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Safety Eligibility Letter CC-68A

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U.S. Department of Transportation
Federal Highway Administration

1200 New Jersey Ave. S.E.
Washington, D.C. 20590

August 19, 2011

In Reply Refer To: HSST/CC-68A

Mr. Dean L. Sicking, Ph.D., P.E. Director,
Midwest Roadside Safety Facility University
of Nebraska - Lincoln P.O. Box 880601
Lincoln, NE 68588-0601

This letter is in response to Messrs. Bielenburg's and Faller's request for the Federal Highway Administration (FHWA) acceptance of an alternative post design for the Thrie beam bullnose median barrier (bullnose attenuator) for use on the National Highway System (NHS).

Name of system: Thrie beam bullnose guardrail system
Type of system: Non-Gating Crash Cushion/Impact Attenuator
Test Level: NCHRP Report 350 Test Level 3
Testing conducted by: Midwest Roadside Safety Facility.
Date of request: December 21, 2010
Request Initially acknowledged: December 22, 2010
Task Force 13 designator: SET03b

You requested that we find a steel post design to be an acceptable alternative to the previously-accepted wood post Thrie beam bullnose attenuator for use on the NHS under the provisions of the National Cooperative Highway Research Program (NCHRP) Report 350.

Roadside safety devices should meet the guidelines contained in NCHRP Report 350 if tested prior to January 1, 2011, and the guidelines in American Association of State Highway Transportation Official's (AASHTO) Manual for Assessing Safety Hardware if tested after that date. The FHWA memorandum "ACTION: Identifying Acceptable Highway Safety Features" of July 24, 1997, provides further guidance on crash testing requirements of longitudinal barriers and crash cushions.

The following device was found acceptable, with details provided below:

The steel post design of the bullnose attenuator is essentially identical to the original wood post design that was accepted for use on the NHS in FHWA acceptance letter CC-68, dated November 8, 2000. The design consists of five slotted Thrie beam rail elements: the nose piece with a 1580-mm (62-3/16-inch) radius, two sections (one on each end of the nose section) with 10400-mm (409-7/16-inch) radii, and one straight rail section on each end of the second sections. Two 5/8-inch diameter steel cables are set behind the top two corrugations in the nose piece. Posts 1 and 2 are wood BCT-type posts and effectively anchor the bullnose in side impacts. In the modified design, the original CRT posts (numbered 3 through 8) have been replaced with the new UBSP. The lower section of the UBSP consists of a 6 inch x 8 inch x 3/16 inch ASTM A500 Grade B steel foundation tube with the lower base plate. The upper portion of the UBSP is a W6 x 8.5 post with the upper base plate. The plates are connected with 7/16-inch diameter ASTM A325 threaded hex bolts. Drawings showing the overall layout of the Bullnose design and the UBSP posts are shown in Enclosure 1.

Crash Testing
Since the UBSP design of the bullnose attenuator was essentially unchanged from the previously accepted CRT post version, you needed only to show that the new post system would not adversely affect the crash performance of the unit. You concluded that NCHRP Report 350 tests 3-30,3-31, and 3-38 were those most likely to be effected by the post substitution and that the remaining tests for a redirective crash cushion need not be rerun. After discussing the test matrix with Mr. Bielenberg, FHWA agreed that the remaining tests would be redundant and accepted the three tests described below as adequate and sufficient to verify crashworthiness of the UBSP design.

Test 3-30 required a 100 km/h impact by a 1800 lb (820-kg) passenger car impacting at 62 mph (100 km/h) directly into the nose of the barrier with a 1/4-point offset. In test no. USPBN-3, the car was captured by the bullnose with little roll or pitch. As shown in Enclosure 2, maximum occupant impact velocity was 33.6 feet/s (10.2 m/s) and the ridedown acceleration was 7.7 g's. The vehicle was brought to a stop approximately 21 ft from the first impact point.

Test 3-31 was run as test no. USPBN-4 and consisted of a 4429-lb (2009-kg) pickup truck impacting the Bullnose Attenuator head-on at 64.5 mph (103.7 km/h). The truck was captured by the barrier and brought to a stop in approximately 51 feet. Occupant Impact Velocity (OIV) was reported to be 21.8 feet/s (6.6 m/s) and the maximum acceleration was under 8 g's. Enclosure 3 is the summary sheet for this test.

Test 3-38 also required a 100 km/h 2000-kg pickup truck test, but into the side of the Bullnose Attenuator at its critical impact point (CIP).  The Midwest Roadside Safety Facility test number USPBN-2 reported a 2029-kg (4472-lb) truck struck the barrier at 101.3 km/h (62.9 km/h) and 21.7 degrees. The OIV was 8.6 m/sec (longitudinal) and the Ridedown Acceleration was approximately 15 g's. Vehicular roll, pitch and yaw were recorded as 12.6, 5.9, and 20.7 degrees, respectively. The truck came to rest within the bullnose envelope, 28 feet (8.7 m) downstream from the initial impact point. Enclosure 4 is the summary sheet for this test.

The Thrie beam Bullnose Attenuator with UBSP as described above is acceptable for use on the NHS as a non-gating crash cushion primarily used to shield rigid objects located in the medians of divided highways.
The conditions listed in the original acceptance letter remain in effect and are repeated here for ready reference:

Please note the following standard provisions that generally apply to all FHWA letters of acceptance:


Sincerely yours,

/* Signature of Michael S. Griffith */
Michael S. Griffith
Director, Office of Safety Technologies
Office of Safety


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