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FHWA Home / Safety / Roadway Departure / Safety Eligibility Letter

Safety Eligibility Letter B-236

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U.S. Department of Transportation
Federal Highway Administration

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

July 10, 2014 HAND CARRIED BY WL

In Reply Refer To:
HSST/B-236 Revised

Mr. Scott Rosenbaugh
Midwest Roadside Safety Facility
130 Whittier Research Center
P. O. Box 830853
Lincoln, NE 68583-0853

Dear Mr. Rosenbaugh:

This letter is in response to your request for revisions to the existing eligibility letter B-236 dated May 30, 2012, and for the Federal Highway Administration (FHWA) to review a roadside safety system for eligibility for reimbursement under the Federal-aid highway program.

Name of system: Wood-Post 31-inch (787-millimeter) Midwest Guardrail System (MGS) to Thrie Beam Approach Guardrail Transition
Type of system: W-Beam Guardrail Transition
Test Level: AASHTO Manual for Assessing Safety Hardware, TL-3
Testing conducted by: Midwest Roadside Safety Facility
Date of Original request: January 19, 2012
DateRevision request: April 10, 2014
Task Force 13 Designator: STG03b

Based on a review of submitted revisions to existing eligibility letter dated May 30, 2012 and crash test results submitted by the manufacturer certifying the device described herein meets the crash test and evaluation criteria of the American Association of State Highway and Transportation Officials (AASHTO) Manual for Assessing Safety Hardware (MASH), the device is eligible for reimbursement under the Federal-aid highway program. Eligibility for reimbursement under the Federal-aid highway program does not establish approval or endorsement by FHWA for any particular purpose or use.

The FHWA, the Department of Transportation, and the United States Government do not endorse products or services and the issuance of a reimbursement eligibility letter is not an endorsement of any product or service

Decision:
The following device is eligible, with details provided below:

Requirements
To be found eligible for Federal-aid funding, roadside safety devices should meet the crash test and evaluation criteria contained in the American Association of State Highway and Transportation Officials’ Manual for Assessing Safety Hardware (MASH).

Description
For many years the roadside safety community has considered 6-in. × 8-in. (152 millimeters × 203 millimeters) wood posts and W6×9 (W152×13.4) steel posts as interchangeable options for 6 feet (1.8 meters) long guardrail posts. However, the posts in these older systems were embedded 43 inches (1,092 millimeters) to 44 inches (1,118 millimeters) in the soil, while MGS posts are embedded only 40 inches (1,016 millimeters). Blockout depth and splice location differences make the behavior of the MGS different from older W-beam systems. Therefore a review of previous testing (post-in-soil component testing and full-scale crash testing) was conducted to compare the performance of 6 inches × 8 inches (152 millimeters × 203 millimeters) wood posts and W6×9 (W152×13.4) steel posts when used in the MGS. However, no such tests have been conducted on either W6×15 (W152×22.3) steel posts or large cross section wood posts. Therefore, a series of dynamic component tests were conducted to determine the post-soil interaction force characteristics for these large post sizes in an effort to find an equivalent wood post for the W6×15 (W152×22.3) steel posts utilized in the steel-post MGS stiffness transition to thrie beam.

This research objective was met through a combination of historical data review, dynamic component testing, and computer simulation and analysis as follows.

  1. Historical Data Review:
    1. W6×9 (W152×13.4) steel posts:
      A literature review was conducted on post-soil resistance for both W6×9 (W152×13.4) steel posts and 6 inches × 8 inches (152 millimeters × 203 millimeters) wood posts and conclusions were made regarding these standard post sizes. In a recent dynamic testing study, two 6 inches × 8 inches (152 millimeters × 203 millimeters) wood posts and two W6×16 (W152×23.8) steel posts were embedded 40 inches (1,016 millimeters) in a highly compacted soil and impacted at 20 mph (32 km/h).  The W6×16 (W152×23.8) posts have the same flange width and overall depth as a W6×9 (W152×13.4) so the soil resistances for the two posts are considered the same.  This testing showed these particular wood and steel posts provided very similar soil resistances throughout the impact event.
    2. W6×15 (W152×22.3) steel posts:
      A literature review conducted on post-soil resistance for W6×15 (W152×22.3) steel posts found no past research was conducted.
    3. MASH Crash Testing:
      Two full-scale MASH 3-11 crash tests were selected to compare the W6×9 (W152×13.4) steel-post and the 6 inches × 8 inches (152 millimeters × 203 millimeters) wood-post performance when installed in the MGS. Test no. 1 (2214MG-2) utilized steel posts, while test no. 2 (MGSWP-1) utilized the wood posts. Both test installations were 181 feet 3 inches (55.2 meters) long
  2. Physical Testing:
    1. Dynamic Component Testing:
      Dynamic component testing was conducted to determine the post-soil resistance characteristics of W6×15 (W152×22.3) steel transition posts embedded 54 inches (1,372 millimeters) in soil as well as wood posts of multiple cross-sections and embedment depths. Twenty dynamic component tests were conducted on W6×15 (W152×22.3) steel posts and various wood-post sizes in soil. The target impact conditions for all tests were 20 mph (32 km/h) at an angle of 0 degrees, creating a classical “head-on” or full-frontal impact and strong axis bending. The posts were impacted 24-7/8 inches (632 millimeters) above the ground line. Four of these dynamic component tests specified AASHTO Grade B Moderate Compaction Soil (NCHRP350), and the remainder of the tests specified AASHTO Grade B Heavy Compaction (AASHTO MASH).
    2. Computer Simulation and Analysis:
      After determining equivalent wood posts for both steel post sizes used in the MGS approach transition, BARRIER VII computer simulations were conducted to compare the performance of the wood and steel post systems.  The steel-post BARRIER VII model was validated against the full-scale crash testing of the steel-post transition system under MASH safety standards (Test no. MWTSP-2) and served as the basis for comparison and evaluation of the wood-post transition system.

After the wood-post transition system was determined to be an adequate alternative via physical component testing and computer simulation and analysis, the final design drawings were created.

Details of this system are included in this correspondence as an enclosure.

Crash Testing
All physical testing was conducted at the test facilities at the Midwest Roadside Safety Facility. This research uses both existing physical cash test results, bogie testing results and BARRIERVII analysis.

  1. Dynamic Component Testing:
    Bogie testing program was conducted to identify a wood post that provided similar force vs. deflection, or energy absorption, characteristics to the 7 feet (2.1 meters) W6×15 (W152×22.3) steel posts utilized in the original MGS approach transition system. Although Grade 1 Southern Yellow Pine posts (SYP) were utilized during all of the tests, wood defects are inevitable in timber posts, especially with the larger cross sectional dimensions. Therefore, posts utilized in actual installations would be expected to have some natural defects that may lead to premature post fracture.  Posts that fracture absorb far less energy and do not provide any resistance after fracture, typically within the first few inches of deflection. From a guardrail transition design perspective, this lack of resistance can have negative effects on the safety performance of the system in this sensitive region of the barrier. Similar performance results are expected for a transition system in which a post fractured prematurely.  Therefore, posts that showed a propensity for fracture before rotating were removed from consideration as equivalent posts to the W6×15 (W152×22.3) steel posts. Post fracture was prevalent in tests conducted on 7 feet (2.1 meters) long versions of 8 inches × 8 inches (203 millimeters × 203 millimeters) and 6 inches × 10 inches (152 millimeters × 254 millimeters) wood posts. As a result, these posts were not recommended for use in the MGS approach transition.

    The individual test results for each post size were averaged together in order to compare the various posts. The 6.5 feet (2.0 meters) long 8 inches × 10 inches (203 millimeters × 254 millimeters) wood posts provide average force characteristics that best match those of W6×15 (W152×22.3) steel posts when the soil was heavily compacted. At 15 inches (381 millimeters) of deflection, the 8 inches × 10 inches (203 millimeters × 254 millimeters) wood posts averaged 17.7 kips (78.8 kN), only 1.1 percent higher than the steel posts. Although the average force of 8 inches × 10 inches (203 millimeters × 254 millimeters) wood posts showed an increase of 15.5 percent over the steel post at 10 inches (254 millimeters) of deflection, the average forces were relatively close.
  2. Physical Crash Testing:
    Two full-scale crash tests were selected to compare the W6×9 (W152×13.4) steel-post and the 6 inches × 8 inches (152 millimeters × 203 millimeters) wood-post performance when installed in the MGS. Test no. 2214MG-2 utilized steel posts, while test no. MGSWP-1 utilized the wood posts. Both 181 feet 3 inches (55.2 meters) long test installations satisfied all MASH safety performance criteria of test designation no. 3-11. The two systems behaved similarly during the test in terms of maximum dynamic deflection, contact length, and exit conditions, as shown in Table 2. Further, the Occupant Impact Velocities (OIV) and Occupant Ridedown Accelerations (ORA) were very similar, thus suggesting the forces imparted to the vehicle were very similar. Similar performance between W6×9 (W152×13.4) steel and 6 inches × 8 inches (152 millimeters × 203 millimeters) wood guardrail posts has been documented in both dynamic component testing and full scale testing. Therefore, the   6 inches × 8 inches (152 millimeters × 203 millimeters) wood posts was selected as the alternative for the W6×9 (W152×13.4) steel posts found in the MGS to thrie beam stiffness transition.
  3. The BARRIER VII analysis simulations used in this research verified that the wood posts did not adversely affect the safety performance of the stiffness transition.

Summary and Standard Provisions

  1. At the conclusion of the bogie testing program, the 8 inches × 10 inches (203 millimeters × 254 millimeters) wood post with an embedment depth of 48 inches (1,219 millimeters) best resembled the performance of the W6×15 (W152×22.3) steel transition post and was recommended for further analysis in the MGS approach transition.
  2. The existing MASH crash testing included both systems that behaved similarly during the test in terms of maximum dynamic deflection, contact length, and exit conditions, as described below.
    • Test no. 2214MG-2featured a 5,174-lb (2,347-kg) 4-door pickup truck that impacted the MGS W6×9 (W152×13.4) Steel post barrier at a speed of 62.8 mph (99.6 km/h) and at an angle of 25.5 degrees. The MGS rail successfully redirected the vehicle while meeting all required safety criteria and sustaining a maximum deflection of 31-5/8 in. (803 mm).
    • Test no. MGSWP-1, featured a 5,174-lb (2,347-kg) 4-door pickup truck that impacted the MGS Wood 6 in. × 8 in. (152 mm × 203 mm) post barrier at a speed of 63.8 mph (99.6 km/h) and at an angle of 25.6 degrees. The MGS rail successfully redirected the vehicle while meeting all required safety criteria and sustaining a maximum deflection of 31-5/8 in. (803 mm).
    Crash Test Summary details of this system are provided as enclosures to this correspondence.
  3. At the conclusion of BARRIERVII analysis, the wood-post MGS stiffness transition outperformed the original steel-post transition system in all three of the evaluation criteria. The maximum deflections for the wood-post system were consistently 15 to 30 percent lower than the original steel-post system. This deflection reduction was the result of the wood posts having a higher stiffness and resistance to rotation than their steel counterparts. The wood-post system also consistently showed a 5 to 25 percent reduction in the maximum pocketing angle. Thus, the wood post system is expected to reduce the risk of vehicle instability. Finally, the propensity for wheel snag was found to be lower for the wood-post system. The reduction in system deflection significantly reduced the estimated wheel snag for the 6-in. × 8-in. (152-mm × 203-mm) wood post. However, the wheel snag estimations for the larger 8-in. × 10-in. (203-mm × 254-mm) wood transition posts were found to be closer to (or slightly higher) the estimations for the steel W6×15 (W152×22.3) steel posts. Thus, the potential benefits (as far wheel snag are concerned) of deflection reduction were offset by the reduction in embedment depth.

Therefore, this system as described is eligible for reimbursement and should be installed 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 eligibility letters:

 

Sincerely yours,

/* Signature of Michael S. Griffith */

Michael S. Griffith
Director, Office of Safety Technologies
Office of Safety

Enclosures

Page last modified on February 22, 2017
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