Federal Highway Administration
400 Seventh St., S.W.
Washington, D.C. 20590
April 23, 2007
In Reply Refer To: HSSD/B-157
Mr. Stephen L. Brown, President
Trinity Highway Products, LLC
P.O. Box 568887
Dallas, Texas 75356-8887
Dear Mr. Brown:
Thank you for your company’s letter of March 20, 2007, requesting the Federal Highway Administration (FHWA) acceptance of modifications to the CASS-TL-4 cable barrier system and the CASS Cable Terminal. Accompanying your letter were drawings of the modified barrier and terminal. You requested that we find these devices acceptable for use on the National Highway System (NHS) under the provisions of the National Cooperative Highway Research Program (NCHRP) Report 350 “Recommended Procedures for the Safety Performance Evaluation of Highway Features.”
The FHWA guidance on crash testing of roadside safety hardware is contained in a memorandum dated July 25, 1997, titled “INFORMATION: Identifying Acceptable Highway Safety Features.”
In the FHWA acceptance letter B-141 dated November 17, 2005, the FHWA accepted the 3-cable CASS-TL-4 Cable Safety System to test level 4 (TL-4) criteria. Your current request is to modify that barrier and terminal design to incorporate a fourth cable at a height of 640 mm (25-3/16 inch), midway between the bottom and middle cables of the original TL-4 CASS.
You also requested a modification to the NCHRP Report 350 TL-3 compliant terminal to accommodate the fourth cable. The modification includes increasing the length of the CASS Cable Terminal through the addition of a fourth Cable Release Post as shown in the enclosed drawing. The fourth cable remains at the 640 mm height on the traffic side of the terminal from post #9 through post #4, at which point it begins to descend towards its terminus at Cable Release Post #1X.
You requested that the CASS-TL-4 Cable Safety System be acceptable with post spacings from 2.0m (6.5 feet) to 9.9m (32.5 feet) and with the same range of post embedment types (direct driven, set in driven tube, set in tube sleeve in concrete foundation.)
Full-scale automobile testing documented under earlier acceptance letters was conducted on 100m (334-foot) long installations of the CASS-TL-3 Cable Safety System with cables spaced at 2m (6.5 feet), 6m (20 feet) and 9.9m (32.5 feet). The cables were not pre-stretched, but were tensioned to 5,600 pounds force for the test. The dynamic deflection for the 100m (330-foot) long test installations were 1.6m (5.3 feet) for the 2m post spacing, 2.3m (7.7 feet) for the 6m post spacing and 3.4m (11.2 feet) for the 9.9m post spacing.
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. 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 100m (330-foot) test length.
In spite of the above caveat, the modification to the CASS TL-4 design described above using 4 cables may be used as either a roadside or median barrier on the 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. The modified CASS-TL-4 end terminal is likewise found to be acceptable.
Because the addition of the fourth cable and associated terminal hardware is not considered to be detrimental to the performance of the crash tested system, and is indeed likely to increase the capacity and improve the performance, the CASS-TL-4 devices described in the requests above and detailed in the enclosed drawings are acceptable for use on the NHS under the range of conditions tested, when proposed by a State.
Please note the following standard provisions that apply to the FHWA letters of acceptance:
John R. Baxter, P.E.
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