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FHWA Home / Safety / Roadway Departure / Safety Eligibility Letter

Safety Eligibility Letter B-187

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U.S. Department of Transportation
Federal Highway Administration

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

February 13, 2009

In Reply Refer To: HSSD/B-187

Ronald K. Faller, Ph.D.
Research Assistant Professor
Midwest Roadside Safety Facility
University of Nebraska-Lincoln
527 Nebraska Hall
Lincoln, NE 68588-0529

Dear Dr. Faller:

This letter is in response to your request for the Federal Highway Administration (FHWA) acceptance of a roadside safety system for use on the National Highway System (NHS).

Name of system: Midwest Guardrail System (MGS) Asymmetrical W-beam to Thrie Beam Transition
Type of system:Longitudinal Barrier
Test Level:NCHRP Report 350 TL3
Testing conducted by:Midwest Roadside Safety Facility
Date of request:October 13, 2008
Date of completed package:November 26, 2007

You requested that we find this system acceptable for use on the NHS under the provisions of National Cooperative Highway Research Program (NCHRP) Report 350 “Recommended Procedures for the Safety Performance Evaluation of Highway Features.”

Requirements
Roadside safety systems should meet the guidelines contained in the NCHRP Report 350. FHWA Memorandum “Action: Identifying Acceptable Highway Safety Features” of July 25, 1997, provides further guidance on crash testing requirements of longitudinal barriers.

Description
The test level 3 (TL-3) Midwest Guardrail System (MGS) Asymmetrical W-beam to Thrie Beam Transition system is 26.67-m (87-ft 6-in.) in length and consists of five major structural components: (1) a thrie beam channel bridge railing system; (2) nested 2.67-mm (12-gauge) thick thrie beam guardrail; (3) standard 2.67-mm (12-gauge) thick thrie beam guardrail; (4) a 3.43-mm (10-gauge) thick MGS asymmetrical W-beam to thrie beam transition section; and (5) standard 2.67-mm (12-gauge) thick W-beam rail attached to a simulated anchorage device. W-beam’s top guardrail height was 787 mm (31 in.) with a 632-mm (24.875-in.) center mounting height.

The post size and embedment depth for this system included construction of three bridge posts and eighteen guardrail posts. Post numbers 1 and 2 were timber posts measuring 140 mm wide x 190 mm deep x 1,156 mm long (5.5 in. x 7.5 in. x 45.5 in.) and were placed in 1,829-mm (6-ft) long steel foundation tubes. The timber posts and foundation tubes were part of an anchorage system used to develop the required tensile capacity of a tangent guardrail terminal.

Post numbers 3 through 8 were galvanized ASTM A36 Steel W152 x 13.4 (W6 x 9) sections measuring 1,829 mm (6 ft) in length. Posts numbers 9 through 15 were galvanized ASTM A36 steel W152 x 17.9 (W6 x 12) sections measuring 2,286 mm (7.5 ft) in length.

Post numbers 16 through 18 were galvanized ASTM A36 steel W152 x 22.3 (W6 x 15) sections measuring 2,134 mm (7 ft) long.

Bridge post numbers 19 through 21 were galvanized ASTM A36 steel W152 x 29.8 (W6 x 20) sections measuring 752 mm (29.625 in.) long.

Post numbers 1 through 6 were spaced 1,905 mm (75 in.) on center, while post numbers
6 through 19 were spaced 953 mm (37.5 in.) on center, as shown in Figure 2. Post numbers
19 through 21 were spaced 1,905 mm (75 in.) on center.

The soil embedment depths for post numbers 3 through 8 was 1,016 mm (40 in.), 9 through 15 was 1,473 mm (58 in.), 16 is 1,391 mm (54.75 in.), 17 is 1,375 mm (54.125 in.), and 18 is
1,403 mm (55.25 in.).

The steel posts were placed in a compacted coarse, crushed limestone material that met Grading B of AASHTO M147-65 (1990) as found in NCHRP Report 350. For post numbers 3 through 11, 152-mm wide x 203-mm deep x 362-mm long (6-in. x 8-in. x 14.25-in.) wood spacer blockouts were used to block the rail away from the front face of the steel posts. For post number 12, a 152-mm wide x 305-mm deep x 483-mm long (6-in. x 12-in. x 19-in.) wood spacer blockout was used to block the rail away from the front face of the steel post. For post numbers 13 through 17, a 152-mm wide x 203-mm deep x 483-mm long (6-in. x 8-in. x 19-in.) wood spacer blockout was used to block the rail away from the front face of the steel post. For post number 18, a 152-mm wide x 203-mm deep x 381 mm long (6-in. x 8-in. x 15-in.) wood spacer blockout was used to block the rail away from the front face of the steel post. For post numbers 19 through 21, a galvanized ASTM A36 steel W152x22.3 (W6x15) sectionspacer blockout measuring 346 mm (13.625 in.) long was used to block the rail away from the front face of the steel bridge post.

Crash Testing
The analysis of the crash test results showed that the MGS W-beam to thrie beam transition element, used in conjunction with an approach guardrail transition system, adequately contained and redirected the vehicle with controlled lateral displacements of the barrier system. There were no detached elements nor fragments which showed potential for penetrating the occupant compartment nor presented undue hazard to other traffic. Deformations of, or intrusion into, the occupant compartment that could have caused serious injury did not occur. The test vehicle did not penetrate nor ride over the barrier system and remained upright during and after the collision. Vehicle roll, pitch, and yaw angular displacements were noted, but they were deemed acceptable because they did not adversely influence occupant risk safety criteria nor cause rollover. After collision, the vehicle’s trajectory revealed minimum intrusion into adjacent traffic lanes. Therefore, this crash test conducted on the MGS W-beam to thrie beam transition element, used in conjunction with an approach guardrail transition system, was determined to be acceptable according to the TL-3 safety performance criteria found in NCHRP Report 350.

Findings
Therefore, the MGS Asymmetrical W-beam to Thrie Beam Transition system described above and detailed in the enclosed drawings is acceptable for use on the NHS under the range of conditions tested, when such use is acceptable to a highway agency.

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

 

Sincerely yours,

Signature of David A. Nicol

David A. Nicol, P.E.
Director, Office of Safety Design
Office of Safety

Enclosures

Page last modified on June 24, 2011.
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