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Rectangular Washers and Reduced Post Spacing

State IL
Description Text

Illinois no longer uses the rectangular plate washers behind the bolt holding guardrail to blockouts for the 6'-3" post spacing.  However, it has been pointed out that we still use this on the double face guardrail, and I have noted that we still use this washer on guardrail installations with 3'-1½" post spacing.


The inquiry regarding the double faced guardrail was referred to FHWA before coming to me, and it was suggested that it would probably be better to eliminate it from that application.


I am aware that the Midwest Roadside Safety Facility has conducted testing of the reduced post spacing of W-beam guardrail on 2:1 slope breaks.  (October 2000 report entitled "Development of a W-Beam Guardrail System for Use on a 2:1 Slope.)


Did that application use the washers at the posts?  From the photos you sent me on disk earlier, it appears that these washers were not used.


Based on the FHWA comments and pending your reply, we are considering removing these washers from our applications of both the double faced guardrail, and from the reduced post spacing design.

  • Guardrail
Other Keywords none
Date November 23, 2004


For the strong-post, W-beam guardrail system installed at the slope break point on a 2H:1V fill slope, you are correct in stating that the system utilized a reduced or half-post spacing. In addition, the noted guardrail system was designed and installed without the use of any rectangular washers on the traffic-side face of the rail element. Rectangular washers are no longer recommended on guardrail systems as their use can lead to the rail being pulled down to the ground during system deformation and post rotation, thus resulting in an increased potential for overrride of semi-rigid barrier systems. The use of these washers are obviously more critical in guardrail systems that are subjected to higher deformations and where guardrail release from the posts is desired. For stiffer guardrail systems, such as the thrie beam approach guardrail transition system, their use is much less of a concern since large barrier deflections generally do not occur.


Over the last several years, MwRSF researchers have been involved in the development, testing, and evaluation of many corrugated steel beam guardrail systems, including those using both standard and reduced post spacing designs. For all of these guardrail systems, MwRSF has not implemented the use of rectangular washers on the rail face and has no plans to do so in the future. As such, we concur with the suggestion made by FHWA.

Date November 24, 2004

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