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Federal Highway Administration |
400 Seventh St., S.W. Washington, D.C. 20590 |
November 17, 2005
In Reply Refer To:HSA-10/B-141
Mr. Stephen L. Brown
President
Trinity Highway Safety Products, Inc.
P.O. Box 568887
Dallas, Texas 75356-8887
Dear Mr. Brown:
In two separate letters to Mr. Richard Powers, both dated November 15, Mr. Brian Smith requested the Federal Highway Administration (FHWA) acceptance of modified designs of your CASS cable barrier, one at test level 3 (TL-3) and one at TL-4. Both designs use S4 x 7.7 structural steel posts to support the high-tensioned cables in lieu of the original CASS C-channel posts. For the TL-3 design, the 3/4-inch diameter cables were set at heights of 21.0, 25.2, and 29.5 inches above the ground surface, measured to the center of each cable. The slot design was also modified and the 47.25-inch long posts were weakened by the addition of two 11/16-inch diameter holes through each flange at ground line. These details are shown in Enclosure 1.
To verify the crashworthiness of the modified CASS design, two initial tests were conducted on a prototype TL-3 design at the Southwest Research Institute in San Antonio, details of which were contained in that agency's September 1, final report entitled "Full-Scale Crash Evaluation of a Tensioned Safety System (TESS) Cable Barrier System - NCHRP Report 350 Tests 3-10 and 3-11". For these tests, the weakening holes in each line post were 9/16-inch diameter and the posts were set on 16'-8" centers. The wire ropes were pretensioned to 5600 pounds. As seen on the test summary sheets (Enclosure 2a and 2b), both tests met the National Cooperative Highway Research Program (NCHRP) Report 350 evaluation criteria, but there was more damage to the small car and more post-crash instability with the pickup truck than normally seen in cable barrier tests. You subsequently increased the hole size to 11/16 inch and increased the post spacing to 20 feet. This final design was tested at the Texas Transportation Institute (TTI) with the pickup truck as described in that agency's November 2005 report, "NCHRP Report 350 Test 3-11 of the Trinity CASS-TL3 Spaced at 6.1 m in Concrete Footings and Sockets with Non-Pre-Stretched Cable". These test results, shown in Enclosure 3, were a significant improvement over the first 3-11 test, and I agree that test 3-10 need not be run on the final "softer" design. The design deflection for the 100-m long test installation was 7.7 feet. All line posts were set into 5-inch x 3-inch x 11 gauge sockets encased in 12-inch diameter x 30-inch deep reinforced concrete foundations.
The TTI also tested the modified CASS with a single-unit truck under TL-4 impact conditions and reported the successful results in its November 2005 report entitled, "NCHRP Report 350 Test 4-12 of the Trinity CASS-TL4 Spaced at 6.1 m in Concrete Footings and Sockets with Non-Pre-Stretched Cable". Enclosure 4 is the summary sheet for this test. The support posts were lengthened to approximately 56 inches and the cable heights were set at 21.0, 29.5, and 38.0 inches. Thus, the bottom cable height remained the same as in the TL-3 design, and the middle cable was the same height as the upper cable in the TL-3 design. Since only the top cable was raised, it is reasonable to assume that the TL-4 design would perform acceptably in tests 3-10 and 3-11, both of which can be waived. The CASS TL-4 uses the same TL-3 anchor developed for the original CASS barrier (FHWA acceptance letter CC-76), and is identical through post 7. Two longer posts are then added to transition the cables gradually to the higher CASS TL-4 cable heights. This design is shown in Enclosure 5. Finally, since the design deflection of longitudinal barrier systems is based on the pickup truck test, which was waived for the CASS TL-4, its dynamic deflection can be assumed to be the same as the CASS TL-3, i.e., 7.7 feet. As with all roadside safety features, the field performance of both designs should be closely monitored to verify acceptable performance under varied field conditions.
The CASS TL-3 and CASS TL-4 designs 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 barrier. You also asked about the acceptability of an alternative post embedment detail, specifically direct-driven posts or posts set in driven sockets. While longer posts embedded directly into the ground or posts in sockets without concrete foundations would probably perform satisfactorily, there is currently no way to predict barrier deflection with different embedment designs. To determine the design deflections for alternative post designs, testing would need to be done.
Please note the following standard provisions that apply to the FHWA letters of acceptance:
Sincerely yours,
(original signed by George Ed. Rice, Jr. for)
John R. Baxter, P.E.
Director, Office of Safety Design
Office of Safety
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