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FHWA Home / Safety / Roadway Departure / Safety Eligibility Letter

Safety Eligibility Letter B-219

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U.S. Department of Transportation
Federal Highway Administration

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

April 4, 2011

In Reply Refer To:
HSSD/B-219

Mr. Mark R. Burkhead
Standards and Criteria Engineer
Pennsylvania Department of Transportation
Roadway Quality Assurance Division
400 North Street, 7th Floor
Harrisburg, PA 17120

Dear Mr. Burkhead:

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: W6 x 8.5 Steel Guiderail Post over Ground Structures
Type of system: Permanent Guiderail Component
Test Level: National Cooperative Highway Research Program (NCHRP) Report 350 Test Level 3 (TL-3)
Testing conducted by: Texas Transportation Institute (TTI)
Date of Request: December 28, 2010
Task Force 13 Designator: SGR04-d

You requested that we find this system acceptable for use on the NHS under the provisions of 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.

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

Description
The Pennsylvania Department of Transportation (PennDOT) currently specifies W6 x 8.5 Steel Guiderail Post over Ground Structures detail in their existing construction standards. The detail includes the W6 x 8.5 steel post (post) positively anchored to a 4 ft x 4 ft x 8 in thick reinforced concrete footing (R.C. footing). The current standard detail shows the R.C. footing is placed at a minimum depth of 6 in in compacted soil over the top of the R.C. footing. The post were welded to 12 in x 12 in x 3/4-in thick base plate, anchored to the R.C. footing with four (4) 7/8-in diameter A325 bolts cast within the R.C. footing. These anchor bolts are 8 in in length and are embedded a minimum of 6 in into the R.C. footings. Concrete compressive strength tests performed the day of the testing yielded an average compressive strength of 3669.0 psi.

Details of this system are provided as enclosure to this correspondence.

Crash Testing
The proposed PennDOT W6 x 8.5 Steel Guiderail Post over Ground Structures (system) was analyzed at the test facilities at the TTI Proving Grounds Riverside Campus. Two pendulum tests were performed on the system. One test specified the standard R.C. footing and one test specified a proposed 5 ft x 5 ft x 8 in depth R.C. footing section. The objective of the pendulum test is to document the impact performance of the system and assess the potential for meeting the recommended safety performance guidelines set forth in the NCHRP Report 350 for TL-3.

The following full-scale pendulum tests were performed on two R.C. footing designs. The post was loaded in bending about the strong axis.

  1. 4 ft x 4 ft x 8 in Thick R.C. Footing
    The pendulum bogie, traveling at an impact speed of 21.5 mph (34.6 kph), impacted the guardrail post with the centerline of the pendulum nose aligned with the centerline of the steel post. The height of the nose was measured at 20.5 inches. At 0.020 s, the steel post began to deform, and at 0.032 s, the concrete footing began to lift up in the soil. The pendulum bogie lost contact with the steel post at 0.144 s, and exit speed at loss of contact was 7.8 mph. The base of the pendulum contacted the steel post at 0.332 s, and then loses contact with the steel post at 0.398 s. At 0.537 s, the base contacted the steel post again, and at 0.562 s, the pendulum went over the steel post. Maximum deflection of the steel post during the test was 29.1 inches, and the final deformation of the post was approximately 6.5 inches. The footing was moved as a result of the pendulum impact on the post. The leading edge of the footing (traffic face edge) was uplifted and came to rest approximately 6.0 inches above the grade surface. After the test, no visible distress was observed in the concrete footing, post base plate, or anchor bolts. Maximum rise of the nose of the pendulum during the test was 23.6 inches, and maximum crush of the honeycomb nose of the pendulum was 10.7 inches. Longitudinal occupant impact velocity was 20.7 ft/s (6.3 m/s), longitudinal ridedown acceleration was -2.9 g, and maximum 50-ms average was -8.5 g. Change in velocity at loss of contact was 20.1 ft/s (6.1 m/s).
  2. 5 ft x 5 ft x 8 ins Thick R.C. Footing:
    The pendulum bogie, traveling at an impact speed of 21.6 mph (34.7 kmh), impacted the guardrail post with the centerline of the pendulum nose aligned with the centerline of the steel post. The center height of the nose was 20.5 inches above finished grade. At 0.005 s, the steel post began to deform, and at 0.046 s, the concrete footing began to lift up in the soil. The pendulum bogie lost contact with the steel post at 0.155 s, and exit speed at loss of contact was 8.5 mph. At 0.260 s, the concrete footing began to rotate, and at 0.352 s, the base of the pendulum contacted the post. The pendulum bogie came to rest on top the steel post at 0.994 s. Maximum dynamic deflection of the steel post during the test was 20.0 inches, and the final deformation of the steel post was approximately 12.25 inches. The footing was moved as a result of the pendulum impact on the post. The leading edge of the footing (traffic face edge) was uplifted and came to rest approximately 3.0 inches above the grade surface. After the test, no visible distress was observed in the concrete footing, post base plate, or anchor bolts. Maximum rise of the nose of the pendulum during the test was 24.1 inches, and maximum crush of the honeycomb nose of the pendulum was 10.1 inches. Longitudinal occupant impact velocity was 24.6 ft/s (7.5 m/s), longitudinal ridedown acceleration was -3.6 g's, and maximum 50-ms average was -8.7 g's. Change in velocity at loss of contact was 19.2 ft/s (5.9 m/s).

In addition to these pendulum tests, a further comparison was made between the posts and concrete foundations tested for this project to standard W-beam guardrail line posts embedded in the NCNRP Report 350 strong soil (strong soil). Two pendulum tests were performed on W6 x 8.5 steel posts embedded approximately 43.4 inches in strong soil (TTl Project Number 220547). For both tests, the center height of the pendulum was 18.0 in above grade. The force versus displacement curves for both posts tested for this project compared very closely to the force versus displacement curves for the same steel posts embedded in strong soil.

Testing details of this system are provided as enclosure to this correspondence.

Findings
The analysis of the systems described herein is as follows:

  1. The 5 ft x 5 ft footing rotated approximately 6 degrees from the pendulum impact. The dynamic rotation of the post was approximately 30 degrees. Plastic failure occurred in the W6 x 8.5 steel posts for this design tested. No distress was observed in the anchor bolts or concrete. The Occupant Risk Value Impact Velocity, longitudinal direction is 24.6 ft/s (7.5 m/s). The Ridedown Acceleration for longitudinal direction is -3.6 g, with a maximum change in velocity of 19.2 f/s.
  2. B. The 4 ft x 4 ft footing rotated approximately 15 degrees from the pendulum impact. The dynamic rotation of the post was in excess of 30 degrees. Plastic failure occurred in the W6 x 8.5 steel posts for this design tested. No distress was observed in the anchor bolts or concrete. The Occupant Risk Value Impact Velocity, Longitudinal direction is 20.7 ft/s (6.3 m/s). The Ridedown Acceleration for Longitudinal direction is -2.9 g, with a maximum change in velocity of 20.1 f/s.

The posts anchored to the footings exhibited greater strength than those tested in strong soil over 0 to 2 ft of displacement. The dynamic rotation of 5 ft x 5 ft R.C. footing was approximately 30 degrees, and greater than 30 degrees for the 4 ft x 4 ft R.C. footing tested for this project. The dynamic rotation of the 4 ft x 4 ft R.C. footing and post was deemed undesirable since the footing came to rest approximately 6 in above grade surface.

Based on the results of this testing, the researcher recommended the W6 x 8.5 steel post anchored to the 5 ft x 5 ft R.C. footing for use in cases where a single post in a guardrail length of need interferes with an underground obstruction. The researcher further recommends a full-scale crash test be performed to validate the performance where multiple posts in succession anchored to shallow 5 ft x 5 ft concrete footing as tested herein is required.

FHWA therefore concurs with recommendation of 5 ft x 5 ft x 8 in R.C. footing system as described in the request 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 FHWA letters of acceptance:

 

Sincerely yours,

/* Signature of Michael S. Griffith */

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

Enclosures

Page last modified on June 24, 2011.
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