110 - Ancillary Structures: Difference between revisions

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==== 110.3.1 Overhead Sign Structure Types and Geometrics ====
==== 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.
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.
===== 110.3.1.1 ''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.
[[File:Bdm-2021-110-1.png|frameless]]
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.
[[File:Bdm-2021-110-2.png|frameless]]
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.
[[File:Bdm-2021-110-3.png|frameless]]
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.
===== 110.3.1.2 ''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.
[[File:Bdm-2021-110-4.png|frameless]]
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.
[[File:Bdm-2021-110-5.png|frameless]]
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.
===== 110.3.1.3 ''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.
[[File:Bdm-2021-110-6.png|frameless|501x501px]]
==== 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 103.3.4.2.1 – 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.
===== 110.3.2.1 ''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.
==== 110.3.2.2 ''Materials'' ====
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.
===== 110.3.2.3 ''Design Criteria'' =====

Revision as of 17:10, 24 November 2021

110.1 Introduction

The purpose of this section is to establish policies and procedures for identifying DelDOT preferences for the final design and detailing of ancillary structures.

110.2 Terms

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.

110.3.1.1 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.


File:Bdm-2021-110-1.png


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.


File:Bdm-2021-110-2.png


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.


File:Bdm-2021-110-3.png


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.

110.3.1.2 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.


File:Bdm-2021-110-4.png


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.


File:Bdm-2021-110-5.png


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.

110.3.1.3 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.


File:Bdm-2021-110-6.png

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 103.3.4.2.1 – 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.

110.3.2.1 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.

110.3.2.2 Materials

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.

110.3.2.3 Design Criteria