Overhead Sign Structure Drawings Sheets
An overhead sign structure drawing is required for each proposed overhead assembly and each existing overhead assembly with proposed sign panels. This drawing shows the roadway cross section and a schematic of the overhead sign structure. Cross sections are obtained from roadway plans or field shots. If the overhead sign structure is designed where roadway cross sections are not available, the design shall be based on field data. Overhead Sign Structure Drawing Sheets shall include the following:
- Overall structure length
- Lane, gore and shoulder widths
- Lane arrows
- Pavement and ground slopes and dimensions
- Support offsets
- S-dimensions at support locations
- Positioning of proposed and future signs relative to travel lanes
- Spacing between proposed and future signs
- Sign messages and/or future messages
- Future signs
- Minimum and maximum vertical clearances
- Existing and proposed guardrail, guiderail, barrier, noise walls, and retaining walls
- Labeling of facility and direction of travel
- Wind load and dead load requirements to be used for proposed structures and footing designs
- Drainage pipe size, type and offset
- Right of Way
- Utility lines and offsets
- All applicable notes.
Sign Placement
Place signs over the travel lanes to which they apply. (Down Arrow according to MUTCD) Down arrows shall be placed centered over the lane to which they apply. Down arrows may be centered up to 2' from the center of the lane if needed. Signs shall not be closer than 0.5' from an adjacent sign. Any sign for the next downstream interchange shall be 0.5' from the exit direction sign for a right exit and centered over the left lane if a left exit. Confirmation (pull through) signs shall be centered over all through-lanes that continue beyond the next exit. Exit Directional signs should be centered over the corresponding lane if structure spacing allows. Exit panels shall be right justified with the primary sign for right exits and left justified with the primary sign for left exits.
Maximum Length
The maximum length for a full span overhead assembly without a center support is 190'. The maximum length for a cantilever support is 52'.
Vertical Clearance
Overhead signs and proposed wind loads require a minimum vertical clearance of 17 feet and maximum vertical clearance of 18 feet from the high point. (Elaborate for sight distance for overhead signs close to signals and bridges)
Lateral Offsets
If guardrail is not included in the Roadway Plans but is required for an overhead sign, a written request for additional guardrail should be made to Roadway Design/PMU/ whoever is responsible. The overhead support and footing should not be located in a ditch bottom as this can obstruct drainage flow. Provide 3' of clearance from the ditch bottom whenever possible.
A chart is provided in the additional resources with guidance on support offsets and guardrail requirements. This chart is intended as a guide and does not replace the AASHTO Roadside Design Guide.
Supports in Barrier
When designing overhead supports to be mounted on median barrier, close coordination between Traffic Engineering and the roadway designers is required. The Structure Design Unit will prepare all details for median barrier supports and these details will be included in Roadway Plans for design and transition of the barrier.
Existing Overhead Structures
Existing structures may be utilized for proposed overhead signs. Proposed signs may be added to existing overhead sign structures if the signs proposed do not exceed the design wind load area for the structure. (Consider the age of the structure before determining if existing structure may be utilized.) Prior to modifying an existing overhead sign assembly to accommodate proposed signs that exceed the original wind load area, a structural analysis of the overhead sign structure shall be performed. All existing overhead sign assemblies determined to be structurally inadequate for the proposed modifications shall be replaced. An existing overhead sign assembly may be modified to accommodate proposed signs that do not exceed the original wind load area without performing the aforementioned structural analysis.
When the structural analysis determines that an existing overhead sign structure is structurally adequate to be retained or the proposed wind load area does not require a structural analysis, proposed signs may be designed to be installed on existing overhead structures. Any pertinent existing overhead signs shall also be replaced, unless otherwise directed by the NCDOT. Structure Drawings for existing overhead structures with new sign panels shall depict the following:
- The existing sign sizes;
- The new sign designs;
- The vertical clearance of all new signs;
- The new signs' positioning over travel lanes;
- The original wind load area;
- The original design information such as the TIP number and the structure letter.
If the Department is unable to provide existing overhead structure drawings or shop drawings of the existing overhead sign structure, a new overhead sign structure shall be designed and installed, and the existing structure shall be removed and disposed of.
The minimum vertical clearance beneath all existing overhead sign assemblies that are retained shall be 17 feet.
Wind load
The 50-year mean recurrence 3-second gust wind speed shall be used in the design of the structure. The wind load numbers are obtained from the Standard Specification for Structural Supports for Highway Signs, Luminaries, and Traffic Signals. No counties in North Carolina have design speed below 90 mph. (See sheet 29 or 30 for wind speed)
Proposed Wind Load Area
Overhead sign structures shall be designed for proposed and future signs. The designer shall determine maximum wind load areas and include the wind load areas on the overhead sign structure drawings. The wind load area for the sign structures shall be determined according to the following:
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Case A - Identified Future Signs: For sign structures that have an identified need for larger future signs shown in the plans, the future signs shall be designed and shown on the overhead sign structure drawings. Future sign messages, sizes, and positions shall be shown on the elevation drawings.
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Case B - General Future Wind Load Area: For overhead signs without identified future signs shown in the plans, the structure shall be designed for a larger wind load area to accommodate future signs that are not identified at the time of the structure design. General future wind load area sizes and positions shall be shown on the elevation drawings. The general future wind load area shall be computed as follows:
- Shape: The wind load area for each group of primary, secondary, and supplemental signs shall be rectangular.
- Width: The wind load area shall extend 2' outside the proposed primary sign width on each side of the sign. In cases where two wind load areas intersect, the taller area shall be used. For cantilever structures, the wind load area shall be flush with the edge of the primary sign at the cantilevered end, such that the wind load areas do not extend past the end of a cantilever sign structure. (Extends 2' or 4' on right side for cantilever)
- Height: The wind load area shall extend 2' below the bottom of each sign and 2' above the top of each sign, including secondary signs, supplemental signs, and the vertical space between signs according to Roadway Standard Drawing No. 904.20. The minimum vertical clearance shall be measured from the bottom of the lowest wind load area.
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Case C, Exceptions from Case B: The following are exempted from Case B, general future wind load areas:
- Arrow Per Lane Signs
- Interchange Sequence Signs
Archiving of Overhead Sign Structure Design Information
Purpose
Information related to the design of each overhead sign structure (full-span, cantilever, pedestal mount) should be archived to be retrievable at any future date. This information is often requested for the analysis of an existing overhead sign structure if there are proposed modifications that will differ from the original design layout and loadings. Modifications may include the replacement of existing signs with larger signs or repositioning of existing signs on the structure.
The Structure Design Unit maintains a database for providing NCDOT with the ability to capture bridge inspection and inventory data, which is known as the
Wearable Inspection and Grading Information Network System (WIGINS) system. Design information for overhead sign structures can be archived within this database. The five design elements that is of interest for archiving is:
- Sign Structure line drawings (Field Verified)
- Sign Designs
- Plan Sheets that include Overhead Assemblies
- Sign Structure Fabrication Drawings approved by Structures Design Unit
- Foundation Footing Details approved by Structures Design Unit
During the design and construction phases of a project, the NCDOT Signing Project Engineer shall ensure the archiving of design elements of interest they are responsible for electronically within the Project Folder on NCDOT Connect, in PDF file format. If there have been revisions made during the construction of the project that affected signing elements, the most up-to-date revised design and plan sheets shall be used. Structures Management Unit will be responsible for archiving of approved Fabrication Drawings and Foundation Footing Details.
If the Signing Project Engineer is not provided any of these items either through a paper or electronic copy, they should make the attempt to solicit copies of these items from the appropriate designer or other sources.
Collection and Recording of Sign Structure Design Data
Signing Project Engineers typically are involved in the design or review of projects that may contain proposed overhead sign structures. These project types include:
- State Transportation Improvement Projects (STIP)
- Design-Build Projects
- Division Design Raleigh Let Projects
- Division Purchase Orders Contracts
- Spot Safety Projects
- Turnpike Projects
Prior to or at the 90 Percent plan submittal, the Signing Project Engineer shall submit to the NCDOT Structures Unit; Structure, Inventory, and Appraisal group (SIA) a spreadsheet listing all proposed overhead sign locations (link for sample spreadsheet). The spreadsheet should be completed by the NCDOT Signing Project Engineer or included within a Firm's Scope of Work for completion and submitted to
SIA@ncdot.gov. The SIA group will assign each structure its Structure Inventory number using the information in the spreadsheet and return that information to the Signing Project Engineer. Note that the inventory number is based on the county the structure resides in and an assigned item number.
Example: Structure 250479 has a county code 25 for Cumberland County, and 0479 is the assigned inventory item number.
The Structure Inventory number shall be included on the final sealed structure line drawing. Geographic coordinate information of the proposed sign structure's location shall be included in the excel spreadsheet but do not have to listed on the structure line drawings. Coordinates should be taken near the center of the structure and reference its approximate Latitude and Longitude.
Once the Structure Inventory number has been provided, the Signing Project Engineer shall also add each overhead structure to the corresponding project's SharePoint Connect site under Project Structures. This generates a list of all structures (overhead signs, bridges, culverts, walls, etc.) that need to be accepted by the Department at the completion of the project.
Removal of Archived Overhead Sign Structure Design Information
Purpose
Design projects may include the complete removal and disposal of existing overhead sign structures. This may be related to the widening of an existing roadway that will compromise the foundations of existing overhead sign structures, or the proposed area of sign panels may exceed an existing structure's design criteria. Removal of these existing overhead sign structures should be relayed to the NCDOT Structures Unit; Structure, Inventory, and Appraisal group, as this group maintains the database for providing NCDOT with the ability to capture bridge inspection and inventory data, which is known as the
Wearable Inspection and Grading Information Network System (WIGINS) system.
Removal of Data within WIGINS
Removal of information from WIGINS for the disposed overhead sign structure will be performed by the Structure, Inventory, and Appraisal group. Prior to the 90 Percent plan submittal, the Signing Project Engineer shall submit a spreadsheet listing all existing overhead sign locations proposed for removal and their appropriate Structure Inventory number. The spreadsheet may be submitted to
SIA@ncdot.gov. The Structure, Inventory, and Appraisal group will determine the appropriate time to remove and archive the existing overhead sign structure data from WIGINS.
Verification of the Structure Inventory numbers can be obtained from the NCDOT
ArcGIS Map under the Structures layer; the Highway Division Traffic Engineers office; or from the Structure, Inventory, and Appraisal group.
Lighting
Lighting is no longer required on proposed NCDOT overhead sign structures. Any existing sign structure on a project that is to be retained, should have the existing lighting, walkways, and lighting electrical service removed, and all sign panels should be updated to NCDOT's current sheeting requirements.
Include reference and description for SP for Cutting of Sign Hangers
DMS
Coordinate with the ITS section of Traffic Mobility and Safety Division for the location of any existing, relocated or proposed DMS structures. All DMS structure drawings shall be shown in the ITS plans. Ensure minimum distance between DMS structures and guide signs are maintained.
Additional Resources
Proposed Overhead Sign Structure Line Drawing Examples
(Link)
Existing Ground-mounted Structure Drawing Example
Windload
Lateral Offset Guidance for Overhead Support Design
NCDOT Construction Manual - Determination of “S"-Dimensions (Link)
Applicable Standard Specifications
SECTION 906 - OVERHEAD
