Mr. Lawrence E. Leahy
President, Xcessories Squared
Auburn, IL 62615
Dear Mr. Leahy:
Thank you for your July 19, 2001, letter requesting Federal Highway Administration (FHWA) acceptance of a modification to our previous action on your company's SB8 slip base. We had previously found your "SB8" breakaway slip bases acceptable for single post supports on September 18, 1996 (Geometric and Roadside Design Acceptance Letter SS-68) and for dual and triple post supports on December 20, 1996 (SS-68A). These designs used triangular plates measuring 8 inches (203 mm) on a side.
Your present request is that the SB8 base, with a revised concrete foundation, be found acceptable for use on the National Highway System (NHS) under the provisions of National Cooperative Highway Research Program (NCHRP) Report 350 "Recommended Procedures for the Safety Performance Evaluation of Highway Features." You submitted calculations showing the increased depth necessary to compensate for the smaller diameter concrete foundation in weak soil.
Testing of breakaway supports is to conform with the guidelines contained in the NCHRP Report 350, Recommended Procedures for the Safety Performance Evaluation of Highway Features. Requirements for breakaway supports are those in the American Association of State Highway and Transportation Officials' (AASHTO) Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals.
In light of the engineering analysis of the strength of soil necessary to adequately support the sign installation the FHWA concurs in your request to reduce the concrete footing diameter to 18 inches and increase the depth to 42 inches when used in weak soil (the worst case soil condition.) Therefore the devices described above and shown in the enclosed drawings for reference are acceptable for use as Test Level 3 devices on the NHS under the range of conditions tested, in either strong or weak soil, when proposed by a State.
Please note the following standard provisions which apply to FHWA letters of acceptance:
Michael L. Halladay .
Acting Program Manager, Safety