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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 May 9, 2007, requesting an update to your Federal Highway Administration (FHWA) acceptance letter of your company’s Poz Loc Slip base Casting 28339 as a breakaway feature for sign supports for use on the National Highway System (NHS). The update of this letter includes changing the reference from casting 2709 to casting 28339 and an updated drawing that is enclosed. Slight design modifications have been made which include adding a fourth set screw. These modifications will not affect the breakaway performance of this device.

Accompanying your original request letter that was received July 14, 2004, was a report from Texas Transportation Institute (TTI) and videos of the crash tests. You requested that we find this slip base 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.” You provided additional information on February 14 and February 22, 2007 in response to our requests.

Testing of the supports 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.

The test article consisted of a 73 mm (2.875 inch) outside diameter schedule 10 pipe support with a crossbar at the top, and a 1200 x 1200 mm x 16 mm (48 x 48 x 5/8 inch) plywood sign panel attached. The cast iron slip base device weighs 5 kg (11 pounds), the pipe was 4.6 kg (10 pounds, 4 oz.), and the sign panel was 14.8 kg (32 pounds, 8 oz.)

Pendulum bogie testing was conducted on your company’s devices. The mass of the test vehicle was 820 kg in all tests. The complete devices as tested are shown in the Enclosures.

Test # NCHRP 350 Speed Impact Height Occup. Speed Delta V
1 3-60 35.1 km/hr 18 inches None 0.47 m/s
2 3-60 35.1 km/hr 24 inches None 0.47 m/s

Occup. Speed: Occupant Impact Speed: Speed at which a theoretical front seat occupant will contact the windshield. In meters per second.
Delta V: Speed change of the test vehicle. In meters per second.

Test #1 was conducted at the typical 18-inch bumper height for small cars (and pendulum bogie noses.) Test #2 was conducted at 24 inches above the ground to investigate a concern of the Texas Department of Transportation regarding the bending of thinner wall tubes. They surmised that low speed impacts with taller vehicles, especially in the light truck class, could cause a bending failure prior to slip activation. The slip base performed as intended at both heights. Calculations based on the FHWA memorandum of November 12, 1993, performed by the TTI show the expected velocity change during a 100-km/hr impact to be 0.48 m/s.

The results of the low-speed bogie testing and the high speed extrapolations met the FHWA requirements and, 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, 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 10 inch round slip base systems and this modification also applies to FHWA Acceptance letter SS-65.

Please note the following standard provisions that apply to the 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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