Ground Mounted Support Designs for Type A and B Signs are based on sign size, wind load, support length, and other information. All sign supports for Type A and B ground mounted signs shall be designed using the NCDOT Sign Support Selection Tool available on the NCDOT Signing and Delineation website. The tool is used to select the proper steel beam or wood supports per the latest edition of the AASHTO's LRFD Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals.
Simple supports are one-piece steel beams set in concrete footings. All Type A and B ground mounted signs on simple (non-breakaway) supports shall be protected by guardrail, barrier or another form of approved positive protection. The minimum lateral distance between the face of the guardrail and the closest sign edge shall be six feet. The signing project engineer shall coordinate with the roadway engineer such that positive protection is provided at all proposed simple support locations.
Steel beam breakaway supports are beams considered to be breakaway per AASHTO and FHWA requirements. They are typically made up of three beam sections with hinges and slip bases and are set in a concrete footing. The top section is from the top of the sign panel to the hinge, which is at the bottom of the sign. The middle section is from the bottom of the sign to the slip base. The bottom section or stub is embedded in the footing.
When signs are installed on breakaway supports on freeways, the minimum lateral offset for Type A and B ground mounted signs on breakaway supports should be 30 feet. Signs can be moved as close as 18' without guardrail protection. The desired offset for Type A and B signs on ramps is 18' for shoulder sections and 12' for curb and gutter sections. If unable to obtain desire spacing, reference MUTCD 2A.19 (Link)
Wood supports may be used for temporary or permanent sign installations. Wood supports shall be protected by guardrail, barrier, or another form of approved positive protection if they are not considered to be break-away. Wood supports are considered to be breakaway supports according to the information below:
- 4" x 4" wood posts can be used at any location, without guardrail protection. Two 4" x 4" posts can be spaced closer than 7' apart.
- 4" x 6" wood posts must be used with guardrail or drilled to make them breakaway. Two posts can be spaced closer than 7' apart.
- 6" x 6" wood posts must be used with guardrail or drilled to make them breakaway. Two posts must be at least 7' apart when used as breakaway.
- 6" X 8" wood posts must be used with guardrail or drilled to make them breakaway. Two posts must be at least 7' apart when used as breakaway.
- 8" X 8" wood posts or larger are not considered breakaway even if drilled. These posts must be behind guardrail.
- LVL (Laminated Veneer Lumber) posts can be used as breakaway up to 8" X 15". See Roadway Standard Drawing 903.20.
Support Design Cross Sections
Typically, for TIP projects, support designs are preliminary, and verification is required prior to construction. The designs are estimated using the roadway design cross sections or measurements from a site visit to the proposed locations. If the sign location is within the roadway cross section limits, the roadway cross section should be used. If the sign location is outside the roadway cross section limits, data from field measurements should be used. The cross section shall have the sign number, sign sizes, project, county, alignment and station, sign offset, support offsets, and s-dimensions shown on the cross section. The PEF is also required to submit the individual worksheet for review. Both are submitted with each submittal starting with the 50% plans.
The s-dimension for ground mounted signs represents an increase (+) or decrease (-) in support length relative to the elevation of the edge of the outside travel lane, not the outside edge of the paved shoulder.
Support Design Worksheets
Information collected in the field or obtained from the cross sections shall be entered into the latest NCDOT Support Selection Tool. The support design tool includes specific instructions on the first tab of the Excel program.
The design must pass all tests including the following as specified by AASHTO (See AASHTO, Roadside Design Guide, Section 4.3.2 for details):
- Mounting Height Test – The hinge shall be at least 7 feet above the ground and should be no more than 10 feet above the edge of travel unless approved by the managing unit. Any deviation needs to be approved by NCDOT. (Max (-) negative s-dimension of -3)
- A single post, if 7 feet or more from another post, should have a weight per foot ratio of less than 45 lb/ft.
- If breakaway, the total weight below the hinge, but above the shear plate of the breakaway base, shall not exceed 600 pounds.
- For two posts are spaced less than 7 feet apart, each post should have a weight per foot less than 18 lb/ft.
- Support Spacing Test – When 3 supports are needed, the distance between them cannot be less than 3.5 ft.
The support safety test does not apply to simple supports. (Simple supports are protected by guardrail.)
Steel and Wood Support Chart Sheet(s)
The support selection tool tabulates the selected supports, footing designs and quantities on two summary tabs at the end of the workbook. One worksheet is provided for signs erected on wood supports, and one worksheet is provided for signs erected on steel supports. If applicable, these two tables shall be copied and pasted directly into the signing plans. As field verifications are performed, these tables shall be updated with the final sign sizes, field verified sign locations, field verified “S" dimensions and field verification dates.
Each support chart sheet shows the version of the support selection tool used and the release date. For each sign location, the support chart sheet shall show the sign number, sign type, sign size, roadway station, number of supports, beam section size, type of sign support, offset from the edge of travel lane, sign attachment method (if applicable), support spacing, sign support S dimensions, lengths, weight, and the amount of concrete needed for the support footings.
The support chart sheet shall include the above information for all proposed signs on proposed supports (100 and 200 series signs), proposed signs on existing supports (700 series signs), and existing signs being relocated to proposed supports (900 series signs).
The support chart sheet shall contain the typical elevation detail for ground mounted signs and the detail for exit gore sign offsets.
Post Letting Updates (“s"-dimension verifications)
The “s"-dimensions and support lengths detailed in the plans are used for estimating purposes and are not to be used for fabricating supports. Prior to sign support fabrication, the s-dimensions shall be verified by field measurements and shall be submitted to the managing unit for review. With the support verification data, the support selection is finalized and a construction revision is completed by either the PEF, if under contract, or the managing unit. Without field verification, signs may be required to be field drilled and any sign damage sustained will be contractor's responsibility.
Prior to establishing the final s-dimensions, stake the horizontal location of the proposed signs for the Engineer's review. Perform a thorough investigation of the proposed sign locations (confirm R/W) or revised locations established during the Engineer's review. Identify any obstructions, either existing or proposed, which may interfere with the proposed sign installation. Such items may include, but are not limited to, existing or proposed drainage systems, underground and/or aboveground utilities, and drainage ditches. If adjustments in the sign locations are warranted to avoid obstacles, advise the Engineer in writing. Include a detailed sketch of the proposed sign location, the obstruction and the new location recommended to avoid the obstruction. Do not revise any sign locations without the written direction of the Engineer.
NCDOT Support Program (Link)
Ground Mounted Sign Cross Section Example
Ground Mounted Sign Support Selection Worksheet Example
Link to Construction Manual (Link)
Applicable Standard Specifications
SECTION 902 FOUNDATIONS FOR GROUND MOUNTED SIGNS
SECTION 903 GROUND MOUNTED SIGN SUPPORTS
901.70 SIGN SUPPORT SPACING
903.10 GROUND MOUNTED SIGN SUPPORTS
903.20 WOOD SIGN POSTS
904.20 SECONDARY SIGN MOUTING
904.30 SUPPLEMENTAL SIGN MOUNTING