Federal Highway Administration
400 Seventh St., S.W.
Washington, D.C. 20590
April 6, 2007
Refer to: HSSD/SS-147
Mr. Herbert G. Bray
1901 Highway 25 North
Buffalo, MN 55313
Dear Mr. Bray:
Thank you for your correspondence via an e-mail of March 21, 2007 requesting the Federal Highway Administration's (FHWA) acceptance of the U-channel post mounted to a perforated square steel tube that is inserted into a permanent base as a breakaway small sign support on the National Highway System (NHS). Accompanying your correspondence were drawings for the support and its foundation. You requested that we find the system acceptable based on observed field performance and by comparison to other breakaway supports under the provisions of National Cooperative Highway Research Program (NCHRP) Report 350 "Recommended Procedures for the Safety Performance Evaluation of Highway Features."
Requirements and warrants 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. Testing of breakaway supports is subject to the guidelines contained in the NCHRP Report 350.
The following is a brief description of your generic design and detailed drawings are enclosed for reference. A standard 2-1/2 pound per foot U-channel post is bolted to a 12 inch long 1-3/4 inch perforated square steel tube (PSST) riser using two 5/16 inch Grade 5 bolts with locking nuts. The bolts should be spaced 4 inches apart and connected such that the U-channel is approximately one inch above the ground line. The PSST riser must fit snug as it is inserted at least 4 inches into the base that is securely held in a concrete footing (minimum 8 inches deep). The PSST riser projects 8 inches above the concrete.
No crash testing of this support has been conducted. However, successful crash tests have been conducted on 1-3/4 inch PSST inserted into 2 inch PSST and securely bolted together near the ground line. The tested PSST support referenced in the FHWA acceptance letter SS-36 (design 92F039), is considered equivalent to your proposed design. Also, based on your observations of field performance, the 1-3/4 inches PSST riser consistently breaks at the ground line with no remaining stub exceeding 4 inches. It is apparent that, in the event of an impact, the
1-3/4 inches PSST riser breaks at the ground line and it would be anticipated that the vehicle velocity change caused by your proposed generic design will be equivalent to that of the successfully tested support.
Since perforated square steel supports mounted to U-channel posts have not been crash tested, it is not known if potential exists for the support or sign to penetrate the occupant compartment. In order to reduce that potential we recommend that the mounting height of the sign be, as a minimum, seven feet to the bottom of the sign. You also requested the installation of two posts within a seven foot path to be used to support a sign. We concur with this request as similar supports were also successfully tested with a dual post configuration.
The support described above and shown in the enclosed drawings for reference can be expected to meet the NCHRP Report 350 requirements and it may be considered acceptable for use as a test level 3 device when installed as shown in the attached drawing. Please note the following standard provisions, which apply to the FHWA letters of acceptance:
/original signed by /
John R. Baxter, P.E.
Director, Office of Safety Design
Office of Safety
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