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FHWA Home / Safety / Roadway Departure / Safety Eligibility Letter

Safety Eligibility Letter B-137C (HSSD)

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U.S. Department of Transportation
Federal Highway Administration

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

February 8, 2008

In Reply Refer To: HSSD/B-137C

Mr. Bill Neusch
President
Gibraltar
320 Southland Road
Burnet, TX 78611

Dear Mr. Neusch:

Thank you for your letter of October 19, 2007, requesting a modification to the anchorage of your company’s 4-cable barrier. Your original test level 4 (TL-4) Gibraltar cable barrier was formally accepted for use on the National Highway System (NHS) in the September 9, 2005, Federal Highway Administration (FHWA) acceptance letter B-137A. The tested design, as described in that letter, consisted of cables placed 20 inches, 30 inches, and 39 inches above the ground line.

In our October 27, 2006, letter B-137A1 we accepted your 4-cable design having the lower three cables at the same heights as your TL-3 design while retaining the top cable at the tested TL-4 height of 39 inches. The addition of the fourth cable required a modified "hairpin" and lock plate to retain the cables at the proper heights. In order to "anchor" the added cable you tapered that 25-inch high cable down to the bottom (20-inch) cable between the first line post and the last terminal post and connected the two cables with a series of four cable clamps. Because of concerns raised over this splicing treatment, especially on downstream terminals, you have proposed to anchor each of the four cables separately.

The modified anchor is shown on the enclosed drawings for reference and features the fourth cable attached directly to the anchor post foundation plate instead of being cable clamped to the bottom cable. You also added a fourth J-bolt to each terminal post to accommodate the fourth cable. We concur in this modification and find it acceptable for use with your four-cable barrier.

The modified Gibraltar Cable Barrier as described above remains a National Cooperative Highway Research Program (NCHRP) Report 350 TL-4 median barrier when the posts are set on alternate sides of the cables or a TL-4 roadside barrier when the cables are all on the traffic side of the C-posts.

Although the barrier performed well under ideal test impact conditions when originally crash tested, the likelihood of passenger car underrides of any cable system may increase as the post spacing increases, particularly when the barrier is installed on non-level or slightly irregular terrain and the cables are not restrained from lifting at each post. Consequently, some transportation agencies have limited post spacing to approximately 6m (20 feet) for cable barriers. The dynamic deflection of the barrier is likely to increase when it is installed along the convex sides of horizontal curves, and when distances between anchorages exceed the 350-foot test length.

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

 

Sincerely yours,

Signature of David A. Nicol
David A. Nicol
Director, Office of Safety Design
Office of Safety

Page last modified on August 16, 2011.
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