piers, all weathering steel members, including bearings, shall be zone-painted to a length of at least 1.5 times the web depth, or a minimum of 10 feet from the face of the concrete substructure, whether a joint is present or not.

The bridge design plans shall indicate or specify the limits of zone painting of weathering steel.

The paint system to be used on weathering steel shall conform to Type 1 or 2, as appropriate.

Weathering steel to concrete mating surfaces and weathering steel to weathering steel faying surfaces are to be left uncoated.

Painting of the interior surfaces of weathering steel tub and box members is recommended for future inspectability.

Prior to recoating of weathering steel, the designer is responsible for evaluation of contaminants, requirements for surface preparation of weathering steel, and specifying a compatible paint system with the substrate.

106.8.7.1.2 Painting of Galvanized Steel

Galvanized steel is only to be painted when appropriate for aesthetic purposes. When painting of galvanized steel is required, galvanized steel surfaces shall be painted with moisture-cured aluminum paint (Paint System Type 4) to ensure adherence to galvanized steel surfaces.

Prior to recoating galvanized steel, the designer is responsible for specifying the requirements for the removal of existing paint, surface preparation, and specifying a paint system.

106.8.7.1.3 Paint Color

The finish coat color for all structural steel shall match chip number 25183 (cyan-blue) of AMS-STD-595A.

When painting just beam ends of weathering steel, the finish coat color shall match chip number 30059 (Brown) of AMS-STD-595A.

Interior surfaces of steel tub and box members shall match chip number 27875 (Insignia White) of AMS-STD-595A. The light color increases illumination inside the tub and box sections, improving detection of corrosion and cracks in the steel members.

The use of other colors requires approval from the Bridge Design Engineer, with documentation as to the reasons for the change.

106.8.7.2 Galvanization

The following items should typically be galvanized:

  1. Bolts, nuts, and washers, except when used with weathering steel
  2. Steel extrusions for strip seal joints
  3. Deck joint structural steel and deck joint support members
  4. Deck joint plates, including tooth dam plates and barrier slider plates
  5. Sign structures
  6. Steel downspouting

Galvanization of other structural steel elements is to be approved by the Bridge Design Engineer.

106.8.8 Steel-Plate Girder and Rolled Beam Bridges

106.8.8.1 Method of Analysis

The method of analysis, 1-D line girder, 2-D grid, or 3-D finite element analysis, used in the design of steel I-beam bridges, is dependent on several factors, including skew and curvature of the structure, span length, bridge width, steel framing, and structure stiffness. Two-dimensional grid analysis is considered a higher level of analysis than line-girder analysis; and 3-D finite element analysis is considered a higher level of analysis than 2-D grid analysis. The appropriate method of analysis required for design should be chosen by the Designer following the procedures outlined in Section 106.8.8.1.1 – Determination of Appropriate Analysis Method using NCHRP Report 725.

For bridges where line-girder analysis is deemed appropriate, it should be used in conjunction with live-load distribution factors and skew adjustment factors found in Section A4.6.2.2.2 – Distribution Factor Method for Moment and Shear and Section A4.6.2.2.3 – Distribution Factor Method for Shear. The V-Load 1-D analysis method is not permitted for final design of curved girder structures.

If using 2-D grid analysis, improvements in the accuracy of the analysis shall be made by incorporating two enhancements into the model:

1. In lieu of the St. Venant torsional constant, , the equivalent torsional constant, , a better approximation of girder torsional stiffness, shall be used.
For cases where the flange warping is fully fixed at the beam ends (interior girder segment), the equivalent torsional constant is equal to:For cases where the flange warping is fully fixed at one end and free at the other end (exterior girder segment), the equivalent torsional constant is equal to:

where:
= distance between cross frame
= St. Venant torsional constant for the girder cross section
=
2. For modeling of a cross-frame as a beam element in a 2-D grid model, the shear deformable (Timoshenko) beam element should be used. Refer to NCHRP Report 725 for the determination of the properties of the Timoshenko beam element.

Analysis shall verify that uplift does not occur at any bearing at any limit state. For curved bridges, the torsion index “IT” should be kept less than 0.65 to avoid uplift at the inside bearings. As a general rule, a minimum of 10 percent of the dead load reaction should be maintained under live load. However, if the bearing design requires a minimum positive reaction beyond 10 percent of the dead load reaction, that minimum vertical reaction should be provided in the design and verified by analysis at the service load limit states.

Refer to Section 106.8.9.1.3 – Cross-Frame Detailing Methods for requirements associated with web-plumbness and presentation of out-of-plane girder rotations for severely skewed and horizontally curved superstructures.

Note that when 2-D grid analysis or 3-D finite element analysis methods are used for the analysis and design of horizontally curved and/or skewed steel superstructures, the designer shall provide a table of live-load distribution factors on the design plans that can be used with a line girder analysis to replicate the response of the structure for the purpose of future simplified analysis and load ratings.

106.8.8.1.1 Determination of Appropriate Analysis Method using NCHRP Report 725

The designer is responsible for selecting an effective and efficient method(s) of analysis for the design of curved and skewed steel girder bridges. The basis of method selection shall be NCHRP Report 725 Guidelines for Analysis Methods and Construction Engineering of Curved and Skewed Steel Girder Bridges (2012), applied to the specific factors of the project in design. Included in this selection should be the selection of quality control checking. The method of analysis is to be identified in the preliminary design phase of the project, as stated in Section 103.3.6 – Bridge Skew.

While NCHRP Report 725 appears to be focused toward determining the level of analysis required for construction analysis, this report is also applicable for assessing the appropriate level of analysis in design. The report, including appendices, is located in the DelDOT DRC.

NCHRP Report 725 uses four key bridge response indices for characterizing the effects of curvature and skew and the ability of simplified methods to capture these effects. The indices and associated ranges where simplified methods of analysis tend to produce unacceptable levels of error associated with various structure responses. Refer to NCHRP Report 725 for further commentary on the following indices and their significance to choosing the appropriate method of analysis. The designer is to use the highest level of analysis recommended among the four indices – with the lowest level of analysis being 1-D line girder analysis and 3-D finite element analysis being the highest. The four indices, with associated ranges identifying where higher level of analysis is appropriate, are provided below:

1. The Skew Index, , defined as:

where:
= width of the bridge measured between the centerline of fascia girders
= largest skew angle of supports
= span length
For multi-span bridges must be calculated for each span and the largest value applied to the entire structure.
For bridges with , the effects of skew are small. For bridges with , skew has a significant influence and flange lateral bending stresses can observe significant errors for lower levels of analysis. When a structure has , the effects of skew are significant, where flange lateral bending stresses, major axis bending stresses and vertical displacements can observe significant errors for lower levels of analysis.
2. The Connectivity Index, , defined as:

where:
= radius of curvature of the bridge centerline in units of feet
= constant equal to 1.0 for simple-span bridges and 2.0 for continuous-span bridges
= number of intermediate cross-frames within the span
For multi-span bridges, and can vary between spans and therefore must be calculated for each span and the largest value applied to the entire structure.
For curved bridges with radial supports and , the anticipated error associated with 2-D grid analysis tends to be small. When a structure has , errors associated with curvature tend to become more significant when using conventional 2-D grid analysis. For bridges with combinations of curvature ( ) and skew (), analysis error tends to become significant when using conventional 2-D grid analysis.
3. The Torsion Index, , is a measure of the potential uplift at bearings and is defined as:

where:
= distance between the centroid of the deck and the chord between the inside fascia girder bearing locations, measured at the bridge mid-span perpendicular to a chord between the intersections of the deck centerline with the bearing lines
= distance between the centroid of the deck and the chord between the outside fascia girder bearing locations, measured at the bridge mid-span perpendicular to a chord between the intersections of the deck centerline with the bearing lines
For bridges with , the structure is not susceptible to uplift at inside bearings. When a structure has , the structure becomes susceptible to uplift at the inside bearings and therefore a higher level analysis should be used to more accurately determine the potential for uplift.
4. The global second-order amplification factor, , which scales the linear response obtained from first-order analyses to determine the second order effects. This index aids the designer in determining whether second-order effects need to be included. The
    amplification factor is defined as:

where:
= maximum total moment supported by the bridge unit for the loading under consideration
= elastic global buckling moment of the bridge unit =
= moment gradient modification factor applied to the full bridge cross section moment diagram
= spacing between the two outside girders of the unit
= modulus of elasticity of steel
= effective moment of inertia of the individual I-girders about their weak axis =
= moment of inertia of the compression flange about the weak axis of the girder cross section
= moment of inertia of the tension flange about the weak axis of the girder cross section
= distance from the mid-thickness of the tension flange to the centroidal axis of the cross section
= distance from the mid-thickness of the compression flange to the centroidal axis of the cross section
= moment of inertia of the individual girders about their major axis of bending
For bridges with , second-order effects are minimal and can be ignored. For bridges with , second-order effects should be included. When a structure has , second-order effects are significant and 3-D finite element analyses should be used.

106.8.8.2 Diaphragms/Cross-Bracing

The recommendations of Section A6.7.4 – Diaphragms and Cross-Frames shall be used in the design, detailing, and spacing of diaphragms and/or cross-frames—with a preference towards minimizing the number of diaphragms or cross-frames for straight multi-beam bridges.

For structures deemed feasible for design by line girder analysis, cross-frame and/or diaphragm members and their connections typically do not need to be designed. Use cross-frames and/or diaphragm typical details from Section 335.01 – Steel Beam Bridge Details if within the geometric limits defined in the typical details.

For horizontally-curved and/or significantly skewed (Is ≥ 0.3) steel superstructures, diaphragm and/or cross-frame members and their connections are considered primary structural members and connections. Therefore, they shall be designed for the loads determined by analysis. Although the configurations and general design and detailing concepts presented in the standard diaphragm and cross-frame details in Section 335.01 – Steel Beam Bridge Details should be followed, the members and their connections shall be designed or verified.

For structures with parallel supports at skews less than or equal to 20 degrees, the cross-frames shall be oriented parallel to the centerline of bearings. For support skews of varying skew angles less than 20 degrees, consideration should be given to framing the interior diaphragms/cross-frames perpendicular to the centerline of the girders. For horizontally curved steel superstructures and/or superstructures with skews greater than 20 degrees, it is generally preferred that the interior cross-frames or diaphragms be oriented radially/perpendicular to the girders. Likewise, when practical, it is preferred that the cross-frames or diaphragms be aligned radially/perpendicular to the girders and framed into the fixed or guided expansion bearings at interior supports. Refer to Section 106.8.8.3 – Bearings for Horizontally Curved and/or Skewed Steel Superstructures for bearing type and bearing configuration recommendations for horizontally curved and/or skewed steel superstructures.

End diaphragms/cross-frames shall always be aligned with the centerline of bearing along the end support.

Selectively omitted diaphragms/cross bracings near supports to reduce unwanted transverse stiffness (“nuisance stiffness”) between girders and the use of “lean-on” type bracing concepts may be permitted only if approved by the Bridge Design Engineer.

106.8.8.3 Bearings for Horizontally Curved and/or Skewed Steel Superstructures

Bearings with multi-rotational capabilities shall be used for horizontally curved and/or skewed (with Is > 0.3) steel superstructures. Multi-rotational bearing types include circular elastomeric bearings and HLMR bearings, which include pot bearings and disc bearings.

For horizontally curved and/or skewed (with Is > 0.3) steel superstructures, the following shall be used as a guide in the layout of bearings at substructure units:

  1. At fixed bearing lines, provide two fixed bearings at the two interior-most girders for cross sections with an even number of girders. Provide three fixed bearings at the three interior-most girders for cross sections with an odd number of girders. Provide unguided expansion bearings for support of the remaining girders in the cross section. For purposes of redundancy (with the exception of extreme load cases), it is recommended that each fixed bearing be designed to carry all of the horizontal loads (lateral and longitudinal loads).
  2. At expansion bearing lines, provide two guided expansion bearings at the two interior-most girders for cross sections with an even number of girders. Provide three guided expansion bearings at the three interior-most girders for cross sections with an odd number of girders. Provide unguided expansion bearings for support of the remaining girders in the cross section. For purposes of redundancy (with the exception of thermal loads and extreme load cases), it is recommended that each guided expansion bearing be designed to carry all of the lateral loads. Align the guides for guided bearings in the direction that the superstructure tends to thermally translate, as though the bearings were unguided. The direction of thermal movement should be determined from the advanced analysis (2-D grid or 3-D finite element analysis) used in the design of the superstructure. The transverse component of movement must be accounted for in the design of the deck joint, as applicable. Note also that if the guides are not oriented in the direction that the bridge tends to move thermally (as if unguided), then the bearings (including the guides) must be designed for thermal forces induced as a result of redirecting the superstructure movement in the guided direction.

Refer to Section 106.10 – Bearings for additional requirements to be incorporated into the final design of bearings.

106.8.9 Erection Analysis and Erection Plans

106.8.9.1 Requirements for Designer

Structural analysis shall be performed and conceptual sequential structural steel erection plans and procedures shall be included on the bridge design drawings for the following conditions:

  1. Structure with one or more spans over 200 feet.
  2. Horizontally curved structures, and/or when advanced analysis (2-D grid or 3-D finite element analysis) is used in the design, per Section 106.8.8.1.1 – Determination of Appropriate Analysis Method using NCHRP Report 725.
  3. Where temporary supports, complex falsework, and/or conditions where multi-crane operations are anticipated to be required for the bridge erection.
  4. For erection over freeways, where MOT and/or lane closures are anticipated to be required during erection.
  5. For erection with potential for conflict and/or coordination with railroads or overhead utilities.
106.8.9.1.1 Erection Plan Details

A conceptual erection plan is to be provided on the Plans and detailed as needed to establish constructability and construction cost. The conceptual erection plan need only portray one possible method of erection. The erection plan should show the following information, as applicable to the project:

  1. Suggested construction sequence for the erection of field sections.
  2. Crane footprint for erection of field sections associated with the suggested erection plan, as needed for constructability and/or MOT.
  3. Crane picks in terms of single-girder or two-girder picks, as applicable or as needed for girder stability. Provide table of associated pick weights.
  4. Suggested layout and conceptual design of temporary support systems, per AASHTO’s Guide Design Specification for Bridge Temporary Works (1995).
  5. Requirements for stability/bracing of girders during erection. The actual number of bolts required at connections for bracing and splices, prior to release of crane, is not to be the requirement of the designer. This shall be the requirement of the Contractor as part of his erection plans.
  6. Limits of right-of-way, suggested means for construction access and staging areas, and limits of temporary easements, as required.
  7. MOT/rail operations during erection of field sections. This should include any requirements for detours, outages, etc. Provide list of outages in terms of number of outages required and requirements for overnight and/or time duration for each outage.
  8. Locations of potential conflicts associated with underground or overhead utilities, specifying means for avoidance, mitigating risk of interference, conceptual layout for temporary utility relocation, and/or design of permanent utility relocation.

Refer to Section 106.8.9.1.3 – Cross-Frame Detailing Methods for additional considerations associated with significantly skewed and horizontally curved superstructures.

Items listed in Section 106.8.9.2 – Erection Submittal Requirements for Contractor’s Engineer that are not listed above shall typically NOT be the responsibility of the design engineer; however, the designer shall ensure that requirements of the Contractor are incorporated into the contract documents, as applicable.

106.8.9.1.2 Construction Loading Conditions

Structural analysis shall be performed considering the effects of applicable dead loads, construction loads and wind on the completed steel framing. Wind loading shall be evaluated per the AASHTO Guide Specifications for Wind Loads on Bridges During Construction (2017). The analysis shall conservatively include the weight of the non-composite deck unless provisions for staged composite behavior are provided and required in the Contract Documents. Means for calculating lateral stresses in girder flanges may be through 2D or 3D analysis methods, or by approximate methods. Lateral flange stresses due to horizontal curvature and/or skew (for skews over 20 degrees) shall be superimposed with lateral stresses due to wind, as applicable.

It is generally not required, and not desired, to include lateral bracing as part of the wind resistance system in the bridge’s final configuration (after construction of a composite deck system). The preference is for the system to be designed to resist wind loads through diaphragm action in the composite bridge deck, and with the cross-frames/diaphragms at the supports being designed to carry the wind loads to the bearings.

If lateral bracing is required to resist wind loads during construction, the design and detailing of the lateral bracing is to be performed during the design phase. The contract drawings must state whether the lateral bracing members are to be removed, or if they are to remain after the construction of the bridge deck. If to remain, the designer must design and detail the lateral system for all applicable AASHTO LRFD limits states, not only for combinations of dead load and wind load. The connection detail shall be such that it does not impart high local stresses in the girder or create problematic distortion-induced fatigue or fracture-prone details.

The designer shall generally prohibit unusual construction loading conditions in the contract documents; however, if unusual construction loading or placement of construction materials on the structure is anticipated to be required for construction, the designer shall verify such conditions during design, and require the Contractor to also verify. Such requirements shall be incorporated into the contract documents.

106.8.9.1.3 Cross-Frame Detailing Methods

For curved and/or skewed bridges meeting conditions specified in Section A6.7.2, the designer is responsible for specifying the dead load condition at which the girder webs are approximately plumb from one of the following conditions:

  • No-load fit (NLF)
  • Steel dead load fit (SDLF)
  • Total dead load fit (TDLF)

The condition specified will determine the initial lack-of-fit, effort to connect the cross-frames to the girders, and resulting locked-in stresses in the final position. Section A.6.7.2 provides guidance on selection of fit condition and when it is appropriate to consider locked-in force effects. For additional background information, refer to NCHRP Project 20-07/Task 355.

106.8.9.2 Erection Submittal Requirements for Contractor’s Engineer

The bridge designer shall anticipate the erection submittal requirements of the Contractor as stipulated in the Standard Specifications, and supplement the requirements with Special Provisions, as recommended in the Manual and/or as required for the project.

For the five conditions stipulated in Section 106.8.9.1 – Requirements for Designer, or as required for the project, the designer shall specify on the Plans that an erection submission by the Contractor be signed and sealed by a Professional Engineer licensed in the State of Delaware, and submitted for approval in accordance with Standard Specifications. The erection submittal shall be specified on the contract drawings to include the following items, as applicable for the project:

  1. Erection plan and sequence for the sequential erection of field sections.
  2. Placement and size of crane for erection of field sections associated with the suggested erection plan.
  3. Indicate crane picks in terms of single-girder or two-girder picks, as applicable or as needed for girder stability. Provide table of associated pick weights. Erection submittal shall include crane charts for review and correlation.
  4. Design of temporary support systems, per Guide Design Specification for Bridge Temporary Works.
  5. Requirements for stability/bracing of girders during erection. Note that when the Contractor intends to partially complete bolted connections during stages of the erection, the Contractor shall provide calculations to support the temporary conditions.
  6. Limits of right-of-way, means for construction access and staging areas, and limits of temporary easements, as required.
  7. Show maintenance of highway traffic and/or railroad limits/rail operations during erection of field sections. This should include any requirements for detours, outages, etc. Provide list of outages in terms of number of outages required and requirements for overnight and/or time duration for each outage.
  8. Locations of potential conflicts associated with underground or overhead utilities, specifying means for avoidance, mitigating risk of interference, and/or temporary or permanent relocations.
  9. The Contractor will also be responsible for the design of temporary support systems and for design of the structure during all stages of construction, including conditions where members and/or connections are partially constructed.
  10. The Contractor’s design shall be in accordance with Section 106.8.9.1 – Requirements for Designer, and no less than what is required in the most recent edition of the Guide Design Specifications for Bridge Temporary Works.
  11. Contract documents shall specify that the contractor shall not perform the erection until review and approval of the erection submittal is received.

106.8.10 Deck Placement Sequence Analysis and Design

Deck placement sequence analysis and design shall be required for multi-span continuous steel bridges. Refer to Section 106.4.2.6 – Deck Placement Sequence for deck placement analysis requirements.

106.9 Prestressed Concrete Bridge Superstructures

106.9.1 Materials

Precast, prestressed concrete members should be designed with structural design strength (f’c) between 5 kips per square inch and 10 kips per square inch. For use of design strengths greater than 8 kips per square inch, there must be a clear economic advantage to be gained. Justification for using structural design strength greater than 8 kips per square inch must be submitted at the TS&L stage for approval. Refer to Section 205.4.2.1 – Compressive Strength for additional information regarding concrete strengths to be used.

Lightweight concrete shall not be used for precast, prestressed concrete beams.

Reinforcing steel shall conform to the requirements of AASHTO M31/M31M, Grade 60. The minimum-size reinforcing shall be No. 4 bar.

Prestressing steel shall be high-strength 7-wire low-relaxation strands, with nominal 0.5- or 0.6-inch diameter, and conform to AASHTO M203, Grade 270, low-relaxation. Do not use stress-relieved strands. The use of straight-strand configurations is preferred over draped-strand configurations.

Bars used for post-tensioning systems should conform to the requirements of ASTM A 722. This specification covers both plain and deformed bars.

For post-tensioned structures, the designer shall specify that all strands will be uncoated, and all ducts shall be pressure-grouted.

Ducts for post-tensioning systems may be either rigid or semi-rigid, and made of ferrous metal or polyethylene. They may also be formed in the concrete with removable cores. The use of polyethylene ducts is generally recommended for corrosive environments. Polyethylene ducts should not be used on radii less than 30 feet because of the polyethylene’s lack of resistance to abrasion during pulling and tensioning the tendons. The inside diameter of the duct should be at least a 1/4 inch larger than the diameter of single-bar or strand tendons. For multiple-bar or strand tendons, the inside cross-sectional area of the duct should be at least twice the net area of the prestressing steel. Where tendons are to be placed by the pull-through method, the duct area should be at least 2.5 times the net area of the prestressing steel.

106.9.2 Design Methodology

Precast, prestressed concrete beams shall be designed for service limit state for allowable stresses and checked for strength limit state for ultimate capacity.

106.9.2.1 Design Methodology

Unless significant differential settlement between supports is anticipated, all multi-span prestressed concrete superstructures shall be made continuous for live load. A minimum girder age of at least 90 days is required when continuity is established. Establishing continuity prior to 90 days requires prior approval from the Bridge Engineer. Minimum age of girder at establishment of continuity shall be shown on the Contract Documents.

DelDOT’s practice is to establish the continuity connection at the same time as the placement of the deck concrete. Therefore, dead load due to the deck concrete will be resisted by the prestressed beams as simply supported. All loads applied after the deck concrete cures will be resisted by the continuous composite structure.

Precast, prestressed concrete beams shall be designed for the envelopes of simple- and continuous-span loadings for all permanent and transient loads. Loads applied prior to establishing continuity need only be applied as a simple-span loading. Continuity reinforcement shall be provided at supports for loads applied after establishing continuity.

106.9.3 Diaphragm Requirements

Diaphragms serve two purposes when used with prestressed beams:

  1. Construction Stage:  During the construction stage, diaphragms help to provide beam stability for pouring the deck slab.
  2. Normal Operation:  During the life of the bridge, diaphragms act to distribute load, and are particularly advantageous for distribution of large overloads. Diaphragms also improve the structures resistance to impact loads from over-height vehicles traveling under the structure.

Diaphragms for prestressed beams shall be cast-in-place or precast concrete for spread box beam and NEXT beam bridges. Diaphragms for PCEF bulb-tee beams may be either cast-in-place concrete, precast concrete, or steel diaphragms. Steel diaphragms for PCEF bulb-tee bridges are permitted with approval of the Bridge Design Engineer. Concrete end diaphragms shall be provided at all bearing lines. Interior diaphragms shall be provided for all prestressed beam bridges with recommended diaphragm spacing, as shown below:

  1. 1/4 points of span for 120 feet < span length ≤ 160 feet
  2. 1/3 points of span for 80 feet < span length ≤ 120 feet
  3. Mid-point of span for 40 feet < span length ≤ 80 feet
  4. No diaphragms required for span lengths ≤ 40 feet

106.9.4 Minimum Spacing of Prestressing Tendons

Spacing of prestressing strands shall typically be at 2-inch increments. The minimum spacing between prestressing strands shall be the larger of:

  1. Center-to-center spacing of 2 inches; or
  2. Clear distance of 2 times the maximum size of aggregate.

Prestressing strands may be bundled in a vertical plane at—and between—hold-down devices, provided that the spacing, specified herein, is maintained between individual strands near the ends of the beams for a distance not less than the maximum shielded length plus development length.

Groups of eight strands of 0.5 or 0.6 inch in diameter or smaller may be bundled linearly in a vertical plane at and between hold-down devices. The number of strands bundled in any other manner shall not exceed four.

106.9.5 Tensile Stresses Due to Prestressing

If higher-than-allowable tensile stresses are encountered during the design of prestressed members (top surface at beam ends), the following design modifications are suggested:

  1. De-bond strands at the end of the unit to reduce the overstress in accordance with Section A5.9.4.3.3 – Debonded Strands.
  2. Drape strands for PCEF bulb-tee beams. When draping is required, the following criteria shall be followed:
    1. The slope of the deflected strands shall be limited to 9 degrees;
    2. The total hold-down force of all draped strands shall not exceed 75 percent of the total beam weight;
    3. When the initial hold-down force exceeds 20 kips, place the following note on the Plans:
    THE HOLD DOWN FORCE DUE TO DRAPED STRANDS IS xxx KIPS.

106.9.6 De-bonding Versus Draping

It is recommended that the designer use the following general guidelines to specify the use of de-bonding versus draping to control stresses:

  1. Draping of strands in slab, NEXT beams and box sections shall not be allowed;
  2. Bulb-tee beams should be de-bonded for beam lengths up to 120 feet; for beam lengths over 120 feet, the designer should use draped strands.
  3. Draping strands is typically more effective for beams that are 87 inches or deeper; and
  4. If de-bonding works with the addition of six strands or less, in comparison to draping, then design using de-bonded strands.

106.9.7 Tensile Stresses at Service Limit State After Losses

Refer to Section 205.9.2.3.2b for the department's tensile stress limit preferences.

106.9.8 Reinforcement

Reinforcement in prestressed beams shall be epoxy-coated.

106.9.8.1 Composite Shear Reinforcement

Composite flexural members consist of prestressed members acting with a cast-in-place or precast-concrete deck. For the deck to act compositely, reinforcement must be provided, extending from the beam into the deck to resist the horizontal shear that develops across this plane. Composite shear reinforcement shall be provided for the full length of the prestressed beam, including the negative moment areas of continuous spans.

106.9.8.2 Anchorage Zone Reinforcement

When prestressing strands are released and the prestressing force is transferred to the hardened concrete, the ends of the beam experiences tensile stresses perpendicular to the direction of prestressing. Anchorage-zone reinforcement shall be provided to resist these stresses. For slabs and box beams, stirrups with multiple legs can be used to accommodate the required reinforcing within the specified distance from the end of the beam

106.9.9 Skew Effects

Skew in prestressed beam bridges affects structural behavior, member analysis, and can complicate construction. The following skew limitations, analysis requirements, and end detailing shall be used to mitigate skew effects for improved design and construction.

  1. Analysis Typically, the effect of skew on beam analysis is accounted for by including the skew correction factor. It is assumed that skew has little effect on normal spans and normal skews. For short, wide spans and for extreme skews (values over 45 degrees), the effect of the skew on structural behavior and load distribution shall be determined by structural analysis. Depending on the beam type, the following skew restrictions apply:
    1. Adjacent box beams:  40 degrees maximum skew
    2. Spread-box beams:  45 degrees maximum skew
    3. I-Beams and bulb-tee beams:  60 degrees maximum skew
    4. NEXT beams: 30 degrees maximum skew
  2. End Detailing:
    1. Box beams To minimize labor costs and to avoid over-stressing, it is preferable that the ends of box beams be skewed. Skewed ends of box beams should match the skew of the substructure unit they rest on at either end.
    2. I-Beams and bulb-tee beams The ends are permitted to be clipped to avoid interference with another beam or backwall. The clipped flange, however, must not extend into the web.

106.9.9.1 Grade and Cross-Slope Effects

Set the transverse beam slope relative to the beam axis as follows:

  1. I-Beams and bulb-tee beams Set beams truly vertical in all cases.
  2. Spread box beams Set beams truly vertical or on a slope to conform to the deck cross-slope. Special consideration should be considered when setting on a slope in areas of super-elevation transition or within a vertical-curve profile with skewed supports. When setting beams vertical, properly consider effects of haunch thickness on beam design and detailing; specifically, the additional weight of concrete and the need for haunch reinforcement.
  3. Adjacent box beams and NEXT beams: Set beams to conform as closely as practical to the deck cross-slope to minimize the haunch thickness and to align holes for the transverse post-tensioning tendons or rods. In areas of super-elevation transition or in a vertical-curve profile with skewed supports, additional haunch or stepped beam seats may be required.

106.9.9.2 Horizontal Curve and Flare Effect

Horizontal curves and tapered roadways each tend to complicate the design of straight beams. Variable overhang dimensions must be investigated for feasibility for structures supporting horizontally curved and tapered roadways. The designer must determine what girder spacing to use for dead- and live-load design, and whether or not a refined analysis that considers actual load application is warranted. The use of parallel beam framing is preferred to splayed framing, when practical. For splayed or variable-width beam spacing, the design girder spacing shall be the two-thirds value between the maximum and minimum spacing values, for the purpose of strength design checks using line girder analysis methodology. Similarly, for variable overhangs, the design overhang shall be the two-thirds value between the maximum and minimum overhang values, for the purpose strength design checks for the exterior beam using line-girder analysis methodology.

106.9.10 Camber

Prestressed beams shall be designed so that the algebraic sum of the beam camber at prestress transfer due to prestress force, the beam dead-load deflections due to non-composite dead load, and superimposed dead-load deflections due to applied superimposed dead loads results in a positive (upward) camber. Camber may increase or decrease with time, depending on the stress distribution across the member under sustained loads. Refer to Section 205.6.3.5 – Deformations for methods of calculating camber and deflections.

The Plans shall show the camber at prestress transfer and the deflections due to non-composite dead load and superimposed dead load.

106.9.10.1 Consideration for Staged Construction Camber

For a given project, fabricators typically cast all of the beams of a given size at the same time to minimize the time required to set up the casting beds. If these beams are subsequently erected at the same time, differential camber between the beams is rarely significant.

On staged construction projects, the precast beams may be fabricated at relatively the same time, and erected months, even years, apart. The haunch provided for spread box beams and PCEF bulb-tees is typically sufficient to accommodate this differential camber growth, and need not be considered. However, for adjacent precast superstructures, the differential camber is significant since they are typically detailed to adjoin and align vertically, and therefore, time dependent camber effects may need to be incorporated in the design.

If camber growth is anticipated between stages, specific measures to control camber growth shall be specified in the contract documents.

106.9.10.2 Simple Spans Made Continuous

Unless significant differential settlement between supports is anticipated, all multi-span prestressed concrete superstructures shall be made continuous for live load. A minimum girder age of at least 90 days is required when continuity is established. Establishing continuity prior to 90 days requires prior approval from the Bridge Engineer. Minimum age of girder at establishment of continuity shall be shown on the Contract Documents.

DelDOT’s practice is to establish the continuity connection at the same time as the placement of the deck concrete. Therefore, dead load due to the deck concrete will be resisted by the prestressed beams as simply supported. All loads applied after the deck concrete cures will be resisted by the continuous composite structure.

106.10 Bearings

Bridge bearings for steel or concrete beams/girders are divided into three categories:  elastomeric, HLMR, and mechanical. These bearing categories are sufficient to cover the vast majority of structures. It is the responsibility of the designer to determine which bearing type is best suited to cost effectively accommodate the requirements of the design.

106.10.1 Elastomeric Bearings

Elastomeric bearings have a low initial cost when compared to other bearing types, and require virtually no long-term maintenance. Elastomeric bearings come in three predominant types:  plain, steel reinforced, and cotton duck. Elastomeric pads shall be steel-reinforced for bridges in Delaware.

106.10.1.1 Steel-Reinforced Elastomeric Bearings

Steel-reinforced elastomeric bearings rely on friction between the contact surfaces, as well as the restraint of the bonded steel shims to resist elastomer bulging. The thin, uniformly spaced elastomer layers allow for higher compressive stresses and higher translation and rotation capacity than plain elastomeric bearing pads (PEPs).

By using multiple layers of elastomer, steel-reinforced elastomeric bearings can handle larger rotations and translations than other types of elastomeric bearings, but the designer needs to ensure stability requirements are satisfied. If horizontal shear force is greater than one-fifth of the minimum permanent dead load, the bearing is subject to slip and shall be secured against horizontal movement per methods described in Section 106.10.9 – Anchorage to Structure. The one-fifth limit is directly related to the design coefficient of friction that can be assumed between elastomer and clean concrete and unpolished, debris-free steel.

106.10.2 High-Load Multi-Rotational Bearings

HLMR bearings are frequently used on modern steel bridges where the number of girders is minimized and the span lengths are maximized. Pot, disc and spherical bearings currently make-up the readily available variety of HLMR bearings that support high loads and that are able to rotate in any direction. They can be fixed or, when fabricated with sliding surfaces, can accommodate translation for use as expansion bearings. In addition, guide bars can be used to restrict movement to one direction.

106.10.2.1 Pot Bearings

Pot bearings subject a confined elastomeric element (disc) to high pressures, effectively causing the disc to behave as a fluid. The neoprene or natural rubber elastomeric disc is confined within a machined pot plate. The vertical force is transmitted to the elastomeric disc via the piston, which sits within the pot. Tight fitting brass sealing rings prevent the elastomer from escaping in the gap between the piston and the pot. Horizontal forces are resisted by contact of the piston face against the pot wall. The vertical and horizontal loads are transmitted from the piston and pot to the sole and masonry plates through bearing and by mechanical connections.

106.10.2.2 Disc Bearings

Disc bearings subject an unconfined elastomeric disc to high pressures. The polyether urethane disc is stiff against compression and rotation, but is free to bulge. Horizontal forces are transmitted from an upper load plate either to a shear pin at the center of the disc or to a restricting ring. The latter is similar in detail to the pot bearing, except that the disc is unconfined with no requirement for sealing rings. If a restricting ring configuration is used, a positive locator device is supplied. The shear pin serves this purpose when it is used to resist the horizontal loads.

106.10.2.3 Spherical Bearings

Spherical bearings transmit all loads, both vertical and horizontal, through the spherical coupling of a convex and concave plate. This interface is typically a mating of low coefficient of friction PTFE and stainless steel. All vertical loads are assumed to be transmitted radially through the interface and all horizontal loads are resisted by the spherical geometry of the plates.

106.10.2.4 Mechanical Bearings

Mechanical bearings (incorporation of bronze plates) or steel bearings distribute forces, both vertical and horizontal, through metal-to-metal contact. Most fixed bearings rely upon a pin or knuckle to allow rotation while restricting translational movement. Rockers, rollers, and sliding types are common expansion types historically used and under certain circumstances can still be used today.

The metal-to-metal contact typically results in corrosion and can eventually lead to “freezing” of the bearing components. Lubricants have been used to mitigate corrosion, but trap debris, which in turn holds moisture and promotes corrosion. Mechanical bearings should not be specified for new designs unless special circumstances exist. For example, this bearing type might be used in a bridge widening where existing bearing styles must be matched.

106.10.3 Guidelines for Bearing Selection

Each bearing type has practical limitations that make it more or less suitable for a particular design. In this section, requirements and appropriateness of bearing types are discussed with respect to design and fabrication.

106.10.3.1 Bearing Type Preferences

Selection of bearing type should be done considering the following guidelines:

  1. Rectangular steel reinforced elastomeric pads shall be used for straight bridges with skews less than or equal to 20 degrees, when structurally feasible.
  2. For skews greater than 20 degrees and horizontally curved girders, use circular elastomeric pads or HLMR bearing types.
  3. When compressive capacity cannot be accommodated by steel reinforced elastomeric pads, select the most cost-effective HLMR bearing type, with a general preference for pot bearings.
  4. When movement capabilities and/or stability checks cannot be achieved with steel reinforced elastomeric pads, the use of PTFE/stainless steel sliding surface details in conjunction with the steel reinforced elastomeric pad shall be considered. If these details cannot be achieved, use the most cost-effective HLMR bearing type.

It is prohibited to mix bearing types along a given substructure bearing line, and it is not recommended to mix bearing types on new bridges.

Refer to Section 345.01 – Elastomeric Bearing Details and Section 345.02 – Pot Bearings for typical bearing details to be used in Delaware when feasible.

106.10.3.2 Feasibility due to Fabrication, Installation and Testing Limitations

Perhaps the single most limiting factor to contribute to a bearing type selection is the feasibility of the bearing to be fabricated and tested.

Steel reinforced elastomeric bearings are molded in the presence of high heat and pressure. AASHTO requires load testing to 150 percent of the maximum design stress. Designs that approach the recommended maximum compressive forces and translations limits should be verified with fabricators at an early stage in design.

The designer shall include a temperature-setting table on the Plans for HLMR expansion bearings. The table should indicate the position of the top plates of the bearing relative to the base plates for different installation temperatures.

106.10.4 Loads, Rotation and Translation

Horizontal loads to the bearing resulting from translation restraint or Extreme Event I (seismic) come from the analysis of the structure. Bridges in Delaware shall meet the requirements of Seismic Zone 1, per A3.10.9.2 – Seismic Zone 1. As such, the horizontal design connection force in the restrained direction(s) shall meet the requirements of A3.10.9.2 – Seismic Zone 1.

Whether or not the bearing is intended to resist movement, the bearing, connections and substructure units should be designed to transfer the forces imparted by the bearing’s resistance to movement. Elastomeric bearings resist movement by shear stiffness. Additionally, the frictional forces of steel bearings and bearings utilizing PTFE/stainless steel sliding surfaces should be considered. The design coefficients of friction should be examined at all compressive load levels and the expected low temperature.

106.10.5 Design Requirements

This section discusses recommendations and considerations for design.

106.10.5.1 Elastomeric Bearings

Steel reinforced elastomeric bearings are to be designed using “Method B.” Based on stability and economics, a limitation of 4 inches of translation is generally practical without the addition of a sliding surface, and rotation is generally limited to 0.02 radians.

Steel reinforced elastomeric bearings are designed for conditions in which the direction of movement and live load rotation is along the same axis and therefore, rectangular shapes are suitable. For horizontally curved and highly skewed structures, these directions may not coincide, or their directions may not be easily defined. In these situations, circular bearings may be considered since they can easily accommodate translation and rotation in any direction.

Shear modulus (G) is a critical material property in the design and performance of elastomeric bearings. The designer shall verify the bearing meets design requirements for the full range of values for G as shown in AASHTO for the prescribed durometer. Fabricators have compounds for different durometer hardness, which in turn have average shear moduli. Although it is possible to specify the elastomer by a shear modulus, check with fabricators to obtain their shear modulus limits. If the elastomer is specified by its shear modulus, AASHTO allows the fabricator to provide a measured shear modulus within fifteen percent of the value specified. Instead, elastomers are typically specified by durometer hardness only. No reference to a required shear modulus should be stated if specifying durometer hardness, and vice versa.

Elastomeric bearings cannot be set with an initial offset to account for varying temperatures at the time of installation. For bearings that must be reset, the contract documents should include provisions for directing the contractor to jack the girders and allowing the bearings to return to their un-deformed shape. If the elastomeric bearing includes a sliding surface, the designer should indicate, in the Plans, the initial offset from centerline to use during erection/installation depending on temperature.

For the initial design attempt, it is recommended that the elastomeric bearings be designed for one-way translation equal to the movement expected through the entire high-low temperature range. This is a conservative approach, but is a practical means for allowing bearings to be set at any temperature without requiring the bearings to be reset at a given mid-range temperature. If a reduced temperature range is required for the design, the designer shall specify on the Plans the maximum temperature ranged permitted at initial set, or require that the bearings be re-set within the permissible temperature range as part of the construction contract.

If under full dead load and at the mean annual temperature, the underside of the girder is out of level by more than 0.01 radians (1 percent), beveled sole plates shall be provided to produce a level-bearing surface at the top of the elastomeric bearing. This implies that beveled sole plates are not required if the out-of-plane rotation is less than 1 percent. If the designer chooses to not use beveled sole plates at slopes less than or equal to 1 percent, then the additional permanent rotation induced by the out-of-plane condition must be added into the required design rotation sum, including the 0.005 radian allowance for uncertainties.

106.10.5.2 High-load Multi-Rotational Bearings

For horizontally restrained spherical bearings with PTFE, the ratio of the maximum horizontal force to the minimum vertical force should not exceed 0.40 to avoid overstressing the PTFE fabric at the spherical interface. If this criterion cannot be met, alternate means to transfer the horizontal forces should be employed.

106.10.5.3 Design Limitations

HLMR bearings designed for expansion with a PTFE/stainless steel sliding surface can nearly accommodate horizontal movements in any range. However, due to the stiffness of the elastomeric element, disc bearings should be limited to a rotation of 0.03 radians. Pot bearings can safely be designed for rotations in the range of 0.04 to 0.05 radians, and spherical bearings can be designed for rotations in excess of 0.05 radians. If the minimum vertical load is less than twenty percent of the vertical design capacity of the bearing, HLMR bearings should not be used, in accordance with AASHTO.

106.10.6 Consecutively Fixed Piers

When it is advantageous to the overall design, consecutively fixed piers should be utilized. It is generally advantageous for tall, slender piers. An analysis should be performed, taking into account the stiffness of the piers, thermal movements and distribution of horizontal forces. The determination of the number of piers to be consecutively fixed must be based on cost-effectiveness.

When consecutively fixed piers are used in a design, instructions for jacking the required deflection into the piers for proper positioning of the bearings under the beams shall be shown on the drawings only if required by pier design. If required, a table of dimensions shall be included showing the relative distance that each pier must be moved for each five degrees in temperature variation from the mid-range of the anticipated temperature extremes.

The theoretical fixed point on the bridge, based on the overall stiffness of the structural system, incorporating the relative stiffness and heights of the piers that are fixed, shall also be shown on the Plans.

106.10.7 Accommodations for Future Bearing Replacement

All bearings should be considered replaceable. Provisions should be made during the design stage to ensure that the superstructure and substructure elements are detailed to accommodate future jacking and removal of each bearing element. Likewise, for HLMR bearings, the entire bearing, or internal elements of the bearing assembly, should be designed for removal and replacement.

106.10.8 Bearings for Horizontally-Curved and/or Skewed Bridges

Refer to Section 106.10.3.1 – Bearing Type Preferences for selection of bearing types for horizontally curved and/or significantly (>20 degrees) skewed concrete and steel superstructures. Refer to Section 106.8.8.3 – Bearings for Horizontally Curved and/or Skewed Steel Superstructures for guidance on the layout of bearings for horizontally-curved and/or skewed (>20 degrees) steel superstructures.

106.10.9 Anchorage to Structure

106.10.9.1 Sole Plates

Sole plates (a plate, typically welded, attached to the bottom flange of a beam that distributes the reaction of the bearing to the beam) are not always required with the design of elastomeric bearings. When they are, beveled sole plates should be used to produce a level bearing surface at the top of the elastomeric bearing when the underside of the girder, under the full dead load and at the mean annual temperature, is out of level by more than 0.01 radians (1 percent). In addition, if the required difference in the sole plate thickness due to the bevel exceeds 0.125 inch, the sole plate should be beveled. Fabricators have the resources to machine nearly any bevel required. The designer shall include the bevel information in the contract documents.

Beveled sole-plate thickness should not be less than 0.75 inch, and should be designed for bending if the width of the elastomeric bearing extends beyond the edges of the girder flange.

The sole plate should extend transversely beyond the edge of the bottom flange of the girder a minimum of 1 inch on each side.

Similar to the connection between the elastomeric pad and the masonry plate, refer to Section 106.10.9.2 – Masonry Plates and Anchor Rods for options to secure the connection between the sole plate and the elastomeric pad, when determined to be required for securing against “walking.” “Walking” refers to slippage or sliding between the bottom or top surface of the elastomeric pad and the concrete or steel surface against which it is bearing.

Overhead welds should be avoided due to limited clearance. The bearing should be detailed with at least 1.5 inches between the elastomer and any field welds. The welds for the sole plate connection should only be along the longitudinal girder axis. Transverse joints should be sealed with an acceptable caulking material.

106.10.9.2 Masonry Plates and Anchor Rods

If the horizontal bearing forces exceed one-fifth the minimum vertical load due to permanent loads, the bearing shall be secured against “walking.” “Walking” refers to slippage or sliding between the bottom or top surface of the elastomeric pad and the concrete or steel surface to which it is bearing against. The designer has three options, listed in order of preference, for securing the bearing against the potential for walking: 1) Vulcanization; 2) use of pintles; and 3.) use of keeper bars. Specifying that the elastomeric bearing be shop–vulcanize-bonded to a masonry plate, which in turn is then anchored to the substructure, will prevent the bearing from walking. In addition to vulcanizing, a pintle can be welded to the masonry or sole plate, which would then be inserted into a hole in the bearing pad to secure it. The effect of the hole must be accounted for in the design of the bearing.

Permanently securing the pads against walking by the use of adhesive is not permitted.

For new construction, anchor rods should generally be detailed for placement in preformed holes using 6-inch-diameter sleeves or block-outs, which are to be grouted with non-shrink grout after installation of the bearings. This detailing allows for adjustment in the placement of the bearings relative to the anchor rods. The designer may consider using reduced-size block-outs to accommodate project-specific pier or abutment top main reinforcement detailing, but the block-outs must be no less than three times the diameter of the anchor rod.

Anchor rods for HLMR bearings should generally be placed beyond the limits of the sole plate to facilitate installation and avoid interference with bearing components during movement and rotation. For HLMR bearings whose components are welded (as opposed to tightly fit within a machined recess) to the sole and masonry plates to allow for future bearing removal, the use of a headed anchor bolt, coupler and anchor rod is suggested; refer to Section 345.02 – Pot Bearing Details. If the anchor assemblies are under the sole plate or other bearing component plates, clearance to install and remove the bolt must be considered. If a headed anchor bolt expects tension, the designer must verify the entire anchor assembly and substructure are also designed for this tension.

106.10.10 Lateral Restraint

For expansion elastomeric bearings, if a restraint system is external to the bearing and stainless steel is required on the guiding system, there shall be a corresponding low coefficient of friction material for it to mate. The stainless steel shall completely cover the material in all movement extremes, and consideration must be given to vertical displacement due to construction and application of the dead loads.

Longitudinally guided expansion bearings on structures with a horizontally curved alignment and structures with non-parallel girders should be guided in the same direction with respect to the centerline of the substructure where the line of bearings is installed. Guiding at differing directions along a bearing line will cause the bearings to bind. It is generally accepted for design purposes that the direction of movement for structures on a horizontally curved alignment is along the chord from the fixed point to the expansion point. In rare occasion, the structure can be forced to move in any direction the designer chooses; however, the resulting forces must be accounted for in the design of the bearing and substructure.

106.10.11 Uplift Restraint

Uplift due to service loads should be avoided with strategic placement of additional dead load. Uplift forces due to construction loads should be offset either by revising the deck pour sequence, or restrained by means other than the bearing.

If uplift at bearings is unavoidable from a practical standpoint, the uplift restraint system for elastomeric bearings should be external to the bearing. Relatively low uplift forces due to construction loads or seismic events can be economically and feasibly built into an HLMR bearing. For HLMR bearings, methods similar to those used with elastomeric bearings can be applied, or the bearing can be designed with hold-down attachments.

106.10.12 Bearing Schedule

Contract documents shall contain a plan indicating the following information, as applicable:

  1. Provide a schedule of all minimum and maximum vertical and horizontal loads for LRFD Load Combinations as shown in Table 106‑2. The schedule shall include all longitudinal and transverse forces, as well as seismic forces. The schedule is not required for elastomeric bearings. Show the location and type of each bearing (fixed, expansion, or guided expansion). Use a bearing framing plan to show this data. Show magnitude and direction of movements at all bearings.
  2. Indicate minimum design rotation requirements of the bearing, including construction tolerances.
  3. Indicate and properly detail all anchorage details and/or requirements for constructability of initial installation and future replacement, and for permanent design requirements.
  4. Provide details and indicate grades, bevels, and slopes for each bearing type.
  5. Indicate the coefficient of friction used in design of the sliding surfaces.
  6. Highlight any special details needed for seismic requirements, such as uplift details, temporary attachments, or other requirements.
  7. Show beam seat elevations based on an assumed total bearing thickness stated in the Plans.


A completed table similar to Table 106-2 shall be provided on the Plans for all bearing types, except for elastomeric bearings. Engineering judgment can be used to eliminate groups that obviously will not control the bearing design to limit the table size.
Table 106-2. Suggested Format for Providing Bearing Schedule Loads
Load Combination Factored Loads (kips)
Vertical Horizontal
DL LL + I Transverse Longitudinal
Min Max Min Max Min Max Min Max

106.11 References

AASHTO, 1990. A Guide for Protective Screening of Overpass Structures, 2nd Edition.

AASHTO, 1995. Guide Design Specifications for Bridge Temporary Works, 1st Edition, 1995 with 2008 Interim Revisions.

AASHTO, 2017. AASHTO LRFD Bridge Design Specifications, 8th Edition.

AREMA, 2015. Manual for Railway Engineering.

AWS, 2020. AWS A2.4, Standard Symbols for Welding, Brazing, and Nondestructive Examination.

DelDOT, n.d. Standard Construction Details.

DelDOT, 2020. Standard Specifications for Road and Bridge Construction, August.

FHWA, 1989. FHWA Technical Advisory 5140.22, Uncoated Weathering Steel in Structures, October 3.

FHWA, 2022. FHWA Memorandum, Inspection of Nonredundant Steel Tension Members, May 9.

FHWA, 2012. Steel Bridge Design Handbook: Redundancy, Publication No. FHWA-IF-12-052, Volume 9, November.

NCHRP, 2012, NCHRP Report 725, Guidelines for Analysis Methods and Construction Engineering of Curved and Skewed Steel Girder Bridges, Transportation Research Board of the National Academies.

NCHRP, 1998. NCHRP Report 406, Redundancy in Highway Bridge Superstructures, Transportation Research Board, National Research Council.

NEPCOAT, 2015. Qualified Products List for Protective Coatings for NEW and 100% BARE EXISTING Steel for Bridges, October 6.

PCI, 2011. Bridge Design Manual, 3rd Edition, November.

Pennsylvania Department of Transportation, 2015. Design Manual Part 4, Structures Procedures – Design – Plans Presentation, Pdt – Pub No. 15M, April 2015 Edition.

Roddis, K., Kulseth, P., and Liu, Z., 2005. Torsional Analysis for Exterior Girders – TAEG 2.1, University of Kansas, K-Tran KU-00-3.

Texas Department of Transportation, 2015. Preferred Practices for Steel Bridge Design, Fabrication, and Erection, February.