110 - Ancillary Structures
The purpose of this section is to establish policies and procedures for identifying DelDOT preferences for the final design and detailing of ancillary structures.
Overhead Sign Structures – Structural supports for any overhead sign that extends over any portion of the roadway, including the shoulders, and provides motorists with a variety of messages.
Sound Barrier Walls – Walls that are erected to attenuate noise created by transportation facilities. These walls are also commonly referred to as noise walls.
Variable Message Sign (VMS) – A programmable sign that can display any combination of characters to present messages to motorists. This section will address those signs that are permanently mounted on overhead structures, although VMSs may be semi-permanent or portable, and are also known as Dynamic Message Sign (DMS) or Changeable Message Sign (CMS).
110.3 Overhead Sign Structures
Sign structures support both overhead and roadside highway signs. Overhead signs are highway signs that extend over any portion of the roadway, including the shoulders, and provide motorists with a variety of messages. Delaware is in the process of transitioning from truss-type overhead sign structures to tubular overhead sign structures. Roadside signs are located outside the roadway and shoulders. The primary focus of this section is to outline the procedures used to design and detail new tubular overhead sign structures, as well as address rehabilitation of existing overhead sign structures. Roadside signs are not explicitly discussed in the following sections; design of these supports shall be in accordance with the references below.
The following subsections are largely based on the information documented in two primary references: AASHTO LRFD Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals, 1st Edition (2015); and AASHTO LRFD, as modified by this Manual.
The Traffic Section is responsible for determining the need, size, and location of signs on a roadway, per the standards and guidelines in the latest version of the Delaware Manual of Uniform Traffic Control Devices (2011) and the FHWA’s Manual of Uniform Traffic Control Devices (2009). It is then the responsibility of the designer to select the appropriate sign structure, given the signage required by the Traffic and Safety Engineer. The following subsection outlines all commonly used overhead sign structures, and the design considerations to be weighed when selecting the most appropriate structure type.
110.3.1 Overhead Sign Structure Types and Geometrics
There are two major types of overhead sign structures—cantilever and span-type—as shown in Figure 110‑1 through Figure 110‑5, and as outlined by LRFD Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals. Cantilever sign supports are typically more appropriate and cost-effective for shorter spans. However, span-type supports become advantageous when more signage is required, or the roadway is wider. Span-type structures shall be selected over cantilevered structures when the required span exceeds 42 feet 6 inches, or 40 percent of the roadway cross section; or when the span-to-height ratio of the cantilever exceeds 1.5.
Cantilever and span-type overhead sign structures can either be ground-mounted or bridge-mounted. Bridge-mounted sign structures can provide information to motorists passing on the structure or passing under the structure. The concepts shown in Figure 110‑1 through Figure 110‑5 are applicable for all cases. Refer to Section 110.3.2 – General Design Considerations for structure type preferences for ground- and bridge-mounted conditions.
In addition to typical overhead sign structures, signs mounted directly to a bridge fascia are commonly used. These signs provide information to motorists passing under a bridge only. An example is shown in Figure 110‑6.
18.104.22.168 Cantilever Sign Structures
Tubular cantilever sign structures consist of four common types: (1) single-cantilever; (2) butterfly; (3) dual-cantilever; and (4) butterfly VMS.
A single-cantilever structure (Figure 110‑1) consists of a curved post field-spliced to a single mast arm, to which the sign is connected.
A butterfly structure, also referred to as a balanced cantilever, (Figure 110‑2) consists of a straight post field-spliced to the base of a mast arm; the signage is connected at (or near) the centroid of the sign structure.
A dual-cantilever structure (also referred to as an unbalanced cantilever Figure 110‑3) is similar to that of a single cantilever, except that two mast arms project from the center post instead of one. Note that the loads and moment arms can be either balanced or unbalanced around its vertical support.
A butterfly VMS (similar to Figure 110‑2) is similar to a butterfly sign structure, except that it supports a VMS instead of a typical roadway sign. VMS-type signs often include service platforms or catwalks; these platforms shall be considered in the design of the sign structure.
22.214.171.124 Span-Type Sign Structures
Tubular span-type sign structures consist of three types: (1) single-mast span-type; (2) double-mast span-type; and (3) span-type VMS.
A single-mast span-type structure (Figure 110‑4) consists of a single mast arm spanning between two curved posts on each side of the roadway.
A double-mast span-type (Figure 110 5), as the name suggests, introduces a second mast arm. When larger signs are required, a double-mast system sign structure may be considered. Sign structures that support sign panels with a height in excess of 20 feet may be best suited as double-mast systems, although the single-mast is preferred.
Lastly, a span-type VMS is similar in structure to the other span-type structures, except that it supports a VMS (see Figure 110‑4 and Figure 110 5). VMS-type overhead signs often include service platforms or catwalks; these platforms shall be considered in the design of the sign structure.
126.96.36.199 Bridge-Mounted Signs
Bridge-mounted signs (Figure 110‑6) are smaller signs that can be directly attached to a fascia girder and/or bridge parapet, typically by structural angles.
110.3.2 General Design Considerations
Overhead sign structures can either be bridge or ground-mounted; signs can also be fastened to a bridge fascia directly without the use of a tubular frame. In general, ground-mounted sign structures are preferred.
Bridge-mounted overhead sign structures are to be avoided where practical, especially on bridges with skews in excess of 30 degrees; the Bridge Design Engineer must approve their use. Bridge-mounted overhead sign structures shall be span-type only; and must be supported on—or at least near—pier caps to reduce vibrations in the sign structure, and to minimize the load effects on the fascia girder. In addition, a 6-inch minimum clear dimension shall be maintained between the outside face of the parapet and the sign structure post to prevent vehicular collision damage to the sign support.
For signs fastened directly to the bridge fascia, the lowest point of a sign or its appurtenances must be 1 foot above the bottom of the superstructure to which it is attached. A 2-inch minimum gap shall be maintained between the bridge fascia and the sign. These signs shall be within a 5 degree skew measured perpendicularly to the roadway below; if this cannot be achieved due to the skew of the bridge relative to the lower roadway or other attachment complications, ground-mounted signs shall be used.
For both bridge-mounted overhead sign structures and signs fastened directly to the bridge fascia, special attention must be paid to the connections to the existing structure. When connecting to existing concrete elements, expansion-type and adhesive- or resin-bonded anchors are disallowed due to pullout and long-term creep concerns. Grouted A307/A325 bolts are the preferred alternative. Anchorage to an existing pre-tensioned or post-tensioned concrete fascia girder is prohibited. Additionally, high-strength bolts are required when fastening to a steel fascia girder.
For ground-mounted overhead sign structures, the minimum vertical clearance between the roadway surface and the bottom of the sign structure and/or sign shall be 17 feet 6 inches for both typical signs and VMS; this dimension must be maintained for the full width of the roadway and shoulder. The sign structure posts shall be placed outside the clear zone, as defined in Section 188.8.131.52.1 – Delaware Clear Zone Concept. Otherwise, they shall be protected with a properly designed traffic barrier.
For all applicable overhead sign structures, handholes shall be placed away from traffic to minimize exposure to de-icing salts, in case the cover is broken and/or not closed. Handholes shall be at least 3 feet 6 inches above the top of the base plate, and be minimum 6½-inch by 3 inch oval holes with 8-inch by 4½-inch oval covers.
Grout pads between the bottom of the steel base plate and the top of the footing shall be avoided where practical. The grout tends to trap water and chlorides, which leads to corrosion of the anchor rods. An open-base post that is supported directly on the anchor bolt leveling nuts is the preferred connection. A protective wire-mesh screening material shall be used to keep birds and rodents out of the void space. If specific conditions warrant the use of a non-shrink grout pad, the grout shall not be considered load-carrying, and an adequate drainage system shall be provided. The Bridge Design Engineer must approve the use of a grout-leveling pad.
184.108.40.206 Designer Responsibility
The designer shall design the entire overhead sign structure in accordance with the design requirements in this section, and prepare a drawing set that includes all materials, connections, and design data. Should DelDOT develop standard drawings and designs for its overhead sign structures, the designer shall conform to those standards. The Engineer of Record (EOR) is also responsible for preparing and sealing all overhead sign structure drawings.
All new overhead sign structures shall consist of steel, tubular sections with a minimum thickness of 1/4 inch. Bolted field splices, bolted base-plates to foundation, and full-penetration shop-welded post-to-base-plate connections are also required for enhanced fatigue performance. All structural steel shall be hot-dip galvanized in accordance with ASTM A123. It is preferable to galvanize sign structure sections in a single dip, as opposed to double-dipping. Double-dipping has caused component failures during the galvanizing process. The designer should discuss with approved local galvanizers their capabilities to single-dip sign structure sections and adjust their design accordingly. A note shall be made on the plans specifying the galvanizing procedure.
Handhole covers shall be made of ASTM A240, Type 302 or Type 304, stainless steel.
Aluminum sections for new structures are not permitted. Aluminum components are only permitted in the rehabilitation of an existing aluminum sign structure.
220.127.116.11 Design Criteria
All overhead sign structures, including service platforms and catwalks, shall be designed in accordance with the LRFD Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals, 1st Edition, with interims through 2019, hereafter LRFD LTS. In addition to the LRFD LTS, the following stipulations also apply:
- Any LRFD LTS optional design parameter noted as “may be used at the discretion of the Owner” or “unless required by the Owner” that is not addressed within Section 110 shall not be required as part of the design.
- Modify Chapter 3 of LRFD LTS as follows:
- Add note under Table 3.8-1 – Assume High risk for NHS routes and Typical risk for all other routes for and all sign structures and traffic signals and DMS and high mast lights that can reach the travel way (height greater than offset distance from travel way). For high mast lights and DMS structures that cannot reach the travel way (height less than offset distance from travel way), assume low risk.
- Replace figures 3.8-1 through 3.8-4 with the following table:
*Values are nominal design 3-s gust wind speeds at 33 feet above ground for Exposure C category **East of US113 includes US113 and associated ramps Basic Wind Speed, V (mph)* County 700-Year MRI 1700-Year MRI 300-Year MRI 10-Year MRI New Castle 115 120 105 76 Kent 115 120 105 76 Sussex, West of US113 120 130 110 76 Sussex – East of US113** 130 140 120 80
- Replace first paragraph in Article 18.104.22.168 with the following: For bridge mounted ancillary structures and other site conditions where structures may be elevated above the surrounding terrain, the height factor used shall be increased to account for increased wind effects.
- Modify Chapter 11 of LRFD LTS as follows:
- Add the following table to end of Article 11.6:
*Span length is defined as center to center of posts for span structures and face of column to tip of arm for cantilever type structures **For twin mast arms, the pole, arms, and connections shall be designed for the applicable fatigue category for the longest arm attached. ***Signal mast arm structures with a DelDOT-approved mitigating device may be designed for Fatigue Category II. Fatigue Importance Categories Structure Type Span Length* (ft) Fatigue Category Dynamic Message Sign (DMS) All span lengths Category I Overhead sign span structure >150 Category I ≤150 Category II Cantilever sign structure >50 Category I ≤50 Category II Butterfly sign structure All span lengths Category II Signal mast arm structure** >50 Category I*** ≤50 Category II Signal mast arm structure on roadways with speed limits of 35 mph or less All span lengths Category III High mast light poles All heights Category I Signal span wires, conventional light poles, and ITS device support poles (excluding DMS) No fatigue design required
- Add the following to the end of Article 22.214.171.124: Galloping loads can be ignored for signal mast arm structures with a DelDOT-approved mitigating device.
- Add the following t the end of Article 11.8: Vertical deflections of the free end of cantilever sign structures and signal mast arm structures due to galloping or truck-induced gust loads shall be limited to 8 inches. This requirement can be ignored for signal mast arm structures with a DelDOT-approved mitigating device.
126.96.36.199 Consideration for Future Conditions
Where feasible, the design of an overhead sign structure shall accommodate future roadway widening, as designated by DelDOT, so that replacement is not required prior to the end of its intended design life. The Traffic Section is responsible for determining the need, size, type, and location of sign panels to be supported by the sign structure, and evaluating whether future sign panel size increases are likely. If changes are expected, the designer shall accommodate the larger of either the current or future sign panel area. To achieve prolonged functionality, overhead sign structures shall be designed for additional sign area 15 percent greater than expected; and the demand-to-capacity ratio shall not exceed 80 percent for Strength Limit State design.
The designer is permitted to incorporate a Stockbridge-type damper as a means of absorbing energy for any new design of any existing sign structure experiencing excessive vibration.
110.3.3 Design Process
The following steps outline the general procedures when designing an overhead sign structure or VMS sign structure:
- The Traffic Section determines the size, type, and location (with respect to the roadway) of sign panels, or VMS assembly to be supported by the sign structure.
- Locate the sign foundation(s) to adequately place them outside of the roadway clear zone where practical. Note that bridge-mounted sign structures are not desirable, and require approval from the Bridge Design Engineer.
- Locate existing and proposed utilities in the area of the sign, and coordinate installation and tie-in of electric, ITS, etc., when required.
- Determine the required span length and sign-panel height based on the minimum required horizontal and vertical clearances, as well as sign-panel dimensions.
- Determine the most appropriate sign structure type based on the design considerations outlined in this section. If using an overhead-type sign structure, determine if a single- or double-mast sign structure is needed based on the sign-panel height.
- Determine post heights. The post height is measured from the bottom of base plate to the centerline of the horizontal mast arm (the lowest mast arm for a double-mast sign structure).
- Design and detail the structure in accordance with Section 110.3.2 – General Design Considerations and Section 110.3.3 – Design Process.
- Complete the overhead sign structure plan set by providing all applicable design criteria and details for the following: materials, design sections, splice and base plate connections, foundation data, catwalk framing and connections, and vibration mitigation.
Specific geotechnical and foundation design issues are addressed in Section 105 – Geotechnical Investigations and Section 107 – Final Design Considerations – Substructure of this Manual.
110.4 Sound Barrier Walls
Noise abatement measures are often used when new construction or widening of an existing roadway results in highway noise impacts to the surrounding community. DelDOT Policy Implement No. D-03 “Highway Transportation Noise Policy” (DelDOT Policy No. D-03, 2011) shall be consulted for determining how and under what circumstances highway- and construction- generated noise shall be mitigated. The feasibility and reasonableness of sound barrier walls as a noise abatement measure is evaluated by this document.
The following measures are typically considered by DelDOT in the design phase: sound barriers (either walls, berms, or a combination), alteration of roadway alignment, traffic management measures, and acquisition of real property for buffer zones. The focus of this subsection is the structural design considerations for sound barrier walls.
The design criteria for these walls are typically separated in two categories: bridge-mounted, and ground-mounted. Bridge-mounted sound barrier walls are fastened to an existing or new bridge structure, typically along its fascia. Ground-mounted sound barrier walls are supported on the ground by foundations, and are typically outside the clear zone.
This section identifies the key issues in the design of sound barrier walls to assist engineers in preparing design drawings. The major focus is on the design criteria and detailing procedures for new sound-barrier wall structures. Because Delaware does not currently own any sound barrier walls, rehabilitation of existing sound barrier walls is not applicable, and therefore will not be addressed herein.
110.4.1 Design Criteria
The subsections herein are specifically based on the information documented in one primary reference: AASHTO LRFD as modified by this Manual. Additional nonstructural design considerations, such as wall aesthetics and acoustics, are addressed in FHWA Highway Noise Barrier Design Handbook (2011).
188.8.131.52 Designer Responsibility
The Bridge Design Engineer and/or the design consultant are to perform a noise study in accordance with DelDOT Policy No. D-03 to determine if a sound barrier wall is warranted. The required height, length, offset, foundation type, wall material types, and geometry of the sound barrier wall to mitigate the noise impact shall be determined in this stage. Sound-barrier wall aesthetics and possibly material type will be coordinated with DelDOT and the public.
Coordination with utilities, right-of-way clearances, and environmental impacts shall be considered in the design, geometry, and location of the sound barrier wall. The Bridge Design Engineer must approve the preliminary sound-barrier wall parameters outlined above before the designer begins final design.
The designer shall design the entire structure in accordance with the design requirements in this section, and prepare a drawing set that includes all materials, connections, and design data. The EOR is responsible for preparing and sealing all sound-barrier wall drawings.
184.108.40.206 General Criteria
In general, ground-mounted walls are preferred to bridge-mounted.
Bridge-mounted sound barrier walls shall be avoided due to additional loading placed on the bridge, additional live load vibration consideration in the sound barrier wall, and proximity to potential traffic-impact damage. The Bridge Design Engineer must approve their use.
Height range for a bridge-mounted sound barrier wall (measured from top of bridge barrier to top of wall) is typically 4 feet minimum to 10 feet maximum. Height range of a ground-mounted sound barrier wall (measured from finished grade elevation to top of wall) is typically 4 feet minimum to 30 feet maximum. Heights of ground-mounted sound barrier walls may be reduced by using earthen berms in conjunction with the sound barrier wall. A minimum height is required to allow for emergency and maintenance access doors to be placed in the area of a single panel, as outlined in Section 15.4.3 of AASHTO LRFD. Post spacing shall be limited to a maximum spacing of 10 feet for bridge-mounted walls, and 20 feet for ground-mounted walls.
Design of bridge-mounted sound barrier walls shall be done with consideration for bridge inspection and maintenance access. The total dead load and wind load per linear foot shall be noted on the bridge plans, and shall be considered in the design of the new bridge or the evaluation of an existing bridge. Bridge-mounted sound barrier walls shall also be designed to account for bridge expansion and contraction movements (see Section 220.127.116.11 – Detailing Connections).
Ground-mounted sound barrier walls shall also be outside of the roadway clear zone, unless dictated by site conditions and/or determined by DelDOT. If the sound barrier wall is within the roadway clear zone, it shall be protected by a properly designed traffic barrier.
Sound barrier walls are typically post-and-panel systems where the panels span between evenly spaced vertical posts. The posts are either supported by a foundation system (ground-mounted) or a component of a bridge fascia (bridge-mounted). Precast concrete panels and rolled-beam steel posts are the preferred sound-barrier-wall system in Delaware; although, other materials and systems are acceptable depending on the specific project parameters. For instance, corrugated metal or lightweight concrete panels with steel posts may be suitable for bridge-mounted walls to reduce dead load on the existing structure. Cast-in-place or precast concrete posts can also be used for ground-mounted sound barrier walls. Proximity to roadway salt spray exposure should be considered in the wall material selection.
All exposed steel, including posts and hardware, shall be galvanized or painted. Anchor bolts, if used, must conform to the ASTM F1554 Grade 55 or 105 specifications.
In general, concrete sound barrier wall panels shall be 8 inches thick at a minimum, with 1.5 inches of concrete cover. Two layers of reinforcement shall be detailed on both faces. Welded-wire fabric is an accepted alternative. This requirement is applicable to precast concrete panels, as well as self-consolidating cast-in-place concrete panels. For conventional cast-in-place concrete, the minimum thickness is increased to 8 inches, with 2 inches of concrete cover.
18.104.22.168 Detailing Connections
Concrete panels are typically connected to steel or concrete posts with compression clamps; metal panels are connected with self-tapping screws. In either case, the designer must provide allowance for expansion of the walls at the joints, as outlined in Section 15.6 of AASHTO LRFD.
As stated above, mounting sound-barrier-wall structures to an existing bridge fascia can be complex. Bridge-mounted sound barrier walls shall be mounted directly to the top of the parapet or to the outside face of the parapet to mitigate vehicular collision effects. Integrally cast anchor bolts are the preferred connection to the parapet. If a sound barrier wall is to be mounted on an existing parapet, use grouted A307/A325 bolts. Epoxy or resin-bonded adhesive and expansion-type anchors are not permitted due to pullout concerns. Use a ½‑inch-minimum thickness closed-cell neoprene sponge (CCNS) bearing pad between the post base and the top of the bridge parapet to mitigate vibration.
Bridge-mounted sound barrier walls shall use steel posts with lock nuts or lock washers due to the vibrations of the bridge. Also, the designer shall incorporate provisions in the design of bridge-mounted walls to prevent the wall from falling below, whether due to traffic impact or wall failure. This can be accomplished using a cable system along the sound barrier wall (cables or wire rope embedded in each panel running between and connected to the steel posts).
For ground-mounted sound barrier walls, steel posts shall be either embedded in the concrete foundation wall/shaft, or anchor-bolted to a base plate cast in the top side of the foundation. Bottom panels for ground-mounted sound barrier walls shall be embedded into the ground a minimum of 6 inches.
All sound barrier walls, bridge- or ground-mounted, shall be designed in accordance with the loading criteria established in AASHTO LRFD. Note that wind pressures derived from this reference are applied uniformly across the post-and-panel system. The panels shall be designed as simply supported spans between posts, unless the designer can justify a more rigid end connection. For bridge-mounted sound barrier walls, the additional dead- and wind-load forces shall also be considered in new bridge superstructure design, as well as evaluation of an existing structure.
Any alternate designs and details not identified in this subsection shall be evaluated, presented to, and approved by the Bridge Design Engineer. For other structural design considerations not presented in this section, such as proper drainage for ground-mounted walls and collision loads, refer to AASHTO LRFD for design guidance. For other general considerations, refer to Highway Noise Barrier Design Handbook.
Sound-absorptive panels may be required to reduce reflective sound when sound barriers are on both sides of a roadway and spaced closer than 100 feet and if required by the noise study.
Aesthetic considerations and architectural treatments shall be coordinated with and approved by DelDOT. This includes items such as texture, surface finish, stain, anti-graffiti coatings, and sealants.
Sight-distance obstruction on horizontal curves shall be considered in the design and location of sound barrier walls.
Specific geotechnical and foundation design guidelines are addressed in Section 105 – Geotechnical Investigations and Section 107 – Final Design Considerations – Substructure of this Manual.
AASHTO, 2015. LRFD Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals, 1st Edition.
AASHTO 2017. LRFD Bridge Design Specifications, 8th Edition.
DelDOT, 2011. Delaware Manual of Uniform Traffic Control Devices
DelDOT, 2011. Policy Implement No. D-03 “Highway Transportation Noise Policy” (DelDOT Policy No. D-03).
FHWA, 2009. Manual of Uniform Traffic Control Devices.