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U.S. Department of Transportation

Federal Highway Administration

400 Seventh St., S.W.
Washington, D.C. 20590

September 5, 2008

In Reply Refer To:

Mr. Raymond Kisiel
Northwest Pipe Company
6307 Toledo Street
P.O. Box 2002
Houston, TX 77252-2002

Dear Mr. Kisiel:

Thank you for your letter of April 30, 2007 requesting the Federal Highway Administration’s (FHWA) acceptance of your company’s Poz-Loc 8 inch square slip base system to be used with solid or perforated square steel tubes as breakaway sign supports for use on the National Highway System (NHS). Accompanying your letter was a report from the Texas Transportation Institute (TTI) of the crash test conducted and drawings of the device. You requested that we find your company’s Poz-Loc 8 inch square slip base systems acceptable for use on the NHS under the provisions of the National Cooperative Highway Research Program (NCHRP) Report 350 “Recommended Procedures for the Safety Performance Evaluation of Highway Features.”

Testing of the device was in compliance 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' Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals.

A bogie pendulum test was recently conducted on your company’s device. The mass of the bogie pendulum was 820 kg. The test was conducted at the typical 18-inch bumper height for small cars. The test article consisted of an 8 inch square slip base system with a single 2-1/2 inch, 12 gauge perforated square steel tube tightened to 40 foot-pounds of torque in standard soil with a winged 7 gauge anchor in standard NCHRP Report 350 soil. A 48 x 48 inch by 5/8 inch thick plywood sign panel was attached to the post at a height of 7 feet from ground level.

The pendulum bogie, traveling at a speed of 21.5 mph (34.6 km/hr), impacted the sign support at 18 inches above ground level. Upon impact, the support slipped away from the base at the designated slip plane and came to rest 50 feet downstream of the impact point. The total crush to the pendulum nose (surrogate bumper) was 1.8 inches. The pendulum bogie test impact displayed a low potential for intrusion into the occupant compartment, measured a maximum acceleration of -0.9 g’s, and a velocity change of -0.2 m/s. The measured velocity and acceleration changes were within acceptable limits, and the remaining stub height was less than 4 inches. The results of testing met the FHWA requirements and, therefore, the device 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, when proposed by a State.

FHWA accepts the results of testing conducted on torque values for slip base systems performed by TTI in 2001 that revealed the torque values for the keeper bolts may be increased from 38 foot-lbs. to 80 foot-lbs. The use of these increased torque values for keeper bolts is acceptable with the Poz-Loc 8-inch and 10-inch square slip base systems and this modification applies to FHWA Acceptance letter SS-130 as well. FHWA also accepts the use of the bolt down stub (Part # SB 2708) for the Poz-Loc 8-inch and 10-inch square as well as the Poz-Loc 10-inch round post slip base systems.

Additionally, you submitted requests for this system to be used with:

The additional requests listed above are acceptable. The section modulus of the tested post was 0.62. Your first two additional requests are acceptable because sign supports with a larger section modulus will increase the rigidity of the support and therefore increase the efficiency of the slip base system. The concrete footing requested above was tested and accepted for a similar slip base when installed in standard or weak soil in an FHWA acceptance letter issued to the
NW Pipe Company on January 5, 2006 (coded SS-130) and is deemed acceptable for this device. Your request to use one, two, or three supports in a 7 foot span is acceptable based on extrapolation of the measured velocity and acceleration changes reported in the pendulum bogie crash test results. Your final two requests to use this system with compatible slip plates (stubs) are acceptable because a secure slip plate foundation will likely have no effect on the breakaway system performance when the 8 inch square slip base system is properly installed.

This letter also amends FHWA Acceptance letters SS-01, 1A, and 65A to include the fact that FHWA accepts 2-3/8 inch diameter, 17 gauge, round supports when no more than 2 are used in a 7-foot path. This acceptance includes the use of these supports in strong and weak soils using the socket and direct bury installations. Your 2-3/8 inch surface mount casting #27593 for use with 13 through 17 gauge round posts is also acceptable for use with previously acceptable round sign posts installed per manufacturer’s instruction.

Please note the following standard provisions that apply to FHWA letters of acceptance:

Sincerely yours,

Signature of George E. Rice, Jr.

George E. Rice, Jr.
Acting Director, Office of Safety Design
Office of Safety                                          


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