collapse Title : 01 INTRODUCTION ‎(5)
collapse Title : 02 PRINCIPALS ‎(7)
collapse Title : 03 PROJECT DELIVERY REQUIREMENTS ‎(9)
collapse Title : 04 SIGNING PLANS ‎(20)
collapse Title : 05 PAVEMENT MARKING AND DELINEATION PLANS ‎(21)
  • 04 SIGNING PLANS

  • 4.7 Overhead Sign Structure Drawings

  • 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: 

    • 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.
    • 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.
    • 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

    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 Structure (Full Span Structure Line Drawing) Example (Link)
    Proposed Overhead Structure (Cantilever Structure Line Drawing ) Example (Link)​

    Existing Overhead Structure Drawing Example
    Wind Speeds and Pressures For Use in Overhead Support Design
    Lateral Offset Guidance for Overhead Support Design
    NCDOT Construction Manual - Determination of “S"-Dimensions  (Link)

    Applicable Standard Specifications
    SECTION 906 - OVERHEAD

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