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FHWA Home / Safety / Roadway Departure / Safety Eligibility Letter SS-164

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U.S. Department of Transportation
Federal Highway Administration

1200 New Jersey Ave. S.E.
Washington, D.C. 20590

June 18, 2010

In Reply Refer To: HSSD/SS-164

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

Dear Mr. Kisiel:

This letter is in response to your request for the Federal Highway Administration (FHWA) acceptance of a roadside safety system for use on the National Highway System (NHS).

Name of system: Poz-Loc 10 inch square slip base system
Type of system: Breakaway Sign Support
Test Level: NCHRP Report 350 Test Level 3
Testing conducted by: N/A
Date of request: December 22, 2009
Request initially acknowledged: December 23, 2009
Date completed package received by FHWA: May 10, 2010

You requested that we find this system 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.”

Roadside safety devices should meet the guidelines contained in the NCHRP Report 350 or the American Association of State Highway and Transportation Officials’ (AASHTO) Manual for Assessing Safety Hardware. Requirements for breakaway supports are those in AASHTO’s Standard Specifications for Structural Supports for Highway Signs, Luminaries and Traffic Signals.

The system consists of an assembly that includes a ductile iron slip base for square tube sign supports. The supports are designed as a slip base casting that adapts thin-walled square tubing to a triangular shaped slip base. The adapter collar allows square tubing to be attached to a triangular slip base, mounted in a foundation. Details for the slip base casting are shown in Enclosure 1.

Assembly of the support system is shown in Enclosure 2. The collar of the slip base casting slides over the end of a thin-walled square tube support (perforated or nonperforated). This system also uses a triangular bolt keeper plate between 26 and 30 gauge separated from the base plate by circular cardboard washers (Enclosure 3). The triangular slip base plates for the support and the ground stub (shown in Enclosure 4) are clamped together using three bolts shown in Enclosure 5.

Crash Testing
No new crash testing was conducted in conjunction with this request. However, variations of this system have been previously crash tested and FHWA has found the test results acceptable according to NCHRP 350 Test Level 3. The findings of this letter recognize and utilize the results of previous testing and associated acceptance letters issued for earlier variations of this system.

Acceptance Letter Number SS-88 SS-130 SS-150 SS-164
Acceptance Letter Date August 15, 2000 January 5, 2006 September 5, 2008 (Current Request)
Size of Slip Base Casting (i.e., 8 or 10) 10 10 8 10
NWPipe Designation SB 6620 16819 31209 36104
Square tube size 2.5 Square 2.5 Square 2.5 Square 3 7 ga Square
Torque on bolts (foot/pound) 40-80 40-60 40-80 40-80
Number of posts within 7-foot span 2 2 3 (see below)

You have requested we accept the use of 3-inch × 3-inch × 7-gauge square sign post (perforated and nonperforated) with the Northwest Pipe Poz-Loc 10-inch square slip base system. This request is accepted. The basis for this acceptance is that FHWA previously has found the system acceptable with 2.5-inch x 2.5-inch x 10-gauge and 12-gauge square tubes in letter SS-130 dated on January 5, 2006. The system described in this submission provides larger section modulus and moment of inertia, which will increase rigidity of the support and not result in any degradation of crash testing performance from that previously demonstrated.

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

These additional requests listed above are also acceptable.

An earlier variation of this system with 2.5-inch x 2.5-inch 10-gauge and 12-gauge square tubes was found acceptable under FHWA acceptance letter SS-130 dated January 5, 2006, where used in a standard Texas Department of Transportation (TxDOT) concrete footing (12-inch diameter and 42 inches deep) placed in the NCHRP 350 Standards Soil. Also, the Northwest Pipe 8-inch Square system was found acceptable to be used with the same concrete footing in either strong or weak soil according to acceptance letter SS-150 dated on September 5, 2008. Furthermore, based on FHWA acceptance letter SS-65A the Poz-Loc slip base system with pipe rather than square tubes was found acceptable to use in weak soil with the 42-inch x 12-inch concrete footing. Based on these earlier tests the system detailed in this letter can be placed in the NCHRP 350 standard and weak soil with a standard TxDOT concrete footing, 12 inches × 42 inches.

The Northwest Pipe 8-inch square slip base system has been found acceptable to be used with one, two, or three supports in a 7-foot span, as noted in the FHWA acceptance letter SS-150 dated on September 5, 2008. Your request to use the 10-inch Square slip base system with one or two 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 reported in previous crash tests. However, the use of three supports in a 7-foot span needs additional justification from the test laboratory in the form of an engineering analysis or a crash test.

Your last request is acceptable on the grounds that a secure slip plate foundation will likely have no effect on the breakaway system performance when the 10 inch square slip base system is properly installed.

Therefore, the system described in the requests above and detailed in the enclosed drawings is acceptable for use on the NHS under the range of conditions tested, when such use is acceptable to a highway agency.

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


Sincerely yours,

/* Signature of David A. Nicol */

David A. Nicol, P.E.
Director, Office of Safety Design
Office of Safety

5 Enclosures

Page last modified on December 17, 2013.
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