U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Ave. S.E.
Washington, D.C. 20590
June 10, 2011
In Reply Refer To:
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
Dear Mr. Sicking:
This letter is in response to Messrs. Robert Bielenberg and Ronald Faller request for the Federal Highway Administration (FHWA) acceptance of a wood post alternative to the steel post Midwest Guardrail System (MGS) installed adjacent to a 2H:1V fill slope.
|Name of system:||Midwest Guardrail System placed adjacent to a 2H:1V fill slope|
|Type of system:||Wood Post and W-beam roadside barrier|
|Test Level:||NCHRP Report 350 Test Level 3|
|Testing conducted by:||Midwest Roadside Safety Facility|
|Date of request:||December 21, 2010|
|Date initially acknowledged:||December 23, 2010|
|Task Force 13 Designator:||SGR38|
You requested that we find a wood post MGS barrier design to be an acceptable alternative to the recently-accepted steel post design when installed at the hinge point of a 2H:1V fill slope. You further requested that the wood post option also be accepted for use on the National Highway System (NHS) as a TL-3 barrier 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 when tested prior to January 1, 2011, and the guidelines in American Association of State Highway Transportation Official's Manual for Assessing Safety Hardware 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 barrier design was found acceptable, with details provided below:
The MGS system has been successfully crash-tested when installed at the hinge point of a 2H:1V fill slope with 9 feet (2.74 meters) long W6 × 9 (W152 × 13.4) steel posts at a standard spacing of 75 inches (1902 millimeters) on centers. This design was accepted by the FHWA for use on the NHS via acceptance letter B-211. Subsequently, the Midwest Roadside Safety Facility was requested to develop a 6 inches × 8 inches (152 millimeters × 203 millimeters) wood post alternative to the steel post design. This alternative design used 7.5 feet (2.29 meters) long wood posts on 75 inches (1902 millimeters) centers to support the W-beam rail element at the MGS height of 31 inches (78.7 millimeters).
Surrogate testing of single posts was performed using a rigid frame bogie weighing approximately 1800 pounds (820 kilograms) and accelerated to 15 mph (24 km/h). Since the objective of the test program was to determine a wood post size that corresponded closely to the 9 feet (2.74 meters) steel posts previously crash-tested, two of the 7 bogie tests used the steel posts set at the 2H:1V slope breakpoint. The remaining five bogie test used 6 inches × 8 inches (152 millimeters × 203 millimeters) standard wood posts with lengths of 7.5 feet (2.29 meters) and 8 feet (2.44 meters) and with embedment depths from 58 to 64 inches (1,473 to 1,626 millimeters). All posts were installed with the center of each post at the slope breakpoint, one foot (30.5 millimeters) away from the road side of the 3 feet (914 millimeters) diameter drilled shafts that were then backfilled with compacted 8 inches (203 millimeters) lifts.
A summary of all bogie testing results is shown in the enclosure to this letter. Review of the data from all seven impact tests found that the 7.5 feet (2.29 meters) long, 6 inches × 8 inches (152 millimeters × 203 millimeters) SYP wood posts provided the best alternative to the 9 feet (2.74 meters) long, W6 × 9 (W152 × 13.4) steel posts. Three tests of 8 feet (2.44 meters) long, 6 inches × 8 inches (152 millimeters × 203 millimeters) SYP wood posts resulted in post fracture due to the post-soil forces exceeding the capacity of the wood post. The wood fracture prevented effective rotation of the post in the soil and resulted in insufficient energy absorption during the impact. Thus, the 8 feet (2.44 meters) long, wood posts were deemed unsuitable for the MGS when installed adjacent to a 2H:1V fill slope. In contrast, the 7.5 feet (2.29 meters) long, 6 inches × 8 inches (152 millimeters × 203 millimeters) SYP wood posts correlated reasonably well with the data obtained from the 9 feet (2.74 meters) long, W6 × 9 (W152 × 13.4) steel post tests. The 7.5 feet (2.29 meters) long posts did not fracture during impact and rotated through the soil. The average peak force for the two 7.5 feet (2.29 meters) long, wood post tests was only 5.7 percent greater than the average peak force of the two W6 × 9 (W152 × 13.4) steel post tests. Similarly, the average total energy of the two 7.5 feet (2.29 meters) long, wood post tests was only 6.5 percent greater than the average total energy of the two W6 × 9 (W152 × 13.4) steel post tests. The average force levels for the 7.5 feet (2.29 meters) long, wood post tests were 23 percent greater through 15 inches (381 millimeters) of deflection than the values obtained from the steel post testing. Thus, the two 7.5 feet (2.29 meters) long, wood posts compared very well with the steel posts in terms of peak force and total energy absorbed, while being slightly higher in terms of average force. It is not believed that the reasonably small differences observed between the 7.5 feet (2.29 meters) long, wood post and the 9 feet (2.74 meters) long, steel post would have any adverse effects on the performance of the MGS system.
In subsequent e-mail correspondence, Mr. Bielenberg noted that a standard W-beam barrier was successfully tested at the breakpoint of a 2H:1V slope using 7 feet (2.13 meters) wood posts at 1/2 the standard post spacing. We believe that the use of slightly longer posts, combined with the additional height of the W-beam and the use of the deeper offset blocks in the MGS system, would result in crash performance similar to that seen in the successfully tested steel post design.
Based on the above considerations, the FHWA agrees that the 7.5 feet (2.29 meters) long, 6 inches × 8 inches (152-millimeter × 203-millimeter) SYP wood post provides a suitable alternative to the 9 feet (2.74 meters) long, W6 × 9 (W152 × 13.4) steel post.
Please note the following standard provisions that generally apply to all the FHWA letters of acceptance:
/* Signature of Michael S. Griffith */
Michael S. Griffith