|
|
June 13, 2005
In Reply Refer To: HSA-10/B-137
Mr. Bill Neusch, President
Gibraltar
320 Southland Road
Burnet, Texas 78611
Dear Mr. Neusch:
In your May 30 letter to Mr. Richard Powers of my staff, you requested the Federal Highway Administration (FHWA) acceptance of a high-tension, 3-strand cable barrier system. Copies of a May 26 report prepared by Karco Engineering and entitled “Crash Test Report for Gibraltar Tested to National Cooperative Highway Research Program (NCHRP) Report 350 Recommendations for test level 3-10 and 3-11 Cable Barrier System” and digital videos of the two tests were also submitted.
Your cable barrier system consists of three ¾-inch 3 X 7 prestretched, post-tensioned galvanized steel cables supported by steel C-posts 3.25 x 2.5 x 0.15-inches thick and 4-ft long, set in HSS 4 x 3 x 3/16 sockets. These sockets were 15-inches deep and placed in reinforced concrete footings 42-inches deep and 12-inches in diameter. Post spacing was 15 feet on centers. The posts were installed on alternate sides of the 3 cables that are held in place by a 7/16-inch diameter x 24-inch long galvanized steel hairpin and lock plate, with which the bottom, middle, and top cable heights are set and held in place at above-ground heights of 20 inches, 25 inches and 30 inches, respectively. These details for the line posts are shown in Enclosure 1. This enclosure also includes drawings of the terminal you developed for use with the Gibraltar Cable Barrier, which will be addressed in a separate acceptance letter in the immediate future. The barrier test installation was 200 feet long and each cable was tensioned to 4800 lbs. prior to the tests.
The NCHRP Report 350 tests 3-10 and 3-11 were both successfully conducted and the summary results of each are shown in Enclosure 2. Dynamic deflection was reported to be 8.5 feet. Based on the test results, the Gibraltar Cable Barrier may be considered an NCHRP Report 350 traffic barrier at test level 3 as a median barrier when the posts are set an alternate sides of the cables or as a roadside barrier when the cables are all on the traffic side of the C-posts.
You also asked about the acceptability of an alternative post embedment detail and the effect additional tension in the cables might have on the dynamic deflection of your barrier. Regarding post embedment details, a 30-inch deep reinforced concrete footing can be used when a mowing strip is used under the barrier. While longer posts embedded directly into the ground would almost certainly work, other factors such as post type and spacing, the use of soil plates, soil conditions, the distance between adjacent terminals or anchors, and the method used to connect the cables to the posts will affect the deflection distance and there is currently no way to predict that deflection with any degree of confidence. Similarly, increasing the cable tension will intuitively decrease deflection, but any such decrease cannot be readily quantified as it, too, is dependent on the factors listed above. To determine the design deflections for alternative post designs or post spacing, testing would need to be done. Design deflections for longitudinal barriers are only a reasonable approximation of what may be seen in the field. Because they are the observed results of a single test, actual deflections for any specific barrier can be much more or much less, depending on the size, speed, and impact angle of the vehicle that strikes it. In locations where deflection is a critical design element, a rigid concrete barrier would be a more logical choice than a flexible or semi-flexible barrier type.
Please note the following standard provisions that apply to the FHWA letters of acceptance:
Sincerely yours,
/original signed by/
John R. Baxter, P.E.
Director, Office of Safety Design
Office of Safety
