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U.S. Department of Transportation

Federal Highway Administration

400 Seventh St., S.W.
Washington, D.C. 20590

June 2, 2006

In Reply Refer To: HSA-10/B-148

Mr. Steve L. Brown
Trinity Highway Safety Products Division
P.O. Box 568887
Dallas, Texas 75356-8887

Dear Mr. Brown:

In his April 28, 2006 letter to Mr. Richard Powers, Mr. Brian Smith requested the Federal Highway Administration (FHWA) acceptance of a new strong post Thrie-beam guardrail design called the T-39. With the letter, he also sent copies of a Texas Transportation Institute (TTI) report dated April 2006, entitled "Testing and Evaluation of the T-39 Thrie-Beam Longitudinal Barrier", and digital videos of the three tests that were run.

The T-39 Thrie-beam guardrail uses standard 12-gauge Thrie-beam panels mounted on modified W6 x 8.5 steel posts with a top-of-rail height of 39 inches. The modified posts, called Steel Yielding Line Posts (SYLP) posts are 6-feet long and set in the ground to a depth of 40 inches on standard 6-ft, 3-inch centers. Each post has four 13/16-inch diameter holes in the flanges at the ground line. The rail is attached to each post without an offset block using a 5/8-inch diameter x 1-3/4-inch long special bolt with a slotted countersunk head. A 6-inch long section of W-beam called a flange protector (backup plate) is used at each post. All splices in the Thrie-beam rail elements fall midspan, between adjacent posts. Design details for the SYLP post and its unique connection bolt are shown on Enclosure 1. Note that the top of the post is 32 inches above the ground and that the top of the Thrie-beam is 7 inches higher. The T-39 railing can be installed as a roadside barrier with rail only on the traffic side of the posts (as shown in Enclosure 1) or as a symmetrical median barrier with Thrie-beam mounted on both sides of the support posts.

Test 3-10 was conducted using the current National Cooperative Highway Research Program Report 350 small car (820C) impacting the T-39 median barrier at a 20-degree angle. The median design was used for this test because the stiffer of the two designs was deemed more likely to cause the small car to impact and interact with the posts. Test 3-11 was conducted with a quad-cab pickup truck having a nominal weight of 2270 kg (currently proposed as the new test vehicle under the Report 350 update) into the roadside version of the T-39, and test 4-11 was run using the standard single-unit truck, also into the roadside design. For the latter two tests, it was believed that the roadside design would be most likely to fail with the larger vehicles due to increased dynamic deflection. My staff agreed with these assumptions. Summary sheets for each of these tests are shown in Enclosure 2.

Based on these test results, both the median and roadside versions of the T-39 guardrail, as described above, may be considered crashworthy under the existing Report 350 guidelines as test level 4 (TL-4) barriers and may be used on the National Highway System when such use is acceptable to the contracting authority. When new testing guidelines are formally adopted, you may choose to rerun the small car test at the 25-degree impact angle proposed by the researchers for test 3-10 since your small car test was deemed marginal (due to passenger compartment deformation) under the current 20-degree impact angle. Because the rail height exceeds that of existing metal beam guardrails, you will need to develop transition designs if the T-39 rail is connected to a bridge parapet or to a crashworthy W-beam terminal. Potential users also need to be aware that under TL-4 impact conditions, the truck box exhibited an estimated roll angle of over 30 degrees (the cab roll angle was reported as 12 degrees), and extended more than 3 feet beyond the deflected rail. This area must be kept free of rigid vertical objects such as overhead/cantilever sign supports and bridge piers.

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

2 Enclosures

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