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August 15, 2000

Refer to: HSA-1/SS-88

Mr. H. Mark Jordan
Sales Manager
Poz-Loc Traffic Systems
Northwest Pipe Company
P.O. Box 2002
Houston, TX 77252-2002

Dear Mr. Jordan:

Thank you for your April 25 letter requesting acceptance of your company's nonproprietary square post slipbase system as a breakaway support for use on the National Highway System (NHS). Accompanying your letter was a report of crash testing done at Texas Transportation Institute dated April 2000, and videos of the crash tests.


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' (AASHTO) Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals.

The test articles were cast-iron slip bases for square tube sign supports. They are designed as a slip-on collar that adapts thin-walled square tubing (nonperforated) to the triangular shaped slip-base. The adapter collar allows square tubing to be attached to triangular slip-base foundations that were originally intended for use with round pipe supports. Details for the slip-on collar are shown in Enclosure 1. The casting is fabricated from ASTM A-536 Grade 65-45-12 ductile iron. Assembly of the slip base system is illustrated in Enclosure 2. The collar of the slip base casting slides over the end of a thin-walled square tube support. A 30 gage galvanized slip plate, shown in Enclosure 3, is inserted into the end of the square tube. The casting, slip plate, and thin walled square tube support are then connected with a 13 mm galvanized steel locking pin.

The triangular slip-base plated for the support and the ground stub (shown in Enclosure 4) are clamped together using three 13 mm diameter x 64 mm long ASTM A-325 hex bolts with a washer under the head and nut of each bolt. The bolts were torqued to 40 foot-pounds. A galvanized bolt keeper plate is placed between the slip planes. Enclosure 5 shows the installed dimensions for single and dual support installations. The single-post installation as tested is shown in Enclosure 6.

The ASTM A-500 Grade B steel posts were 63.5 mm square, with a wall thickness of 3.3 mm (10 gage). You also requested acceptance for dual support installations based upon a comparison of tests using two round pipe posts on a similar slip-base system.


A summary of the crash tests of the single square tube posts is presented in the following table:

Test #



NCHRP 350 Designation



Vehicle Mass

820 kg

820 kg

Vehicle Impact Speed

35.0 km/hr

99.7 km/hr

Soil Type



Impact Angle

0 degrees

0 degrees

Test Article Mass

41 kg

41 kg

Occupant Impact Speed


0.9 m/s

Vehicle Velocity Change

0.75 m/s

0.81 m/s

Windshield Damage


No contact

Vehicle Deformation

20 mm roof dent

160 mm dents to bumper

Stub Height

86 mm

86 mm


The tested supports met the change in velocity and stub height requirements of the AASHTO Standard Specifications and NCHRP Report 350. There were no excessive deformations of the passenger compartment and no damage to the windshield. Therefore, these slip-base supports are acceptable for use on the (NHS) within the range of conditions tested, when requested by a State. Because the supports were tested in standard soil and there were little or no motion of the stubs in the soil as a result of the test, this breakaway support will be acceptable when placed in a concrete foundation in standard soil.

The tested square steel tube posts have a larger cross-sectional area and section modulus than many square tubes used as sign supports. Smaller tubes could behave differently when struck, that is, they could begin to bend before the slip base is activated. Therefore, this slip base should be used with posts of comparable stiffness to the tested post to ensure that the energy of the impacting vehicle will be transferred directly into activating the slip base.

The test report also presents a comparison of the single square tube support versus the single and dual schedule 80 pipe support. The researchers conclude that based on the current testing covered by the table above and the previous testing conducted on the similar Poz-Loc slip base system for round pipe supports the new slip base collar for square tubing will perform satisfactorily in both single and dual post application. We concur in that finding and find these single or dual supports acceptable in standard soil.

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

        Our acceptance is limited to the crashworthiness characteristics of the devices and does not cover their structural features, nor conformity with the Manual on Uniform Traffic Control Devices.

        Any changes that may adversely influence the crashworthiness of the device will require a new acceptance letter.

        Should the FHWA discover that the qualification testing was flawed, that in-service performance reveals unacceptable safety problems, or that the device being marketed is significantly different from the version that was crash tested, it reserves the right to modify or revoke its acceptance.

        You will be expected to supply potential users with sufficient information on design and installation requirements to ensure proper performance.

        You will be expected to certify to potential users that the hardware furnished has essentially the same chemistry, mechanical properties, and geometry as that submitted for acceptance, and that they will meet the crashworthiness requirements of FHWA and NCHRP Report 350.

Sincerely yours,

Frederick G. Wright, Jr.
Program Manager, Safety

6 Enclosures

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