Federal Highway Administration
400 Seventh St., S.W.
Washington, D.C. 20590
May 8, 2006
In Reply Refer To:HSA-10/B-141B
Mr. Stephen L. Brown
Trinity Highway Safety Products, Inc.
P.O. Box 568887
Dallas, Texas 75356-8887
Dear Mr. Brown:
In my November 17, 2005, acceptance letter B-141, I agreed that a 3-rope CASS cable barrier using weakened S4 x 7.7 structural steel posts set in concrete footings on 20-foot centers met the National Cooperative Highway Research Program (NCHRP) Report 350 evaluation criteria as a test level 3 (TL-3) traffic barrier. In your April 28, 2006 letter to Mr. Richard Powers, you requested that the same system with the posts set on 32.5-foot centers also be accepted as an NCHRP Report 350 TL-3 design. To support this request, the Texas Transportation Institute conducted a test on the modified system on March 10, 2006, and detailed the results in an April 2006 report, "NCHRP Report 350 Test 3-11 of the CASS-TL3 with Posts in Sockets Spaced at 9.9 M." For this test, the three 3/4-inch diameter cables were set at the same heights as in the earlier test (i.e., 21.0, 25.2, and 29.5 inches above the ground surface, measured to the center of each cable) and each line post was weakened by adding two 11/16-inch diameter holes through each flange at ground. The cables were not pre-stretched, but were tensioned to 5,600 pounds force for the test. The dynamic deflection for the 330-foot long test installation was 11.2 feet. By comparison, the deflection in your earlier test with a 20-foot post spacing was 7.7 feet. As can be seen in the attached test summary sheet, all Report 350 evaluation criteria for test 3-11 were satisfactorily met. I would be remiss, however, if I did not express my concern for the performance of this design when it is installed under typical field conditions. Although the barrier performed well under ideal test impact conditions with the pickup truck, 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. With fewer posts, the dynamic deflection of the barrier is also likely to increase when it is installed along the convex sides of horizontal curves. Consequently, some transportation agencies have limited post spacing to approximately 20 feet for cable barriers.
In spite of the above caveat, the CASS TL-3 design described above may be used as either a roadside or median barrier on the National Highway System (NHS) when such use is acceptable to the contracting agency. Although the cables used in the test were not pre-stretched, this acceptance is also valid if and when pre-stretched cables are used, assuming that the recommended post-tensioning is applied to the cables.
Please note the following standard provisions that apply to the FHWA letters of acceptance:
/original signed by/
John R. Baxter, P.E.
Director, Office of Safety Design
Office of Safety
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