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MWTSP with vertical-faced bridge rail

Question
State IA
Description Text What would it take to evaluate the attachment of the steel post Midwest transition to a vertical-faced concrete bridge rail (no tubes/channels connecting the bridge rail to the first guardrail post)? I’m specifically referring to the bottom drawing of Figure 96 on page 167 of TRP-03-210-10. I am interested in attaching this directly to a vertical-shape bridge end post with perhaps some sort of chamfer at the end to reduce snagging potential. Would this be something you might be able to recommend based on some investigation and simulation, or would it require a crash test (or two)?
Keywords
  • Approach Guardrail Transitions
Other Keywords none
Date April 11, 2012


Response
Response

In general, I believe that acceptable safety performance would be obtained with the attachment of the AGT (bottom – Figure 96) to a vertical concrete buttress with the minor flared back end. We know that this detail has been successfully evaluated when attached to steel post - steel beam bridge railing systems with an additional tube or channel member carried from the bridge to the first few transition posts. We believe that the removal of the backside beams and attachment to a concrete parapet would allow slightly more thrie beam deflection relative to the more rigid bridge system. However, we do not believe that wheel snag, excessive barrier deflections, or vehicle pocketing would cause any concerns.

 

It may be possible to compare this design to other wood post AGTs which use the half-post spacing to help determine maximum barrier deflections. If available, this information would be used to make comparisons to this design and hopeful show that similar or lower barrier deflections would be obtained with the larger steel posts at half-post spacing. Comparisons would then be made to the original Iowa transition which was accepted for use with safety shape ends and flared vertical ends. If needed, BARRIER VII computer simulations could be used to estimate the increased barrier deflections between original steel-post AGT designs with tubes/channels and modified AGT with additional hardware removed. Once those results were obtained, a comparison of simulation results would be made between Figure 96 and Iowa transition as well as an evaluation of wheel snag.

 

If needed, the last resort would be an actual 2270P test on a flared vertical concrete buttress.

 

Does this make sense?

Date April 12, 2012


Response
Response

Yes, it makes sense.  In your opinion, would this design require a curb at the point of attachment to the bridge (or might FHWA require such)?

 

And on a related note, do you have a minimum recommendation for the design of the chamfer (i.e. flare or turnout) to be used at the approach end of a vertical bridge rail?  I know we have several different designs that we use: one flares back 2 inches in 12 inches while another flares back 6 inches in 24 inches…

 

Date April 12, 2012


Response
Response

In my opinion, I would not think that a curb would be required. I believe that a 2-in. minimum flare back would be required. I have enclosed a few samples that have been included in FHWA acceptance letters for which I am familiar.

 

Date April 12, 2012
Attachment b-47a.pdf
Attachment b47b.pdf
Attachment b105.pdf


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