HSA-10/B-147

May 8, 2006

In Reply Refer To:HSA-10/B-147

Subject: INFORMATION: Cable Barrier Transitions to W-Beam Guardrail

From:
/original signed by/
John R. Baxter, P.E.
Director, Office of Safety Design
Office of Safety

To:
Director of Field Services
Resource Center Managers
Division Administrators
Safety Field

In the late 1980's, the Southwest Research Institute conducted successful crash tests on a cable barrier transition to a strong-post W-beam guardrail using a passenger sedan. In 1998, the Midwest Roadside Safety Facility (MwRSF) tested this transition design using NCHRP Report 350 test vehicles and concluded that it met all appropriate evaluation criteria at test level 3 (TL-3). The non-proprietary design tested in both cases was one developed by the South Dakota Department of Transportation. This design transitioned the generic 3-strand cable barrier over and under the W-beam using special steel straps and connected each cable to a standard concrete anchor block located behind the W-beam. A reduced post spacing was used as the cable barrier approached the W-beam installation, which was itself anchored with a Breakaway Cable Terminal (BCT) offset 4 feet from the cable barrier. In 2002, the MwRSF again tested this design successfully with the 2000-kg pickup truck, but with a FLEAT guardrail terminal anchoring the W-beam installation in lieu of a BCT. Although the South Dakota design remains an acceptable TL-3 transition, the 6-foot or more dynamic deflection of the cable barrier in each of the Report 350 crash tests allowed direct vehicular contact with the W-beam terminals, resulting in considerable pitch, yaw, and roll to the pickup trucks. This design also requires installation of a downstream anchor for the cable barrier.

Recently, the manufacturers of several of the high-tensioned proprietary cable barrier systems have requested acceptance of unique transition designs for connecting their barriers to strong-post W-beam and/or Thrie-beam guardrail. Each of these eliminates the need for a separate downstream cable anchor by attaching each cable directly to the metal beam rail element. Like the South Dakota design, the cable rail post spacing is also reduced to further limit deflection of the cable in the transition area. Reduced dynamic deflection can also be expected with the greater cable tension specified for all of the proprietary designs.

Each of these designs has been previously accepted as a TL-3 transition based on our review of the designs themselves and the crash testing done on the South Dakota design. Copies of the letters sent to each manufacturer are attached for ready reference. Note that for each specific design, the W-beam terminal itself should be offset at least 4 feet. Because the cable will prevent virtually all head-on impacts into the W-beam terminal, a light-weight, non-energy absorbing terminal would be the preferred method of anchoring the W-beam barrier. The MELT, which has been accepted as a TL-2 design, could be used to anchor the W-beam if a generic terminal is desired.

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