103 - Bridge Geometry and Structure Type Selection
The purpose of this section is to establish policies and procedures for identifying DelDOT preferences for the geometric layout and selection of structure types for standard bridges in Delaware.
Considerations for bridge geometry shall take into account issues of highway safety, including sight distance, adequate horizontal and vertical clearances, and bridge geometry compatible with the approach roadway and/or with minimum standards as indicated herein.
Considerations for structure type selection include economics, constructability, inspectability, and design in accordance with established standards for design and construction to facilitate inspection and future maintenance.
- AASHTO LRFD
- Reference to the AASHTO LRFD within this section shall be considered a reference to AASHTO LRFD Bridge Design Specifications, 8th Edition, 2017.
- AASHTO Green Book
- Reference to the AASHTO Green Book or Green Book within this section shall be considered a reference to AASHTO: A Policy on Geometric Design of Highways and Streets, 7th Edition, 2018.The FHWA recognizes the AASHTO Green Book as a general set of guidelines for the design of highways and streets.
- ABC Rating Score
- A quantitative rating system that assesses the applicability of ABC to a bridge construction project and helps to determine which construction projects are more suited to ABC methods than conventional methods.
- Accelerated Bridge Construction (ABC)
- Bridge construction that uses innovative planning, design, materials, and construction methods in a safe and cost-effective manner to reduce the onsite construction time that occurs when building new bridges or replacing and rehabilitating existing bridges.
- The American Railway Engineering and Maintenance-of-Way Association, but for the purpose of this Manual, AREMA shall refer to the latest published version of the AREMA Manual for Railway Engineering.
- Bridge Management System (BMS)
- The system used by the Department to manage and track the inventory of bridges and their associated repair needs for the bridges in Delaware. DelDOT uses AASHTOWareTM Bridge Management software BrM (formerly PONTIS software) for the bridge management system.
- Clear Zone
- An unobstructed, traversable area provided beyond the edge of the traveled way for the recovery of errant vehicles. For the purpose of this Manual, this term refers to the horizontal clear distance between the edge of the traveled way and the nearest point of the closest adjacent structure (typically substructure) element.
- Geosynthetic Reinforced Soil / Integrated Bridge System (GRS/IBS)
- A popular type of ABC technology, GRS consists of closely spaced layers of geosynthetic reinforcement and compacted granular fill material and is commonly used in constructing bridge abutments. GRS/IBS includes a reinforced soil foundation, a GRS abutment, and a GRS-integrated approach.
- Horizontal Clearance
- Horizontal clearance under a bridge is measured as the perpendicular distance from the edge of the traveled way below the bridge (or from the centerline of track for bridges over a railroad) to the nearest point along the adjacent abutment face or bridge pier within the associated vertical clearance envelope.
- Link Slabs
- This term refers to bridge superstructures that provide for the construction of a continuous deck over interior supports, but do so while accommodating simple-span beam end rotations (i.e., no superstructure moment continuity over the interior supports) for all dead loads and live loads. A section of deck slab over the interior support is typically constructed after the remainder of the deck is placed and designed to accommodate the beam end rotations due to superimposed dead loads and live loads. Link slab bridges work to eliminate deck joints over interior supports and accommodate longitudinal translations over the entire length of the superstructure unit, as defined by the limits of continuous deck. Link slab bridges can typically offer a construction time-savings advantage over simple-made-continuous type construction.
- Mean High Water (MHW)
- Average of all the high-water heights observed over a period of several years.
- Mean Low Water (MLW)
- Average of all low-water heights observed over a period of several years.
- Non-redundant Steel Tension Member (NSTM)
- A structural member in tension or with a tension element whose failure would likely cause a portion of or the entire bridge to collapse (formerly referred to as fracture critical member or FCM).
- Prefabricated Bridge Elements and Systems (PBES)
- A common ABC approach that involves transporting prefabricated elements and systems from an off-site location to the final bridge site.
- This term, in reference to structural systems, refers to structures that are configured or designed such that the failure of any one member or connection will not lead to the overall failure, or collapse, of the entire structural system.
- Self-Propelled Modular Transporters (SPMT)
- A popular ABC structural placement method, a SPMT is a high-capacity transport trailer that can lift and move prefabricated elements with a high degree of precision and maneuverability.
- Simple-Made-Continuous Construction
- This term refers to bridge superstructures that are constructed as simple spans for beam self-weight and concrete deck slab weight, and made continuous for superimposed dead loads and live loads. This type of construction is more typical for prestressed concrete bridges, but can also be used for steel bridges. Although similar, simple-made-continuous construction is not to be confused with link-slab designs. Refer to the definition for link slab above for comparison.
- DelDOT and AASHTO define skew angle as the angle between the centerline of a support and a line normal to the roadway baseline, which shall be the angle denotation used in this Manual. Refer to Figure 103‑2 for an illustration of bridge skew.
- Skew Index Factor (Is)
- The skew index factor is defined in National Cooperative Highway Research Program (NCHRP) Report 725: Guidelines for Analysis Methods and Construction Engineering of Curved and Skewed Steel Girder Bridges (2012). The skew index factor aids in the determination of recommended methods of analysis for skewed bridges. Refer to Section 126.96.36.199.1 – Determination of Appropriate Analysis Method using NCHRP Report 725 for a method of calculating the skew index factor.
- Traveled Way
- The portion of the roadway for the movement of vehicles, excluding the shoulders. As such, the traveled way is the horizontal limits within roadway lane(s).
- Vertical Clearance
- The vertical clearance for bridges is measured as the minimum vertical dimension between the roadway (or railroad tracks) under the bridge and the closest bridge element. The horizontal limits of the vertical clearance envelope below the bridge shall include the entire traveled way and the limits of the paved shoulders for the roadway below the bridge. The designers shall refer to AREMA Chapter 28 (or as required by the Railroad, whichever controls) for description and diagrams depicting the required vertical clearance envelope for railroads under bridges.
103.3 Bridge Geometric Design Requirements
103.3.1 Bridge Length
In general, bridge limits shall be established incorporating the following considerations:
- For underpass roadways, provide span lengths as required to meet current roadway geometric design requirements as specified in the DelDOT Road Design Manual (2004).
- Set span configurations to achieve the horizontal clearance requirements for underpass roadways, railroads, and waterways as specified in Section 103.3.4 – Horizontal Clearance and Pier Protection.
- Consider the potential for future widening of roadways below the bridge.
- Design the structure to limits that minimize the total project costs. Depending on approach roadway construction requirements, including the construction of embankments and retaining walls, the least bridge cost does not always equate to the least project cost.
- Design to meet the “Clear Zone Concept,” as deemed applicable for a particular project. Refer to Section 188.8.131.52.1 – Delaware Clear Zone Concept for description of the Delaware Clear Zone Concept.
103.3.2 Minimum Width of Bridges
Minimum bridge width is a function of the roadway classification, average daily traffic (ADT), design speed, existing roadway features, and the proposed roadway improvements.
Bridge width for this section of the manual shall be defined as the clear distance between the gutter lines on the bridge. This will include the traveled way and the shoulder width on each side of the traveled way.
For new bridges on new alignments, the minimum bridge width, as measured from curb to curb over the bridge, shall match that of the approach roadway width. The approach roadway width is defined as the width of the approach traveled way plus approach paved shoulder width(s).
For construction projects where existing bridges are rehabilitated (i.e., bridge to remain with new deck or superstructure) and bridge replacement projects on an existing alignment, the bridge width shall match that of the width requirements for a new bridge, where feasible. Regardless of approach roadway width, the following minimum bridge width should be provided as indicated in Table 103‑1, unless otherwise approved by the Bridge Design Engineer.
The use of a projected 20-year ADT shall be used in determining the minimum bridge width for all projects.
For long bridges (greater than 200 feet in length) supporting collector and local roads, consideration may be given to reducing the minimum roadway width over the bridge to the width of the approach travel way plus 3-foot shoulders, with approval of the Bridge Design Engineer. In no cases shall the bridge width be less than the approach traveled way.
Cases where additional roadway width over the bridge may be required in comparison to the minimum widths provided in Table 103‑1 include, but are not limited to:
- Additional shoulder width for bridge deck drainage, in accordance with Section 184.108.40.206 – Shoulder Width Requirements for Deck Drainage
- Additional shoulder width over the bridge for horizontal sight distance
- Safety considerations for shoulder widths over bridges; shoulder widths between 4 feet and 6 feet should generally be avoided where there is a possibility for vehicular shoulder use (travel, parking, or disabled vehicle use) adjacent to the bridge rail
- Proposed or future re-decking considerations
- Future widening considerations
- As required by roadway design
- Potential for future shared use path
- Inspection/maintenance activities
|TABLE 103-1. MINIMUM WIDTH CRITERIA FOR BRIDGES1|
|Bridges To Remain2||Reconstructed Bridges3|
|Collector & Local Roads||Arterials & Expressways||Collector & Local Roads||Arterials & Expressways|
|Traffic Volumes (Future ADT)||Min. Bridge Width (2 Lanes)||Min. Bridge Width (2 Lanes)||Min. Bridge Width||Min. Bridge Width|
|400 and under||22 ft4||28 ft||Traveled Way + 4 ft||Note 7|
|401 to 1500||22 ft||30 ft||Traveled Way + 6 ft||Note 7|
|1,501 to 2,000||24 ft||30 ft||Traveled Way + 8 ft6||Note 7|
|Over 2,000||28 ft5||30 ft||Approach Roadway Width7||Note 7|
220.127.116.11 Shoulder Width Requirements for Deck Drainage
For bridges where the highway design speed is less than 45 miles per hour, the size and number of deck drains shall be such that the spread of deck drainage does not encroach on more than one-half the width of any designated traffic lane.
For bridges where the highway design speed is not less than 45 miles per hour, the spread of deck drainage should not encroach on any portion of the designated traffic lanes.
Hydraulic computations for the assessment of bridge deck drainage shall be in accordance with FHWA Hydraulic Engineering Circular No. 22 (HEC-22), Urban Drainage Design Manual (2009).
In addition to using the design methods presented in HEC-22 for evaluating rainfall and runoff magnitude and determining gutter flow, bridge deck drainage systems are also to be designed in conformance with the HEC-21, Design of Bridge Deck Drainage (1993). HEC-21 presents the hydraulic design requirements from the viewpoints of bridge hydraulic capacity, traffic safety, structural integrity, practical maintenance, and architectural aesthetics. System hardware components, such as inlets, pipes, and downspouts, are described in HEC-21. Guidance for selecting a design gutter spread and flood frequency is also provided.
If the hydraulic computations determine that bridge deck drainage is required, the length of the deck overhang and the placement of the fascia stringer/girder shall be optimized to accommodate the drains and downspouts.
Refer to Section 6.3 of the Road Design Manual for the design storm frequency to be used in the bridge deck hydraulic computations.
Unless otherwise approved by the Department, the width of sidewalks on bridges should match the width of sidewalk on the approach roadway, but should not be less than 5 feet, or as required by the Pedestrian Accessibility Standards Manual.
Consideration can be given to providing a 4-foot sidewalk if a 5-foot wide by 5-foot long passing area is provided every 200 feet. If the bridge is less than 200 feet long, then the use of 4-foot sidewalks can be considered when there are constraints preventing practical application of a 5-foot wide sidewalk. Use of less than 5-foot wide sidewalk must be approved by the Bridge Design Engineer and documented in accordance with the Pedestrian Accessibility Standards Manual.
Note that bridge sidewalk width does not include the width of a raised curb or protective barrier.
On bridges greater than 200 feet in length with two approach sidewalks, consideration can be given to providing a single sidewalk on one side of the bridge if safe crossings are provided at both ends of the bridge. Refer to the Pedestrian Accessibility Standards Manual, Road Design Manual, and the AASHTO Green Book for further guidance.
A protective barrier with minimum height of 42 inches between the traveled way and the sidewalk is required where roadway design speeds are 40 miles per hour or greater and should be assessed on a case-by-case basis for other conditions.
In cases where roadway design speeds are less than 40 miles per hour and a protective barrier is not proposed for use between the traveled way and the sidewalk, the minimum sidewalk width must factor in the 8-inch-wide curb poured monolithically with the sidewalk. For example, a monolithic sidewalk/curb that is 5 feet 8 inches in width provides the same functional width provided by a sidewalk 5 feet wide with a curb 8 inches wide.
The need for a sidewalk on the bridge where there is no approach sidewalk should be assessed on a case-by-case basis. The assessment should consider the potential for future approach sidewalk construction, cost, and right-of-way in accordance with the Pedestrian Accessibility Standards Manual and the Road Design Manual.
Requirements for bicycle and shared use facilities are outlined in the Road Design Manual, Section 5.1.2. The bicycle and shared use facilities provided on the approach roadway shall be provided on the bridge.
Where possible, transitioning of superelevation shall be completed outside of the limits of the bridge, including the limits of the approach slabs. If a superelevation transition within the limits of the bridge and approach slabs cannot be avoided, the designer must take great care to evaluate bridge deck elevations to ensure proper deck drainage. Superelevation transitions within the limits of the bridge can create flat spots on bridge decks that collect water and create hazardous driving conditions.
103.3.3 Protection for Median Gap of Parallel Structures
Where the distance between back-to-back barriers on parallel structures is between 6 inches and 15 feet and the bridge deck is greater than 6 feet above the ground or the resulting fall could result in serious bodily injury or death, the minimum barrier height for the median barrier will be 54 inches. Where required, the minimum median barrier height can be provided by a single full height barrier, full-height railing or the combination of a barrier equipped with a crash tested traffic railing. When implementing this standard, the design should adhere to the typical design criteria as applicable for the site specific conditions, such as horizontal sight distance, which may be impaired by the 54-inch median-side barrier. If the design criteria cannot be met, a design variance will be required, which should include an alternate means of fall protection, such as safety netting. The height of the fascia barrier for each bound of the parallel structures is not affected by this design requirement.
103.3.4 Horizontal Clearance and Pier Protection
18.104.22.168 Over Rivers, Streams, Wetlands, and Floodplains
Structures spanning waterways shall be designed to meet the specific H&H needs of the site. Refer to A22.214.171.124 – Waterway and Floodplain Crossing for the establishment of bridge length and for abutment and pier locations, as applicable. Refer to Section 104 – Hydrology and Hydraulics for design requirements.
Refer to A126.96.36.199 – Navigational, AC188.8.131.52, and AC184.108.40.206.
For new bridges over navigable waterways, designers should be cognizant of the requirements for vessel collision resistance or protection, as specified in A220.127.116.11.5 – Vessel Collisions, AC18.104.22.168.5, and A3.14 – Vessel Collision: CV and Section 203.14 – Vessel Collision: CV. Span configurations over navigable channels are subject to review by the U.S. Coast Guard and shall meet the requirements of vessel collision risk analysis as specified in A3.14 – Vessel Collision: CV. Note that these provisions often lead the design toward the placement of substructure units outside of the navigable waterway, where practical.
The assessment of vessel collision risk analysis and/or for the design of vessel collision protection systems for existing bridges is at the discretion of the Department, to be assessed on a project-by-project basis.
22.214.171.124 Over Roadways / Grade Crossings
The horizontal clearance for grade separation structures is measured as the perpendicular distance from the edge of the traveled way (lanes) below the bridge to the nearest point along abutment face or bridge pier within the vertical clearance envelope.
Refer to A126.96.36.199 – Highway Horizontal and Section 3.5.1 of the Road Design Manual. Where the desired clear zone limits cannot be obtained, protection (rigid barrier or guardrail) between the edge of shoulder below the bridge and the face of the closest adjacent substructure unit is to be provided, unless the substructure unit was designed for or verified to resist the calculated collision load as specified in A3.6.5 – Vehicular Collision Force: CT. Even if the substructure unit was designed for the collision load, protection of the blunt end within the clear zone must be provided.
When a substructure unit falls within the clear zone, a minimum horizontal clearance of 14 feet is desirable, but shall not be less than what is required to provide for the normal shoulder width of the roadway below the bridge, plus the width and deflection requirements for the protection device (rigid barrier or guardrail) between the edge of shoulder and the substructure.
Refer to A3.6.5 – Vehicular Collision Force: CT for provisions for protection from and/or incorporation of vehicular collision forces into the design of abutments and piers. The means of pier protection from vehicular collision and the incorporation of vehicular collision forces as per A3.6.5 – Vehicular Collision Force: CT, are to be determined as part of the preliminary design phase.
188.8.131.52.1 Delaware Clear Zone Concept
Delaware has adopted a policy known as the Clear Zone Concept, which is an acceptable application for projects involving the replacement of short-span structures. As with all roadside safety decisions, each project should be evaluated on a case-by-case basis and should be designed in accordance with appropriate DelDOT, AASHTO, and FHWA design manuals. In general, the Clear Zone Concept is a design option where the structure width is extended to provide the minimum design clear zone in lieu of installing a guardrail or rigid barrier.
- Background: The clear zone is an unobstructed, traversable area provided beyond the edge of the through traveled way for the recovery of errant vehicles. The provision of a clear zone is applicable to new construction and re-construction projects pursuant to guidance outlined in the AASHTO Roadside Design Guide (2011). On existing roads, primarily those of an older or lower-order nature, a clear area has been established through maintenance activities. While this practice is strongly encouraged, these areas should not be construed as providing the same safety benefit as clear zones. In general, the clear zone, or forgiving roadside concept is the preferred method of achieving roadside safety. The four methods of establishing a clear zone are listed here in order of preference: eliminate obstacles; redesign obstacles so they can be safely traversed; relocate obstacles to a location where they are less likely to be struck; or reduce the impact severity of obstacles by using appropriate breakaway devices.
- Bridge Types: Only bridge types eligible to be coded as “19” (Culverts, which include pipe, box, and frame culverts) for Main Span Design Type in accordance with the FHWA Recording and Coding Guide for the Structure Inventory and Appraisal of the Nation’s Bridges (Report No. FHWA-PD-96-001) will be considered for designing according to the Clear Zone Concept.
- Bridge Lengths: All crossroad pipes (single cell and multiple cells) are eligible for consideration for designing for the Clear Zone Concept. All box, frame, and arch structures with a structure length less than 25 feet will also be eligible for consideration for designing for the Clear Zone Concept.
- Roadway ADT: Roadways with a design ADT of 400 or less should be given first consideration for designing for the Clear Zone Concept. Roadways with a design annual average daily traffic (AADT) of 1,000 or less are also eligible for consideration for designing for the Clear Zone Concept.
- Existing Conditions: Unless removal is warranted and documented through the design process, roadways with existing roadside protection should be designed to include roadside protection. Designers should propose to meet existing conditions at a minimum, if design standards cannot be achieved.
Consideration should also be given to horizontal and vertical alignment (sight distance), accident data, and the surrounding terrain, including utilities, environmental impacts, and location of entrances, in determining whether the “Clear Zone Concept” is applicable for a specific site. The Horizontal Curve Adjustment Factors shown in Table 3‐2 of the AASHTO Roadside Design Guide should be considered in developing the roadside design for the project.
184.108.40.206 Over Railroads
For highway structures passing over railroads, the horizontal clearance is measured as the perpendicular distance from the centerline of the nearest track to the nearest point along a bridge pier or abutment face below the bridge within the required limits of railroad vertical clearance envelope. See Figure 103‑1 for required limits.
Refer to A220.127.116.11 – Railroad Overpass and AC18.104.22.168.
Horizontal clearance and crash protection requirements for piers and abutments adjacent to railroads are subject to the standards of the specific railroad being overpassed for a given project location.
However, the minimum horizontal clearance, specified and provided, shall not be less than that shown in AREMA Chapter 28. An 18-foot lateral clearance from the centerline of track shall be provided for off-track equipment on one side, if requested by the railroad. Class 1 (major) railroads may require additional lateral clearance depending on the need for drainage ditches, an access roadway, and/or for off-track equipment. The requirements for crash walls for the protection of piers, in accordance with AREMA and as required by the specific railroad, are to be followed. Also, refer to A3.6.5 – Vehicular Collision Force: CT and AC22.214.171.124 for horizontal clearance limits where the incorporation of railroad collision forces into the design of abutments and piers is required, when crash protection is not provided.
The minimum horizontal clearance shall be shown for each track on the drawings. If track and abutment or piers are skewed relative to each other, horizontal clearances to the extremities of the structure shall also be shown. If the track is on a curve within 80 feet of the crossing, additional horizontal clearance is required to compensate for the curve (refer to AREMA, Volume 4, Chapter 28). If a railroad requests clearance in excess of the above, complete justification of this request shall be provided. The agreement on the lateral and vertical clearances shall be reached with the operating railroad and shall be secured prior to the TS&L submission.
Refer to Sections 126.96.36.199 – Over Railroads and 103.10 – Requirements for the Design of Highway Bridges over Railroads for further requirements for the design of bridges over railroads.
103.3.5 Vertical Clearance
188.8.131.52 Over Rivers, Streams, Wetlands, Floodplains
Structures spanning waterways shall be designed to meet the specific H&H needs of the site.
As a minimum for inspection, bridges shall provide a minimum of 4 feet of vertical clearance above mean water levels to allow for inspection with a boat. For bridges over tidal waterways, provide at least 4 feet of vertical clearance above MLW and at least 1 foot of vertical clearance above MHW. Provide for a minimum vertical opening of 4 feet in box culverts and rigid frames, unless approved by the Bridge Design Engineer.
Refer to Section 104 – Hydrology and Hydraulics for design requirements.
Refer to A184.108.40.206 – Navigational.
220.127.116.11 Over Roadways / Grade Crossings
Vertical clearance over roadways is defined as the minimum vertical distance between points on the roadway (lanes and shoulders) below the bridge and the corresponding bottom of the bridge superstructure.
Refer to A18.104.22.168 – Highway Vertical and the Road Design Manual for vertical clearance requirements. The design vertical clearances for new and reconstructed bridges shall provide for an additional 6 inches of clearance from the minimum values to allow for future roadway resurfacing.
Unless otherwise indicated by the reference manuals and codes listed above, the minimum vertical clearance for bridges over an expressway, an arterial, and a collector roadway facility is 16 feet 6 inches. The minimum vertical clearance over local roads is 14 feet 6 inches. Pedestrian bridges and overhead sign structures shall provide 17 feet 6 inches vertical clearance for all roads. The clearances listed above include the additional 6 inches of clearance for future roadway resurfacing. However, additional clearance should be considered during preliminary design to account for potential final design changes and construction tolerances.
22.214.171.124 Over Railroads
Refer to A126.96.36.199 – Railroad Overpass.
The requirements for vertical clearance over railroads are subject to the requirements of the railroad being overpassed for a given project location. Coordination with the owner of the railroad is required for all projects over or adjacent to railroads.
At a minimum, for structures carrying highways over railroad tracks, the vertical clearance, specified and provided, shall not be less than that which is shown in AREMA Chapter 28. Provide for an additional 12 inches of vertical clearance in the design from the minimum required clearance to allow for construction tolerances and future track re-profiling. See Figure 103‑1 for minimum vertical clearance dimensions.
Refer to Sections 188.8.131.52 – Over Railroads and 103.10 – Requirements for the Design of Highway Bridges over Railroads for further requirement for the design of bridges over railroads.
103.3.6 Bridge Skew
Bridge skew is defined as the angle between the centerline of a support and a line normal to the centerline of roadway, as illustrated in Figure 103‑2.
The selection of the magnitude of skew to provide is dependent on the type of feature(s) crossed; however, the designer should make every effort to minimize the bridge skew to 30 degrees or less to reduce the potential for deck cracking, minimize diaphragm or cross-frame loading, minimize the potential for uplift at acute corner end supports and minimize the potential for increased shears in members at obtuse corners. Reduction of bridge skew, and preferably the elimination of bridge skews, will also improve and simplify design, detailing, fabrication, and construction, as well as reduce future maintenance costs. In addition, substructure quantities and costs increase sharply with skews over 30 degrees.
New bridge substructures with skew angles greater than 0 and less than 10 degrees should generally not be proposed. Given the simplicity of fabrication and construction for zero skew bridges, substructure layouts between 0 and 10 degrees should be revised such that the skew angle is 0 degrees, when feasible.
For straight steel bridges whose Skew Index (Is) is greater than 0.30, the designer shall identify and submit for approval the method (2-D grillage or three-dimensional [3-D] finite element) and software to be used to analyze the structure as part of the design, load rating, and assessment of bridge constructability. The method of analysis shall be in accordance with the recommendations of NCHRP Report 725 – Guidelines for Analysis Methods and Construction Engineering of Curved and Skewed Steel Girder Bridges. Refer to Section 184.108.40.206.1 – Determination of Appropriate Analysis Method using NCHRP Report 725 for further description regarding the selection of appropriate analysis method for skewed steel I-beam bridges. Approval for the analysis method is to be obtained as part of the TS&L submission (or the preliminary plans submission when a TS&L submission is not required).
Refer to Section 106.9.8 – Skew Effects for maximum permissible skews for various prestressed concrete bridge types. For prestressed concrete bridges with skews greater than 45 degrees, the designer shall submit for approval the method of analysis for the design of the prestressed concrete beams and bracing members. The advanced analysis shall be used to assess the stability of structure during construction and for the design of the structure in its final condition. Approval for the analysis method is to be obtained as part of the TS&L submission (or the preliminary plans submission when a TS&L submission is not required).
103.3.7 Approach Slabs
Approach slabs shall be provided on all bridges supporting arterials, freeways, and interstates. For collectors and local roads, approach slabs shall be provided if the full range of thermal movement of the superstructure is greater than ½ inch.
103.4 Structure Type Selection
103.4.1 Bridge Types
The bridge types listed in this section represent the bridge types commonly utilized in Delaware. These bridge types are not bridges that would be classified as unusual or complex, as defined in Section 101.6.1 – Bridge Types.
220.127.116.11 Structural Steel
Typical steel bridges used in Delaware include rolled I-beam, plate-girder, and box-girder bridges. The use of rolled beams is preferred to plate girders, unless span length, material or section availability, or construction lead time dictates otherwise.
Composite girders, with no fewer than four girders in the bridge cross section, are required, unless approved by the Bridge Design Engineer. Constant depth girders are preferred over haunched girders. Haunched girders should only be considered for unique site-specific conditions, such as vertical clearance concerns, or where aesthetics and/or economic considerations render them competitive.
The use of steel pin-hanger structures and “piggy-back” type construction are prohibited for new construction and should be replaced or retrofitted, when practical. Bridge types that contain NSTM, are not permitted, unless otherwise approved by the Bridge Design Engineer.
Continuous spans shall be used for multiple span bridges. The ratio of the length of end spans to the intermediate spans should be 0.7 to 0.8. The latter ratio is preferred because it nearly equalizes the maximum positive moment of all spans. While three- and four-span continuous units tend to be more structurally efficient in comparison to single-span and two-span continuous units, the most-cost effective span configuration may simply be a function of the features crossed.
Always consider the presence of uplift at ends of continuous girders, particularly with light, rolled beam units or short end spans. AC3.4.1 indicates uplift to be checked as a strength load combination and provides guidance in the appropriate use of minimum and maximum load factors. Uplift restraint, when needed (this is not common), should satisfy the strength limit state and the fatigue and fracture limit state. Spans should be proportioned to avoid the presence of uplift at supports.
The minimum depths for constant depth superstructures, as presented in Table A18.104.22.168.3-1 must be met. As a general rule, a well-proportioned straight multi-girder composite steel superstructure should have a total section depth (slab plus girder) in the range of 0.035 to 0.038 for continuous spans and 0.044 to 0.048 for simple spans. The AASHTO minimum depths for straight girders should be increased by a minimum of 10 percent for skewed and curved girder bridges, typically increasing in relation to severity of the curvature and/or skew. The 10 percent increase is a guideline for establishing a starting point for preliminary design. The overall superstructure depth will be determined by satisfying all strength and service limit states.
For plate girder structures, high-performance steel (HPS Grade 70) may be considered, where structurally prudent or where an economic advantage can be achieved. As a general rule, when the use of Grade 50 steel requires flange thicknesses greater than 3 inches, Grade 70 steel should be considered. Note that when high-strength steels are used, deflection criteria tend to control the design. Compliance with live load deflection criteria should be confirmed along with structural capacity. The use of HPS Grade 100 shall not be allowed without prior approval of the Bridge Design Engineer.
Refer to Section 106.8.7 – Protective Coatings for consideration for steel coatings and considerations for the use of weathering steel.
22.214.171.124 Concrete Bridges
126.96.36.199.1 Reinforced Concrete Slab Bridges
This superstructure type is not recommended for new construction and should only be considered for widening of existing reinforced concrete slab bridges when replacement with concrete box culvert or prestressed plank superstructure types are not feasible or economical. Instead of widening, existing reinforced slab bridges should generally be replaced when economically feasible.
188.8.131.52.2 Reinforced Concrete T-Beam Bridges
This type of superstructure is not recommended for new construction. Replacement of these bridge types should consider prestressed box beams or precast prestressed double-tee sections (i.e., NEXT beams) developed by the Precast/Prestressed Concrete Institute Northeast (PCINE).
184.108.40.206.3 Prestressed Concrete Bridges
Precast prestressed concrete members are economical and especially advantageous in situations where quick erection is desired. Precast concrete members can be fabricated year-round and can be delivered, erected and put into service in a very short time. Precasting permits better material quality control and helps provide for a maintenance-free service life.
Prestressed concrete beams shall be considered advantageous for spans over water and electrified railroads to reduce the hazards and disruptions to rail operations and/or costs associated with future painting of steel structures.
For multi-span units, simple-made-continuous design is the recommended structure configuration. Generally, multiple simple spans should be avoided where practical, due to reduced structural efficiency and the need for deck joints between each span. Continuous superstructure units of more than six spans are generally not preferable.
For the purpose of conceptual design and bridge alternative studies, beam charts from Chapter 6 of the Precast/Prestressed Concrete Institute (PCI) Bridge Design Manual and Table A220.127.116.11.3-1 can be used for preliminary beam sizing and spacing. Refer to Section 106.9 – Prestressed Concrete Bridge Superstructures for Delaware-specific design requirements for the final design of prestressed concrete bridges.
Where practical and deemed economically advantageous, configuring interior spans within multi-span units as equal spans is preferrable. Proportioning end spans from 0 percent to 20 percent less than the interior spans is also preferable for efficient use of superstructure material.
All concrete bridge beams will be precast and prestressed. Post-tensioning may be justified on a case-by-case basis.
Refer to Section 18.104.22.168 – Compressive Strength for concrete design strengths (f’c), which are to be established during the preliminary design/TS&L stage.
22.214.171.124.3.1 Beam Types
Delaware uses a number of precast prestressed concrete beam types:
- Voided or solid slabs: AASHTO has standardized a number of sections to accommodate a variety of bridge widths and span lengths in the 30- to 50-foot range. The sections are 36 to 48 inches wide with depths of 15, 18, and 21 inches. Thinner 12-inch sections may be designed by eliminating the voids. Adjacent prestressed concrete slab units are preferred at stream crossings having limited freeboard because they provide a continuous flat surface along the bottom of the superstructure that prevents debris from becoming trapped under the bridge and impeding the hydraulic flow. Voided slabs are prohibited over waterways that frequently flood and submerge the superstructure.
- NEXT beams: These beams are used for short- to medium-span length bridges (30- to 90-foot range). The beams can be produced in a variety of lengths and widths, with the capability of spanning either longitudinally or transversely with respect to traffic. The beams offer an economical alternative to traditional concrete box beams. The NEXT beams comes in two configurations: an “F” (Form) option with a partial-depth flange serving as the formwork for a cast-in-place concrete deck and a “D” (Deck) option with a full-depth flange, which requires the installation of a membrane-wearing surface system.
- Adjacent and spread box beams: These beams are used for short- to medium-span length bridges (50- to 130-foot range). Similar to the voided slabs, AASHTO has developed a series of standard box sections. Standard sections are available in 36- and 48-inch widths and a variety of depths to accommodate various bridge widths and span lengths.
- PCEF bulb-tee beams: These beams are used for medium span length bridges (90- to 170-foot range). Similar to the AASHTO I-beams, bulb-tee beams can be modified to accommodate longer spans. The FHWA Mid-Atlantic States Prestressed Concrete Committee for Economic Fabrication (PCEF) has developed a series of bulb-tee beams that offer a wide range of beam depths, flange widths, and web thicknesses. While AASHTO I-beams may be considered when determined to be more structurally or economically feasible, the PCEF bulb-tee beams generally provide a more economical use of materials than the AASHTO I-beams and are the preferred choice of the Department.
Refer to Sections 330.01 – 330.04 for sections properties and details for the typical prestressed beam types used in Delaware, as listed above.
126.96.36.199.3.2 Spliced Prestressed I-Beam Superstructures
Prestressed concrete bridge beams may be spliced by joining two or more beam segments to form one beam. Typically, splicing is achieved by cast-in-place concrete along with longitudinal post-tensioning. Splicing of bridge beams is generally used for one or more of the following reasons:
- Increasing span lengths to reduce the number of substructure units and total project costs;
- Reducing the beam length and weight to facilitate transport from the fabricator to the bridge site;
- Increasing the girder spacing to reduce the number of girder lines and total project costs;
- Increasing span lengths to improve safety by eliminating shoulder piers or interior supports;
- Minimizing structure depth to obtain required vertical clearance over highway and/or rail traffic, waterways, etc.;
- Avoiding the placement of piers in water to reduce environmental impact and total project costs;
- Placing piers to avoid obstacles on the ground, such as railroad tracks, roadways, and utilities;
- Improving aesthetics through various design enhancements, such as more slender superstructures, longer spans, and haunched sections at piers; and
- Eliminating joints for improved structural performance, reduced long-term maintenance/increased service life, and improved rideability.
When possible, the full portion of the longitudinal post-tensioning to be applied after the deck is poured shall not be applied until after the deck reaches its specified compressive strength, so that the net tension on top of the deck surface is less than or equal to the modulus of rupture.
The contract plans shall show one suggested erection method and the associated post-tensioning sequence. The structural analysis should consider the effects of fabrication and erection tolerances on bridge performance.
188.8.131.52.3.3 Segmental Concrete Structures
A segmental precast box girder superstructure may be a viable and economical alternative for the following types of structures:
- Long Multi-Span Bridges: Segmental precast box girders are well suited for long multi-span bridges on straight or slightly curved alignments in locations where maintenance and protection of traffic issues and/or environmental concerns require that field work be minimized. Repeated use of and erection set-up for the box girder segments is the main advantage. The span-by-span method of erection is generally used for these types of bridges.
- Long-Span Bridge on High Curvatures: Segmental precast box girders are well suited to accommodate high curvatures on long spans due to high torsional stability. The balanced cantilever method of erection is generally used for these types of bridges.
When long open spans with clean visual lines are desired, segmental precast box girder superstructures are a good solution. Haunching of the segmental girders to improve the visual impact and structural efficiency is possible with this type of superstructure.
The expected durability of segmental box girder bridges is relatively high. These types of structures utilize post-tensioning in both the longitudinal and transverse directions to be free of tensile cracks. This results in an expected substantial increase in the durability of the overall structure. However, there are unique areas of vulnerability for these types of structures:
- Since the deck is an integral part of the box girder system, the complete replacement of the bridge deck is extremely difficult. To increase long-term durability and design life, the structure should be designed so there is no tensile stress at the top surface of the segment under service load conditions, both including and excluding time-dependent effects.
- Deck run-off should not be allowed to flow over the grouted block-outs for tendon anchorages. When end anchorages are located in vulnerable areas, such as beneath a deck expansion joint, additional protective measures shall be provided.
184.108.40.206.3.4 Prestressed Concrete Superstructure Type Selection
The cost of the girders is a major portion of the overall cost of a bridge superstructure. Therefore, much care is warranted in the selection of the type of girders and in optimizing their position within the structure. The following guidelines should be considered:
- Beam Type: All beams in a bridge should be the same type and size, unless approved otherwise by the Bridge Design Engineer. If vertical clearance is not a problem, a larger beam size, utilizing fewer beams lines may be a desirable solution. Fewer beam lines may result in additional reinforcement and concrete, but less forming costs.
- Beam Concrete Strength: Higher concrete strength should be specified where that strength can be effectively used to reduce the number of beam lines. Refer to Section 220.127.116.11 – Compressive Strength for additional information on concrete strengths.
- Beam Spacing: Consideration shall be given to the deck slab cantilever length to determine the most economical girder spacing. The deck slab cantilever should be maximized if a line of girders can be saved. When the amount of top transverse reinforcement in the deck overhang is controlled by vehicular collision forces on the traffic barrier, increasing the overhang width to the maximum that can be supported by the reinforcement is desirable. However, it is recommended that the overhang length, when measured from the edge of slab to the centerline of the exterior beam, be less than 40 percent of the interior beam spacing. Under this cross-sectional configuration, the design loads for the exterior and interior beams typically match well. The following guidance is suggested:
- Tapered Spans: On tapered roadways, the minimum number of beam lines should be established by using flared beam lines. Place as few beams as possible within the limitations of the beam capacity. Deck slab thickness may need to be increased.
- Curved Spans: When straight prestressed beams are used to support a curved roadway, the overhang will vary. The designer shall strive to match the maximum deck slab overhang at the centerline of the span at the outside of the curve with that of the overhang at the piers on the inside of the curve. At the point of minimum overhang, the edge of the beam top flange should be no closer than 1 foot from the deck slab edge. Where curvature is extreme, other types of girders and/or girder material should be considered. Straight beam bridges on highly curved alignments have a poor appearance and also tend to become structurally less efficient.
- Geometrically Complex Spans: Complex spans that are combinations of taper and curves require careful consideration to develop the most effective and economical girder arrangement. Beam lengths and number of strands (straight or draped) should be made the same for as many beams as possible within each span.
- Deck Slab Cantilevers: Some considerations that affect deck slab cantilevers are noted below:
- Appearance: Normally, for best appearance, the largest deck slab overhang that is practical should be used.
- Economy: The condition that provides the best appearance is also that which will normally afford maximum economy. A larger overhang typically means that a line of girders can be eliminated, especially when combined with higher concrete strengths.
- Deck Slab Strength: The deck slab cantilever may be critical and may require thickening.
- Drainage: A large deck slab cantilever may severely affect where deck drainage can be placed. Therefore, when deck drainage is required, it must be considered when determining exterior beam location.
18.104.22.168 Timber Bridges
Existing timber bridges in Delaware include timber trusses, timber, and glulam beam structures. However, the use of similar timber bridge types for new construction should only be considered on local roads with ADT < 750 and less than 10 percent truck traffic.
Culverts are typically rectangular, circular, or elliptical structures that are buried and designed when flowing full to be submerged and under hydraulic pressure. Types of culverts used in Delaware include pipes, boxes, rigid frames, and arches. DelDOT prefers pipe culverts constructed of concrete. Thermoplastic pipes are allowed for maintenance projects or where site constraints may allow. Metal culverts are prohibited. Metal culverts are prohibited.
Most small culverts in Delaware are constructed with round or elliptical pipes. Only culverts or a series of culverts with a total opening of 20 square feet or greater are classified as bridges in Delaware. For openings larger than 20 square feet, concrete box culverts, per ASTM C1577, rigid frames, or arches are usually preferred. Culverts of 20 square feet or greater require load ratings, as per Section 101.6.1 – Bridge Types. The use of concrete box culverts, or concrete arches versus larger multiple pipes is based on a number of factors, including hydraulic efficiency, compaction around the structure, height of fill required, and total width of multiple cells. No more than three adjacent pipes are permitted at a given location.
Three-sided rigid frames or arches may be considered for projects where a natural stream bottom and/or a low-flow channel are required. The bottom slab of a box culvert can also be depressed (typically 12 inches) to promote the development of a natural stream bottom. Refer to Section 22.214.171.124 – Over Rivers, Streams, Wetlands, Floodplains for minimum vertical clearance and vertical opening requirements for rigid frames, arches, and box culverts.
Culverts shall be designed to meet the current and future hydraulic needs as discussed in Section 104 – Hydrology and Hydraulics.
103.4.2 Selection of Superstructure Type
When comparing among structure alternatives, the selection of the recommended structure type for a given project shall include the following, as applicable to a given project. The relative importance of each criterion may vary among projects.
- Least overall project cost (note that the least structure cost typically matches that of the least project cost, but other project costs, when varying among structure alternatives, should also be considered in the alternatives cost analysis)
- Lowest life-cycle cost
- Construction and/or construction schedule
- Maintenance of traffic (MOT) during construction
- Minimum number of deck joints
- Future maintenance
- Aesthetics and/or maintaining locally used bridge substructure types
The following provides approximate guidelines for use in the consideration and selection of appropriate structure types for a given span range.
126.96.36.199 Spans less than 20 feet
In this span range, precast reinforced concrete culverts or pipes, precast reinforced concrete boxes, per ASTM C1577, and prestressed solid or voided plank beam bridges are typically considered more economical structures than cast-in-place reinforced concrete box culverts and cast-in-place reinforced concrete rigid frame (RCRF) structures. Voided plank beams shall not be used over waterways that frequently flood and submerge the superstructure.
188.8.131.52 Spans from 20 feet to 30 feet
In this span range, arch culverts, cast-in-place concrete box culverts, prestressed solid or voided slab beam, and prestressed box beam bridges are generally more economical than steel I-beam bridges. Consideration should also be given to multiple precast reinforced concrete boxes in lieu of a single-span bridge. Physical constraints, characteristics of the project site, such as debris potential and aquatic habitat need to be considered. Voided slabs and box beams shall not be used over waterways that frequently flood and submerge the superstructure.
184.108.40.206 Spans from 30 feet to 90 feet
In this span range, prestressed box beam, NEXT beam, or PCEF bulb-tee beam bridges are generally more economical superstructures in comparison with steel superstructures. However, changing market conditions and bridge site conditions, such as low under-clearance, steel beam bridges in this span range may also merit consideration.
220.127.116.11 Spans from 90 feet to 165 feet
In this span range, prestressed box beam and PCEF bulb-tee beam bridges tend to be cost effective. The final selection should be based on the cost analysis for each bridge type for each location. Similar to the 30- to 90-foot range, given changing market conditions and bridge site conditions, multi-girder steel beam bridges may also merit consideration.
18.104.22.168 Spans greater than 165 feet
Bridges with span lengths over 165 feet are more complex structures. The process of selecting the most economical type of structure will require that the designer develop a preliminary design using different superstructure types, span arrangements, and substructure types. Generally, for spans up to 250 feet, multi-girder steel bridges are an economical type of bridge. Haunched steel plate girders are generally not preferred, unless unique site specific conditions, such as vertical clearance concerns, aesthetics, and/or economic considerations, render them competitive.
Consideration should also be given to long-span prestressed concrete bridges and spliced prestressed concrete girders for spans in this range.
Construction issues should include, but not be limited to, future re-decking, future-widening, deck drainage, hauling restrictions (permit loads), erection weights and maintenance and protection of traffic. Each of these should be investigated to ensure constructability and to minimize or eliminate “surprises” during construction.
103.5.1 Future Re-decking Considerations
The feasibility for future re-decking of the bridge shall be established in the preliminary design phase. Requirements may include:
- Maximum number of permissible construction stages
- Number of required lanes
- Minimum lane width(s)
- Lane location limitations
- Need to maintain pedestrian traffic
- Minimum number of beams
The need to accommodate a future re-decking sequence can affect the number of stringers/beams required. In addition, construction joints shall be placed over stringer/beam lines; therefore, stage limits will impact location and spacing of stringers/beams. For cases where future re-decking consideration is controlling the number of stringers/beams required, or where multiple stages are required, a cross section(s) showing the re-decking sequence shall be included in the preliminary and final plans.
In addition, if the future re-decking is to be performed in stages; the loading on the structure for each stage should be investigated to determine the controlling loading condition. A temporary stage for future re-decking can control the design for a given structure layout. The appropriate load combinations shall be discussed with the Bridge Design Engineer during the preliminary design phase.
103.5.2 Consideration for Future Widening
When widening is anticipated within 10 years of completion of construction of the original design, the substructure for the widening should be included in the original design. When widening is anticipated beyond 10 years, design should facilitate splicing the rebar and adding to the substructure details.
When considering future widening, consideration of vertical clearance is important. The vertical clearance needs to be high enough on the original portion to permit adequate clearance for the widened portion, while maintaining the deck cross-slope.
103.5.3 Hauling Permits
Longer span prestressed AASHTO I-beams, prestressed PCEF bulb-tee beams and steel girders require careful consideration with regards to transportation needs and the ability to obtain hauling permits. The State of Delaware classifies a “superload” as a field section that is at least 120 feet long or at least 15 feet wide or at least 15 feet high or over 120,000 pounds, which requires a special hauling permit. In particular, the permitting requirements and the feasibility of shipping superloads, when proposed, shall be investigated during the TS&L /preliminary planning stage and approval for their use must be obtained from the Bridge Design Engineer. Use of beams exceeding 120 feet is permitted in alternate designs by the Contractor if it is not restricted by the contract, as long as all hauling restrictions are obeyed and a hauling permit can be obtained. Refer to the most recent edition of DelDOT’s Oversize/Overweight Hauling Permit Policy and Procedures Manual for additional information on oversized and overweight permit vehicle provisions for Delaware.
103.5.4 Maintenance of Traffic
The MOT during construction may be a significant consideration in the selection of the preferred alternative, as well as affect the cost and scope of the work. The method of MOT for a project should be determined as part of the preliminary design phase. Similarly, requirements for staged bridge construction, as applicable, may have a significant impact on controlling design cases for the superstructure and/or substructure design. In addition, pedestrian MOT should be considered where applicable. Refer to Section 22.214.171.124 – Deck Placement Sequence and 126.96.36.199 – Deck Overhangs for design considerations associated with temporary load conditions, including load cases during staged construction.
Generally, maintenance and protection of traffic will be based on one or more of the following options: detour, staged construction, temporary on-site detour bridge, and new alignment, such that the existing bridge can be used to maintain traffic.
Coordination of the MOT plan with the Traffic Safety Section needs to occur early in the design process.
In addition to construction, inspectability of the structure also must be considered. Maintenance and inspection access requirements should be included in the preliminary design phase. Provisions for maintenance and inspection access should be provided for NSTM girders, cross-girders, and bents that cannot be inspected from a snooper. Inspection handrails, safety cables, and other fall arrest systems, all secure from trespass, should be considered in addition to catwalks.
When using concrete box or steel tub girders, inspection access shall be provided to the interior of the girders.
103.6 Substructure Type Selection
103.6.1 General Considerations
Substructure units should be optimized and standardized in shape and size to ease construction and economize quantity.
Minimizing the number of substructure units typically produces a more economical bridge, particularly where tall piers are required and where deep foundations are recommended.
Preference should be given to substructure types that eliminate deck joints within the limits of the bridge. See Section 106.6.1 Jointless Bridges.
Special forms should be avoided unless for aesthetic or other special reasons. However, site conditions must be satisfied.
Radial supports (i.e., 90 degrees as measured from the centerline of bearing to the baseline tangent) are preferred for curved structures.
Long-term settlement and service life are to be considered in selecting the substructure type.
The effect of scour shall be considered when selecting the substructure type.
103.6.2 Abutments and Wingwalls
188.8.131.52 Abutment Types
- Type I
- Integral Stub Abutments
- Type IIA
- Semi-Integral Stub Abutment with Deck Slab Pourover Details
- Type IIB
- Semi-Integral Conventional Abutment with Deck Slab Pourover Details
- Type IIC
- Semi-Integral Stub or Conventional Abutment with Full Height End Diaphragm
- Type IIIA
- Stub Abutment with Deck Extension Details
- Type IIIB
- Conventional Abutment with Deck Extension Details
- Type IV
- A Project-Specific Abutment when none of the above are applicable.
- Type V
- Traditional Abutment with Deck Joint on Bridge Side of Backwall
184.108.40.206 Abutment Type Definitions
- Stub abutments are placed at the top of an embankment slope or located behind a proprietary wall. Stub abutments are typically on piles but may be founded on spread footings provided adequate consideration is given to settlement. Lateral loads for stub abutments constructed in combination with proprietary walls shall be resisted by horizontal straps fastened directly to the rear face of the abutment. Stub abutments are generally more economical than cast-in-place concrete cantilever abutments. See Section 310.02 Stub Abutment Details.
- Conventional abutments can be used when the abutment cannot be placed near the top an embankment. A shallow spread footing on rock or good founding material is usually the most economical foundation type. However, potential settlement and potential scour depth concerns may require a deep foundation. When on piles, use a minimum of 2 rows. See Section 310.01 Cantilever Abutment Details.
- Integral abutments have the superstructure fully connected and fixed to the substructure such that expansion/contraction movement and rotation are transferred to the substructure. The longitudinal movements are accommodated by the flexibility of the abutment foundations in the longitudinal direction. Integral abutments must be supported by a single row of piles such as a stub abutment. The piles shall be oriented for bending mainly about their weak axis. Integral abutments shall not be used for curved structures and at sites where there are concerns about settlement or differential settlement. See Section 220.127.116.11 Integral Abutments for more information.
- A Semi-Integral Abutment creates a jointless bridge with a stationary abutment. A full depth end diaphragm encapsulates the ends of the beams, but is not connected to the abutment. To utilize a semi-integral design, either a stub or cantilever abutment can be used. An approach slab is included or omitted based on project needs. The geometry and design of the approach slab, wingwalls and parapets must be compatible with the movement required for the superstructure to translate longitudinally. This design should not be used for curved structures and at sites where there are concerns about settlement or differential settlement. Spread footings may be appropriate for semi-integral abutments but settlement should be evaluated. See Section 18.104.22.168 Semi-Integral Abutments for more information.
- The Deck Slab Pourover details eliminate the joint between the end of the deck and the backwall by incorporating the backwall into the superstructure. The backwall becomes an end diaphragm cast as part of the bridge deck. The horizontal joint between abutment and superstructure must be protected against water penetration. See Section 325.01 – Concrete Deck Details, sheet 2 for more information. The deck slab pourover details are typically used with adjacent concrete box beam superstructures where an approach slab is not needed.
- The Deck Extension details also eliminate the joint between the end of the deck and the backwall. Instead, the bridge deck is connected directly to the approach slab above a shortened backwall. The approach slab is free to translate across the top of the backwall due to a 1” min. gap between these elements. The joint to accommodate superstructure movement is transferred off of the bridge and is placed between approach slab and sleeper slab. These details can be utilized on a stub abutment or a conventional abutment. See Section 310 details.
- Traditional Abutment with Deck Joint on Bridge Side of Backwall can be found on many existing bridges in both stub abutment and conventional abutment configurations. It is included here in consideration of future rehabilitation projects on existing bridges. These abutments feature an open joint between end of the bridge deck and the backwall. The joint should be closed with a device such as expansion joint or a strip seal, but is also a frequent maintenance problem. The approach slab is an independent structure located on the opposite side of the backwall. This abutment type should be used on new construction as a last resort.
22.214.171.124 Abutment Type Preferences
- Preference should be given to substructure types that eliminate deck joints in accordance with Section 106.6.1 Jointless Bridges. Apply Abutment Types IIA, IIIA and IIIB as appropriate to the site location and structure type. When a semi-integral abutment is preferred, but the deck slab pourover details are not applicable, use Abutment Type IIC.
- Where design considerations can be satisfied, a Type I Integral Abutment can be used in lieu of other abutment types. Refer to Section 126.96.36.199 Integral Abutments for design considerations.
- Where none of the above are appropriate, a project specific abutment type should be proposed, again with a preference for a jointless bridge or a joint moved away from the bearing side of the abutment. For example, a superstructure that has to accommodate more horizontal movement (longitudinal or transverse) than is feasible for the above abutment types.
- Abutment Type IIB may be encountered in some rehabilitation or superstructure replacement projects. A traditional abutment can be converted into a jointless bridge of this type. However, it is not preferred for new construction.
- Abutment Type V may be encountered in some rehabilitation or superstructure replacement projects. If feasible, convert to a jointless bridge. This type should be used on new construction as a last resort.
Many types and configurations of wingwalls are available. Choose the wingwalls most appropriate to the structure and site conditions. See Section 103.7 Retaining Walls and Section 310 details for more information.
188.8.131.52 Other Considerations
- When suitable rock is available at an average depth of less than 10 feet below the proposed bottom of footing, a pedestal foundation or foundation that is made possible by removal of the overburden and backfilling with lean concrete or suitable material is typically more economical than using piling or drilled shafts. For depths greater than 10 feet, the piling is usually more economical than the drilled shafts, except where “pullout” is a concern. However, in special situations (where piles cannot be driven due to site conditions), micropiles or drilled shafts may prove to be more economical.
- Slopes at abutments and wingwalls should be maintained at 2H:1V. Use riprap slope protection in lieu of concrete slope protection. Steeper slopes may be utilized, but must be justified through geotechnical investigations, approved by the Bridge Design Engineer, and protected by properly designed stone riprap.
- A bench shall be provided at the top of all slopes adjacent to abutments, wingwalls, and retaining structures. The bench will provide for improved access for inspections. A 4-foot-wide bench is desirable, but the bench shall be no less than 2 feet wide. A minimum vertical clearance of 1 foot shall be provided from the top of the bench to the underside of the superstructure.
The following guidelines shall be considered for the selection and design of bridge piers:
- For highway-grade separations, the pier type should generally be cap-and-column piers supported on a minimum of three columns (multi-column bent). Note that this requirement may be waived for temporary construction conditions that require caps supported on less than three columns. Typically, the columns are circular and the pier cap ends should be cantilevered and have rounded ends.
- For cap-and-column piers to be generally cost effective, the column height should be less than 30 feet with column spacing between 15 and 20 feet.
- For cap-and-column piers, continuous, isolated or pile/drilled shaft foundations may be specified. The engineer should determine estimated costs for all foundation configurations and choose the most economical. Where the clear distance between isolated footings is less than 4 feet 6 inches, a continuous footing shall be specified.
- On wide structures with more than five columns and/or cap lengths greater than 80 feet, the engineer should consider whether to split a cap-and-column pier into two piers, especially where columns are short and contraction/expansion of the pier cap results in large internal forces. For cap-and-column piers with more than six columns and/or cap lengths greater than 100 feet, two piers are required. Consideration should also be given to limiting the skew with respect to flow for wide piers to reduce scour effects.
- Where cap-and-column piers are used, the potential for vehicular collision should be evaluated, and when deemed necessary, crash-wall type or partial-height solid wall piers should be used.
- For tall piers over 50 feet in height, two-column bents tend to be more economically feasible than cap-and-column piers. For piers over 75 feet in height, single-column bents (hammerhead) tend to be the most cost-effective pier type, as a rule of thumb. For tall piers or for piers that will be costly for other reasons, such as access (e.g., water, rail, traffic control) or unique foundation issues, reduction in the number of piers (i.e., longer spans) should be considered to achieve the least overall cost of the structure.
- For bridges over railroads, solid-wall type piers are preferred. Protective pier crash-walls should be considered and designed in accordance with AREMA specifications.
- For bridges over waterways, the following pier types should be considered:
- Pile bents: The unsupported pile length should generally be limited to a length of 20 feet. The engineer should investigate both the existing ground and scoured condition when determining the unsupported length, as the assumed point of fixity for the piles can vary substantially.
- Hammer-head piers
- Solid wall piers: When using wall piers in waterways, the potential for channel migration should be considered.
- Cap-and-column pier: For this pier type, the engineer must consider the potential for increased scour associated with vortexes forming around columns. Designers may consider the construction of a solid wall section with columns constructed above the water line.
- Note that the use of hammer-head type piers, or other pier types with large overhangs, inhibits the removal of debris at the pier face from the bridge deck. For low stream crossings with debris flow problems and where access to the piers from the stream is limited, hammer-head type piers, or other similar pier types, should not be used.
- Piers within navigable waters should be solid to a height of 3 feet above maximum navigable elevation or 2 feet above the 100-year flood or flood of record, whichever is higher. If the remaining height of pier above the solid stem is 16 feet or less, piers should be made completely solid.
- The upstream face of water piers should be rounded or V-shaped to improve hydraulics. If debris and/or ice is a problem, the upstream face should be battered 15 degrees and armored with a steel angle to a point 3 feet above the design high water elevation. This allows the debris to ride up the pier face.
- For unusual conditions, other pier types may be acceptable. In the design of piers that are readily visible to the public, aesthetics should be considered if it does not add appreciably to the cost of the pier.
103.7 Retaining Walls
103.7.1 Wall Types
The following are some commonly used types of retaining wall structures available for the designer to consider in a specific design: post and plank, sheet pile (either cantilevered or anchored), reinforced cast-in-place concrete, soil-nail walls, mechanically stabilized earth (MSE), and proprietary retaining walls.
184.108.40.206 Post and Plank Walls
Post and plank walls shall consist of steel H-piles driven or augured at designated spacing. The piles may be anchored using tie-back type anchors. The spaces between the piles are spanned with structural elements, such as wood (typically only for temporary structures), reinforced concrete, precast or cast-in-place concrete lagging, or steel members, to retain the soil.
220.127.116.11 Sheet Pile Walls
Sheet piling walls may be either exposed cantilever or anchored design. Sheet piling is driven in a continuous line to form a wall. Exposed cantilever walls shall be limited to 15 feet in height. In anchored design, deadmen or tie-backs are used to support the wall. The top of a permanent steel sheet pile wall must be constructed with a concrete cap so that the top of the sheeting is not exposed.
Steel sheet pile retaining walls are used as sea walls and for similar types of shore protection, such as flood walls, levees, and dike walls used to reclaim lowlands. If driven sheet pile walls are constructed as part of an abutment, the steel sheeting shall not be used as a support for the bridge vertical loads. Refer to the United States Steel (USS) Sheet Piling Design Manual (1984) for further information.
Concrete sheet piles are precast, prestressed concrete members designed to carry vertical and lateral earth pressure loads. These members shall be connected by a keyed vertical joint between two adjacent sheets. Geotextile fabric or suitable joint sealer is used to prevent loss of backfill material through these joints. The sheets are driven to ultimate bearing capacity using water jets, except the last 12 to 15 feet are driven using a suitable hammer. The use of concrete sheet piles is permissible in sandy soils only with approval of the Bridge Design Engineer.
18.104.22.168 Reinforced Concrete Walls
Reinforced concrete gravity or cantilever walls may be constructed using cast-in-place or precast concrete elements. They may be constructed on spread footings or footings on piles. They derive their capacity through combinations of self-weight, backfill, and structural resistance.
22.214.171.124 Anchored Walls
Anchored walls may be considered for both temporary and permanent support of stable and unstable soil and rock masses. Depending on soil conditions, anchors may be used to support both temporary and permanent non-gravity cantilevered walls higher than 15 feet.
The availability or ability to obtain underground easements and proximity of buried facilities to anchor locations shall be considered by the engineer when assessing the feasibility of anchored walls.
126.96.36.199 Proprietary Retaining Walls
In locations where retaining walls are needed to reduce span lengths or facilitate construction, proprietary walls may be considered. Economics, location, construction requirements, and aesthetics should be considered in the evaluation. These walls have proprietary patented systems for retaining soil. Two types of systems used in Delaware are gravity and mechanically stabilized. Gravity walls generally use interlocking, soil-filled reinforced concrete bins or modular blocks to resist earth and water pressures; they depend on dead load for their capacity. Mechanically stabilized walls use metallic or polymeric tensile reinforcement in the soil mass and modular precast concrete panels to retain the soil.
This type of construction can also reduce span lengths, thus saving on superstructure construction costs. Proprietary retaining walls can be economical where high wall heights are dictated by field conditions.
Locations where proprietary walls should be considered are based on the following requirements: readily available acceptable backfill material, available site working area, insufficient right-of-way for embankments or construction of alternative wall types, and fill conditions.
Each design location must be evaluated based on the advantages and disadvantages of the specific construction being considered. This is particularly important when a mechanically stabilized wall is being considered for a roadway crossing over a waterway. Close consideration must be given to long-term stability, stream flow, and storm flows. Positive erosion control, such as riprap placement, in addition to geotechnical fabric, shall be provided as deemed necessary. These walls should not be used in tidal areas or other locations where water might reach the wall.
Refer to the list of approved proprietary wall types in the Standard Specifications.
103.8 Bridge Rehabilitation versus Replacement Selection Guidelines
Several factors must be considered in decisions involving rehabilitation versus replacement. Each factor must be investigated and considered separately and collectively. The most common factors are noted below. LRFD design methodology should be used for all structure comparisions.
The estimating of both rehabilitation and replacement costs is usually performed after all other factors have been evaluated because the other factors may affect the scope of the rehabilitation or replacement option. The replacement estimate is to be done in accordance with procedures outlined in this Manual for new bridges.
When considering rehabilitation, the first step is to check the load rating. If the bridge is posted or if the current load rating appears suspect, rerate the bridge before proceeding with the estimate. A rehabilitation estimate is more difficult to develop as it cannot be developed from the biennial inspection report. It requires close inspection and examination of the bridge. This inspection must be of sufficient detail to develop a practical idea of the extent of the necessary work. The inspector should keep in mind that the actual rehabilitation work will most likely not be done for several years. Consequently, the estimate of quantities should have reasonable projections to compensate for continued deterioration. The BMS contains historical data for deterioration rates.
Like the replacement estimate, the bridge rehabilitation estimate should include highway and project costs necessary to provide a fair, relative cost comparison.
For comparison of rehabilitation versus replacement, cost estimates should be performed using LCCA. Refer to FHWA publication Life-Cycle Cost Analysis Primer (2002) available from the Office of Asset Management for more information (http://www.fhwa.dot.gov/infrastructure/asstmgmt/lcca.cfm). For the purposes of these guidelines, “user costs” are not included in the total costs associated with rehabilitation or replacement because, in both cases, traffic is usually restored to the same level of service that existed before construction. It may be necessary to take user costs into account on bridge removal and bridge capacity improvement projects because there would be a change that would affect the traveling public on a permanent basis. Therefore, user costs should be considered on an individual project basis and usually significant in only a small percentage of cases.
The next step is to compare rehabilitation and replacement costs related to the bridge assuming both alternatives are viable possibilities. The comparison should be based on life-cycle costs developed for each alternative. This relationship should be established in terms of the rehabilitation cost being a percentage of the replacement cost (RH/RP). Given the inherent uncertainties of estimating, relative costs may generally be separated into three ranges.
- RH/RP < 65%. The preliminary choice is rehabilitation.
- 65% < RH/RP < 85%. Rehabilitation or replacement may be the preliminary choice.
- RH/RP > 85%. The preliminary choice is replacement.
For all three ranges, other factors must be examined for compatibility with the rehabilitation or replacement selection. For example, detouring traffic in highly urbanized areas may not be feasible from a capacity point of view and constructing a temporary structure may not be possible from a right-of-way point of view. Construction of a replacement bridge alongside the existing bridge may not be possible due to right-of-way restrictions, even with staged construction.
Crash history and potential should be examined for the project bridge, with crash history being the more important of the two. Crash history can be determined by examining the crash reports on file, which are available upon request from the Traffic Safety Section. The review should look for trends in crash patterns that would point to whether the bridge caused or contributed to the crashes. Geometrics that contain clear potential for crash problems should also be considered for improvement. The review of geometrics should include, but not be limited to, sight distance, bridge width, horizontal clearances, and alignments. These elements should be compared to the standards and evaluated with regard to crash potential.
If either the crash history or crash potential indicates the bridge geometrics are unacceptable, the safety problem must be addressed by either widening the structure under rehabilitation or replacing the existing bridge with a wider structure.
103.8.3 Bridge Type
Some bridges, by their very type, will indicate a probable rehabilitation or replacement selection. For example, the Department gives special attention to non-redundant bridges where failure of one primary member would result in collapse or an unserviceable condition of the bridge. This factor includes a review of the sensitivity to being non-redundant, the consequences of no action, and the possibility of adding redundancy to the bridge. The rehabilitation versus replacement decision should take into account the redundancy of the bridge. Non-redundancy should be a factor in favor of replacement.
The type of construction of some bridges makes replacement a better choice than rehabilitation. For example, concrete arches and rigid frames are difficult and expensive to rehabilitate because of their monolithic construction. Past rehabilitation work on these types of bridges has been costly, so they are generally not rehabilitated. Also, because of their endurance, letting the life of these bridges simply expire is often more cost effective. However, considerations for historical needs may override economic feasibility associated with rehabilitation versus replacement decisions.
Another example is existing substructure foundations without piles that exhibit scour problems. This condition may push the decision toward replacement.
Constructability must be considered when deciding to rehabilitate or replace. The environment around the bridge may have changed dramatically since it was first constructed. The presence of critical utilities, right-of-way restrictions, adjacent infrastructure, and site access should be considered.
103.8.4 Bridge Standards
When any bridge is considered for rehabilitation, it should be reviewed for compliance with current standards. Existing vertical clearance, horizontal clearance, load capacity, freeboard, seismic capacity, lane width, and shoulder width should be compared to current standards. The hydraulic history of the bridge should also be reviewed. If the existing features are nonstandard, consideration should be given to improving them under rehabilitation or by replacing the bridge. If improvements cannot be made or only substandard improvements are possible, a nonstandard feature justification will be required. Refer to the  for further information on justification of design exceptions.
103.8.5 Feature Crossed
The feature crossed can have a significant effect on the type of work chosen and its cost. As an example, environmental concerns may push the rehabilitation versus replacement decision in the direction of rehabilitation, while hydraulic inadequacies and poor stream alignment may push the decision toward replacement.
103.8.6 Comprehensive Assessment of Rehabilitation versus Replacement
Other considerations in the rehabilitation versus replacement decision may have little to do with the structural adequacy, functionality, or safety associated with the structure. These considerations may include historical, social, political, utilities, and environmental considerations. These considerations can influence the rehabilitation versus replacement decision on individual bridge projects. They are difficult to categorize into specific indicators that trigger a particular decision; consequently, they have not been included in Table 103‑2. When these or any other considerations surface on a project, they should be treated as additional subjective factors and given the weight they deserve.
There may be additional factors on a specific bridge, such as the functional importance of the bridge and how important the bridge is to the overall transportation system of the area. Because many factors involve subjectivity, the people and agencies involved may reach different conclusions. This can present an opportunity to discuss differing viewpoints and gain the knowledge and experience of others. All conclusions drawn in the replacement versus rehabilitation discussion process must be fully documented in the TS&L Report.
|Table 103-2. Bridge Rehabilitation (RH) vs Replacement (RP) Worksheet1|
|Cost||A||Is the rehabilitation cost < 0.65 of the replacement cost?||Yes ......................................................RH
No ..................................Proceed to step B
|B||Is the rehabilitation cost between 0.65 and 0.85 of the replacement cost?||Yes .........................Consider other factors
No ..................................Proceed to step C
|C||Is the rehabilitation cost > 0.85 of the replacement cost?||Yes .......................................................RP|
|Safety||A||Are there accidents attributable to the bridge geometry or highway approach geometry?||Yes .................................Proceed to step B
No .............................................RP or RH
|B||If there were accidents, were there any fatalities or is the number of accidents above the statewide average?||Yes ...........................................,,RP or RH with corrections to the safety problem
No ..............................................RP or RH
|C||Is there an accident potential? (highway, waterway, or railroad)||Yes .............................................RP or RH with corrections to accident potential problems
No ..............................................RP or RH
|Bridge Type||A||Is the bridge nonredundant?||Yes..............................................RP or RH, including adding redundancy
No ..............................................RP or RH
|B||Does the bridge have fatigue sensitive details?||Yes ............................................RP or RH removing or modifying critical details
No .............................................RP or RH
|C||Is the bridge concrete arch, concrete rigid frame, etc.?||Yes ......................................................RP
No .............................................RP or RH
|Standards||A||Does existing bridge conform to all current standards?||Yes ............................................RP or RH
No ..................................Proceed to step B
|B||Can bridge be rehabilitated and brought up to standards?||Yes ...........................Bridge may be RH’ed
No .........................Bridge should be RP’ed
|C||Can the nonstandard feature be justified?||Yes ...........................Bridge may be RH’ed
No .........................Bridge should be RP’ed
|A||If existing bridge is over water, have there been hydraulic problems indicating an inadequate opening or poor stream alignment that would require a span adjustment?||Yes.......................................................RP
No ..............................................RP or RH
|B||Does existing bridge span have anything that requires special treatment or have special conditions associated with it such as railroad, or is historically, environmentally or politically sensitive?||Yes.............................................RP or RH*
No ..............................................RP or RH
*The sensitive feature must be thoroughly examined and considered in RH/RP analysis with special attention to the cost necessary to accommodate the sensitivity.
|MOT||A||Can traffic be detoured off the project site?||Yes ..............................................RP or RH
No ..................................Proceed to step B
|B||Can traffic be maintained on the existing bridge with a new bridge built alongside?||Yes ......................................................RP
No ..................................Proceed to step C
|C||Can construction be staged?||Yes ..............................................RP or RH
No ..................................Proceed to step D
|D||Can a temporary structure be used on the project site?||Yes ...............................................RP or RH
No. STOP. All traffic strategies have been rejected.
103.9 Accelerated Bridge Construction
ABC is construction that utilizes innovative planning, design, materials, and construction methods in a safe and cost-effective manner to reduce the on-site construction time of bridge projects. These innovative techniques include PBES, bridge movement methods and equipment to set into place complete substructures and superstructures built at offsite locations, and fast-track contracting procedures to rapidly replace or rehabilitate a highway bridge structure. The use of ABC techniques can improve worker and motorist safety, improve material quality and constructability, reduce right-of-way and environmental impacts, and minimize traffic disruption and cost, and should be investigated where appropriate following the guidelines contained herein. ABC techniques can be utilized on a range of repair, rehabilitation or replacement projects.
Design and construction guidance for ABC technologies and components shall be in accordance with AASHTO LRFD, as modified by this Manual. As the number of bridges constructed with ABC increases, innovation in the field will continue to grow and develop. As such, many ABC technologies are new and untested, and their use shall be coordinated closely with DelDOT. Because of the relative newness of some ABC technologies, the bridge designer shall consider incorporating long-term performance provisions when implementing ABC into projects. These provisions may include but are not limited to: additional concrete cover, high-performance concrete, corrosion-resistant rebar, and concrete sealers. Information on the subject of ABC and PBES can be found in the following FHWA references:
- Accelerated Bridge Construction – Experience in Design, Fabrication and Erection of Prefabricated Bridge Elements and Systems (2011 edition; abbreviated as FHWA ABC herein).
- Decision-Making Framework for Prefabricated Bridge Elements and Systems, Publication Number FHWA-HIF-06-030 (2006; abbreviated as FHWA Decision-Making herein).
- Manual on the Use of Self-Propelled Modular Transporters to Remove and Replace Bridges, Publication Number FHWA-HIF-07-022 (2007).
- Connection Details for Prefabricated Bridge Elements and Systems, Publication Number FHWA-IF-09-010 (2009).
103.9.1 Decision-Making/Planning Process
Except for emergency projects (Section 103.9.3 Emergency Projects), the typical approach to evaluating projects is multi-phased. It involves a concept team consisting of DelDOT representatives and/or other key stakeholders. FHWA Decision-Making provides a guide for the concept team to select viable ABC alternatives early in the process and determine their potential benefits over conventional methods.
All bridge projects are eligible for ABC techniques, and more than one ABC technology is typically feasible at a site. Therefore, prior to implementing these techniques, it is important that all ABC technologies be thoroughly weighed in the concept phase of the project.
The concept team will prioritize the list of ABC candidates once the evaluation process is complete based on scheduling issues and funding. If one or more alternatives are accepted, then the project-specific ABC technique(s) will be further developed in a TS&L plan by the bridge designer in accordance with Section 102 – Bridge Design Submission Requirements.
If additional evaluation is desired, the decision-making process can be supplemented by the DelDOT ABC Rating Score and the DelDOT ABC Decision Flowchart. These tools can aid the team in prioritizing ABC techniques and are available on the DelDOT DRC.
188.8.131.52 DelDOT ABC Rating Score and Decision Flowchart
The applicability of ABC to a bridge construction project can be further assessed by its ABC Rating Score. This rating system helps to determine which construction projects are more suited to ABC methods than conventional methods. The factors considered include ADT, allowable lane and road closures, phased construction options, impacts to local businesses and residents, construction durations, seasonal limitations, environmental impacts, safety, costs, and risk management. These factors are then individually weighted to reflect their relative impact on the construction and project planning process. Note that DelDOT Design Guidance Memorandum No. 1-24, Road User Cost Analysis (DelDOT DGM 1-24) can be consulted for guidance on calculating user cost for road construction projects.
The rating system yields a weighted score out of 100. Bridges with scores exceeding 50 are recommended for use of ABC technologies. Bridges with scores below the threshold can be further evaluated as required; unique circumstances not addressed in the rating may enter the discussion at this time. However, bridges with an ABC rating score below 20 are typically relegated to conventional construction methods.
103.9.2 ABC Methods/Techniques
FHWA ABC sorts the available ABC technologies into five distinct categories:
- foundation and wall elements,
- rapid embankment construction,
- structural placement methods, and
- fast-track contracting.
The first four components focus primarily on methods designed to expedite the on-site construction process; the fifth component is aimed to expedite the project delivery through use of innovative contracting methods.
The following subsections are intended to highlight the technologies successfully constructed in Delaware, as well as untried technologies that are viewed as attractive alternatives for future use. Refer to FHWA ABC and AASHTO LRFD as modified by this Manual for information not provided on the design and construction of the outlined technologies.
184.108.40.206 Foundation and Wall Elements
Some innovative foundation materials and construction methods useful in ABC are listed below:
- Continuous flight auger piles
- Prefabricated pier cofferdams
- Precast abutments
- Prefabricated foundation elements (see below)
220.127.116.11 Rapid Embankment Construction
Several ABC techniques are available to rapidly and more efficiently construct embankments; the most widely used are listed below:
- Expanded polystyrene (EPS) geofoam
- Accelerated embankment preload techniques
- Column-supported embankment technique
- Flowable fill
18.104.22.168 Prefabricated Bridge Elements and Systems
The most common form of ABC involves connecting prefabricated elements at the site to form a bridge. FHWA ABC summarizes the available ABC technologies into four main categories: materials, superstructure elements, substructure elements, and foundations.
As previously stated, the intent of the following lists is to highlight the technologies that have potential for widespread use in Delaware. The following is not meant to be an exhaustive list, but covers some of the more common ABC elements and systems. Use of ABC is highly encouraged when applicable; as such, all technologies outlined herein and in FHWA ABC are acceptable for consideration during the concept phase of the project pending the approval of DelDOT.
- Ultra-high performance concrete (UHPC) is capable of achieving very high flexural strengths and ductility. The material has shown great promise for several applications, including closure pours between adjacent elements such as prestressed girders, connections between precast deck panels, repair and rehabilitation of existing elements, and deck overlays. Despite being a costly material, UHPC has high potential for use in ABC and has already been successfully implemented on projects in Delaware and across the country.
- High-early strength concrete uses a combination of engineered materials and accelerating admixtures to create concrete that gains strength quicker the traditional cast-in-place concrete. When cast-in-place concrete is needed within an ABC project, High-early concrete can be utilized to minimize curing times.
- Superstructure elements
- Prefabricated and precast beam and girders, including NEXT beam bridges.
- Stay-in-place deck forming, including partial-depth, precast concrete deck panels.
- Full-depth precast deck panels for superstructure replacements.
- Modular superstructure systems include topped multi-steel beam units and precast concrete systems, such as double tees, bulb-tees, and segmental construction. Accelerated construction is achieved because the decking surface is connected to the beams and girders during fabrication. These prefabricated elements can be erected using ABC large-scale placement methods (Section 22.214.171.124 Structural Placement Methods).
- Substructure elements (in conjunction with Section 126.96.36.199 – Foundation and Wall Elements)
- Precast concrete pier elements including columns, caps and walls.
- Prefabricated abutment elements including caps and associated wingwalls.
- Prefabricated retaining walls, such as MSE walls.
- Modular culvert and arch systems.
- Foundations (in conjunction with Section 188.8.131.52 – Foundation and Wall Elements)
- Pile bents with precast concrete piles. Piles are driven to a smaller tolerance when using a prefabricated cap.
- Precast concrete spread footings.
- Note that with all prefabricated systems there should be a huge emphasis on the field connections between elements.
- In general, use UHPC for connections between prefabricated superstructure elements.
- In general, use high-early-strength concrete for connections between prefabricated foundation and substructure elements.
184.108.40.206 Structural Placement Methods
ABC not only involves materials and prefabricated elements, but also rapid large-scale movement techniques of structural systems and even complete bridges. The most common placement practices are achieved by one of the following:
- Longitudinal launching
- Horizontal skidding or sliding
- Other heavy lifting equipment and methods, including pipe and culvert jacking, strand jacks, climbing jacks, pivoting, and gantry cranes
220.127.116.11 Fast-Track Contracting
Innovative contracting methods are often used to expedite the project, both in terms of in-field construction time and planning/design time. Traditional design-bid-build methods require design and construction to take place sequentially. ABC alternative project delivery (APD) methods generally allow design and construction to take place concurrently, thereby requiring less time to complete a project. Acceptable APD methods in Delaware include Design-Build (D-B) and Construction Manager/General Contractor (CMGC). Consult the DelDOT Project Delivery Selection Process for guidance on selection of APD methods.
In conjunction with the delivery methods, a variety of contracting provisions are often used on ABC projects to place emphasis on the need to complete the project quickly. These are listed below:
- Best value selection
- A+B and A+B+C bidding
- Continuity of the construction process
- Incentive/disincentive clauses
- Lane rental
103.9.3 Emergency Projects
Emergency repair or replacement projects are typically the result of extreme events, such as flood damage, fire, vehicle impact, and waterway vessel collision. The goal for any emergency project is to quickly restore the affected portion of the transportation network back to full capacity, regardless of the cause. Establishing detour routes, and making these routes public knowledge promptly, will minimize impacts and ease traffic congestion.
Because of the immediate need imposed by an emergency, the decision-making and design processes are accelerated. Large-scale or uncommon emergencies may require an emergency response team to be assembled from DelDOT officials, design consultants, and contractors. The response team will quickly make planning and design decisions with the primary focus on public safety and mitigation of traffic disruption.
To expedite the planning, design, and construction processes, a thorough damage assessment must be determined quickly to establish a scope for the project. Likewise, conduct a thorough site review to identify potential conflicts that could delay the project. ABC techniques can still be utilized on an emergency project to complete construction speedily. However, the lead time associated in procuring prefabricated bridge elements needs to be considered in planning and design decisions. Use of advance contracting methods can allow for fabrication to happen concurrently with other elements of the design project, unlike traditional design-bid-build project delivery.
103.10 Requirements for the Design of Highway Bridges over Railroads
Coordination with the owner of the railroad is required for all projects over, under, or adjacent to a railroad. Regular communication with the railroad is needed throughout the entire project development process to ensure time-sensitive approval from the railroad.
Refer to Sections 18.104.22.168 – Over Railroads and 22.214.171.124– Over Railroads for horizontal and vertical clearance requirements adjacent to and above railroads. Refer to Sections 126.96.36.199 – Over Railroads for crash wall requirements for bridge piers constructed adjacent to the railroad.
Care must be taken to ensure that survey data of rails are accurate. The top of rail must be properly surveyed in order to accurately calculate the vertical clearance of highway bridges over railroads. Survey shots shall be taken on both rails at spacing not to exceed 25 feet in the area under the bridge. The surveyor shall take three shots at each location, one on the top of the rail and one at the tie on both sides of the rail so that the engineer is certain he has a shot on the top of the rail. A pre-survey field meeting should take place between the designer and survey group. Additional survey shall be taken to double check against the original survey immediately prior to start of construction. This is to safeguard the Department from errors and/or differences in rail elevations due to survey errors or when the owner of the railroad re-tracks the rails during the design phase. A check for clearance shall also be performed after the beams are set. This check shall take into account the expected loss of camber due to the application of all dead loads.
Where a drainage ditch is to be provided parallel to the track, the elevation of the top of footing adjacent to track shall be at least 3 feet 6 inches below the elevation of the top of rail, unless rock is encountered. The edge of the footing shall be at least 7 feet from the centerline of adjacent track.
Bridge scuppers shall not drain onto railroad tracks or ballast. Provisions shall be made to direct surface water from the bridge area into an adequate drainage facility away from the railroad track and will require railroad approval.
Safety provisions required during excavation in the vicinity of railroad tracks and substructures shall be in accordance with a special provision for the maintenance and protection of railroad traffic. Sheet piling walls or other approved support systems, as required for excavation support for the protection of railroad tracks and substructure, shall be designed according to AREMA specifications and shall be subject to approval by the railroad company.
Complete details of temporary track(s) or a temporary railroad bridge to be constructed by the contractor shall be shown on the design drawings, if applicable. Applicable railroad design standards or design drawings shall be referred to or duplicated on the design drawings.
For NHS structures crossing over railroads, protective screening/fencing shall be provided per the railroad’s requirements (e.g., both sides, sidewalk only) for the portion of the structure (spans) over the railroad. For non-NHS structures with sidewalks, the protective fencing shall be provided only on the sidewalk side of the structure, for the portion of the structure (spans) over the railroad. For non-NHS structures crossing over railroads where protective fencing is not required by Department criteria, the railroad may request the installation of the protective fence for the portion of the structure (spans) over the railroad, if the railroad agrees to reimburse the Department for the installation of the protective fence.
For electrified railroad tracks, these additional requirements apply:
- If a railroad is electrified, the preliminary plans submitted for TS&L approval should note that.
- A protective barrier shall be provided on spans or on part of spans for structures over electrified railroads, as directed by the railroad company. The protective barrier shall extend at least 10 feet beyond the point at which any electrified railroad wire passes under the bridge. However, in no case shall the end of the protective barrier be less than 10 feet from the wire measured in a horizontal plane and normal to the wire outside of the limit of the bridge, and less than 6 feet from the wire within the limit of the bridge. Refer to Section 325.02 – Bridge Railing Details for protective barrier details.
- All open or expansion joints in the concrete portion of barriers, divisors, sidewalks, and curbs within the limits of the barrier shall be covered or closed with joint materials. Details of such joints shall be shown on the design drawings.
- The details of catenary attachments and their locations, if attached or pertinent to the structure, shall be shown on the plans. Consideration shall be given to realign the catenary by installing support columns on each side of the bridge to avoid catenary attachments to the bridge. Normally, ground cable attachments, cables, and miscellaneous materials are supplied by the contractor and are installed by the railroad. The Plans shall show a separate block identifying the materials required, a description of materials, the railroad reference number for materials, and the party responsible for providing or installing materials. Approval of grounding plans shall be obtained from the railroad concurrently with approval of the structure drawings.
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AREMA, 2015. Manual for Railway Engineering.
DelDOT, 2022. Road Design Manual, September.
DelDOT, 2010. Design Guidance Memorandum No. 1-24, Road User Cost Analysis, April.
DelDOT, 2015. Oversize/Overweight Hauling Permit Policy and Procedures Manual, October.
DelDOT, 2020. Standard Specifications for Road and Bridge Construction, August.
FHWA, 1993. HEC-21, Design of Bridge Deck Drainage, May.
FHWA, 2002. Life-Cycle Cost Analysis Primer, Office of Asset Management, August.
FHWA, 2005. Barrier Guide for Low Volume and Low Speed Roads, Publication Number FHWA-CFL/TD-05-009, November. https://flh.fhwa.dot.gov/resources/design/library/FLH-Barrier-Guide.pdf.
FHWA, 2006. Decision-Making Framework for Prefabricated Bridge Elements and Systems Publication Number FHWA-HIF-06-030.
FHWA, 2007. Manual on the Use of Self-Propelled Modular Transporters to Remove and Replace Bridges, Publication Number FHWA-HIF-07-022.
FHWA, 2009. Connection Details for Prefabricated Bridge Elements and Systems, Publication Number FHWA-IF-09-010.
FHWA, 2009. HEC-22, Urban Drainage Design Manual, 3rd Edition, Publication No. FHWA-NHI-10-009, September.
FHWA, 2011. Accelerated Bridge Construction – Experience in Design, Fabrication and Erection of Prefabricated Bridge Elements and Systems.
FHWA, 2014. Specification for the National Bridge Inventory Bridge Elements, January 21.
Iowa DOT, 2018. Instructional Memorandum No. 3.240 – Clear Zone Guidelines, May 18. https://www.iowadot.gov/local_systems/publications/im/3240.pdf.
MnDOT, 2005. Final Report 2005-39 – The Safety and Cost-Effectiveness of Bridge-Approach Guardrail for County State-Aid (CSAH) Bridges in Minnesota. http://www.dot.state.mn.us/research/TS/2005/200539.pdf.
NCHRP, 1997. Research Results Digest 220 – Strategies for Improving Roadside Safety, November. http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rrd_220.pdf.
NCHRP, 2012. NCHRP Report 725 – Guidelines for Analysis Methods and Construction Engineering of Curved and Skewed Steel Girder Bridges, Transportation Research Board.
PCI, 2011. Bridge Design Manual, 3rd Edition, November.