April 4, 2004
Refer to: HSA-10/CC-61B
Dean L. Sicking, PhD., P.E.
Director, Midwest Roadside Safety Facility
W. 328.1 Nebraska Hall
PO Box 880529
Lincoln, NE 68588-0529
Dear Dr. Sicking:
The Federal Highway Administration allowed conditional use of a two-bolt breakaway post with the Sequential Kinking Terminal (SKT) terminal in Ms. Carol Jacoby's October 30, 2002, acceptance letter CC-61A, but requested an additional test before this post was used with the flared FLEAT terminal. Your March 25, 2004, letter transmitted the results of this additional test as well as a copy of the Midwest Roadside Safety Facility's report entitled "Performance Evaluation of the FLEAT-MGS End Terminal - NCHRP Test No. 3-35."
For this test, the original FLEAT was modified to connect to your Midwest Guardrail System (MGS), a strong post W-beam barrier that has been successfully tested to NCHRP Report 350 guidelines, but has not yet been submitted for formal FHWA acceptance. To match the design features of the MGS barrier, the FLEAT posts were increased to an above-ground height (breakaway height) of 33.5 inches to attain a top rail height of 31 inches, posts 3 through 6 were fitted with the 6-in wide by 12-in deep x 14-in long routed wood offset blocks used throughout the MGS barrier, and the lead section of W-beam was a 16.67-ft long section of 12-gauge W-beam with five pairs of slots on the upstream end of the rail. Use of this initial length section of rail resulted in all rail splices falling midway between the support posts throughout the terminal and the barrier proper, a key characteristic of the MGS design. In addition, the splice bolts used in the breakaway line posts were reduced to a 9/16-inch diameter from the original 5/8-inch diameter bolts. Enclosure 1 shows the layout of the test installation.
Test 3-35 was conducted on a barrier layout with the FLEAT offset 4 feet from the line of the MGS barrier. The 2000P truck was contained and redirected upright. Enclosure 2 is the test summary page from the aforementioned report.
Although the test was run on a FLEAT that was modified as noted above, you wrote that the higher mounting height produces greater deflection in the terminal and is, therefore, more critical than the standard mounting height. I concur with that statement but would note that the use of deeper offset blocks and the positive effect of moving the W-beam splices from a post location to mid-span are two factors that enhance the performance of the tested design and which are not present when the FLEAT is used to terminate a standard W-beam installation. Nevertheless, based on your test results, I am herewith rescinding the October 30, 2002 conditional acceptance which limited the use of your two-bolt breakaway posts to the SKT terminal and find this design to be fully acceptable for use on the National Highway System with both the SKT and a standard FLEAT installation. The field performance of both terminals using the two-bolt post design should continue to be monitored to verify acceptable crash performance under field conditions.
(Original Signed By Hari Kalla)
John R. Baxter
Director, Safety Design
Office of Safety
Safety Home | FHWA Home | Feedback