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w-beam back-up plate

State WI
Description Text

In TRP-03-226-10 Development of a Low-Cost Energy-Absorbing Bridge Rail, the back up plate shown is 6" wide.

I believe that at a pooled fund meeting there was some discussion about the need to make the back up plate wider.  What width should the back-up plate be?

  • Bridge Rails
  • Guardrail
Other Keywords w-beam back-up plate
Date January 29, 2016


You are correct that recent testing as part of the MGS installed in mow strips project led us to recommend revised backup plates for weak post MGS systems using the S3x5.7 post. The use of 12-in. (305-mm) long backup plates behind the rail was recommended. The partial rail tearing observed during test no. MGSMS-1 was caused when the test vehicle impacted a post and caused it to deflect downstream and twist such that its flange contacted the bottom of the rail directly below the downstream splice bolts. Then, as the vehicle’s right-front bumper and fender loaded the splice, the tear propagated to span half of the rail height. If a long backup plate had been installed at this location, the tear may have never occurred.

The original MGS bridge rail utilized 6-in. (152-mm) long backup plates at every post, including splice locations since the splice bolts are 8 in. (203 mm) apart. Unfortunately, the design drawings for the full-scale test specified 12-in. (305-mm) backup plates (taken from the non-blocked MGS drawings) instead of the 6-in. (152 mm) backup plates, and these larger backup plates could not be installed over the splice bolts, which are 8½ in. (216 mm) apart, without additional holes in the plate. As such, backup plates were not installed at locations where posts coincided with rail splices. The lack of backup plate material may have contributed to the partial rail tearing in test no. MGSMS-1. However, the tearing would have likely still occurred had 6-in. (152-mm) backup plates been utilized, because the 6-in. (152-mm) backup plates do not extend below the splice bolts where the tear initiated. Similar rail tearing has been observed in other 2270P testing on S3x5.7 (S76x8.5) weak-post guardrail systems that utilized 5⅝-in (143-mm) backup plates at all post locations.

To prevent rail tearing due to post contact near rail splices, a longer backup should be utilized to protect the rail around all posts, especially at splice locations. Therefore, the utilization of a 12-in. (305-mm) long backup plate is recommended for the MGS weak-post guardrail systems, including the original MGS bridge rail and the weak-post guardrail attached to culverts.

Since 12-in. (305-mm) long backup plates are unable to be installed at guardrail splices, holes or slots need to be cut into the backup plate to allow the guardrail bolts to pass through the plate. The backup plates could utilize the same splice bolt slot pattern that is currently punched into the ends of every guardrail segment. Utilizing this design, the backup plate could be attached to the guardrail and assembled as a part of the splice. Alternatively, a backup plate could be configured to fit over the back of assembled guardrail splices at the time of mounting the rail to a post. Under these conditions, the slots would need to be enlarged to fit around the splice bolts and nuts. Both of these design options are shown in the attached figure and should be equally effective in reducing the risk of rail tearing.
Date January 29, 2016
Attachment backup plates.jpg

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