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Nested Guardrail Recommendation for Thrie Beam Transition - Task Force 13 Presentation

Question
State FL
Description Text At the last Task Force 13 meeting in College Station (see attached agenda), Karla gave a presentation on guardrail and discussed crash test failure of the W-beam to Asymmetric Thrie-beam connection. During that presentation she mentioned that subsequent successful crash tests were conducted with the W-beam nested. I am requesting  the crash test results which show the successful crash testing of that Critical Impact Point with the nested W-beam.

I have incorporated a nested 12’-6” W-beam into our FDOT Design Standards and require the crash test results and/or research report for my files. If there are issues with this design, then I need to understand what they are and what can be done to rectify the issue. I have included a copy of our Detail “J” from Index 400, Index 402 and Index 430 for your review and input. These designs are based on the TxDOT and TTI crash tested system with 8” block-outs and panels splices set to mid-span. These designs incorporate the addition of a nested W-beam as based on Karla’s TF13 presentation and was reviewed by the Washington Safety Office (Dick Albin) with approval for release by our FHWA Division Office. Please, advise on the status of the requested documentation.

Note:
We have released these drawings based on crash test information provided by TTI and discussions with MwRSF staff and final approval from FHWA.
31” guardrail designs are currently underway which depict the nested W-beam and these designs will begin construction for projects to begin on or after July 2014.
I would like to put this documentation request to rest. Your input and support is greatly appreciated.
Keywords
  • Approach Guardrail Transitions
Other Keywords none
Date March 18, 2014


Response
Response The report is not complete yet. It is still in internal review within MwRSF and then will be sent to the Midwest States Pooled fund states for review. We can forward it to you when it is final. The report along with the CAD and videos will be uploaded to our Research Hub site when it is final. In the mean time, we can forward the videos of that test if you would like.

After reviewing the materials you provided, we wanted to bring a few things to your attention. The transition system that included the nested W-beam was a transition used in conjunction with a curb beneath it. If the transition does not include a curb, you would want to use the details contained in the following report: http://mwrsf.unl.edu/reportresult.php?reportId=38&search-textbox=transition

Within this report are implementation guidelines (Chapter 13) and adaptation recommendations to different approach transitions than what was used during testing (Chapter 14). Within the recommendations are recommendations regarding the use of flared guardrail in the transition. For flared guardrail applications, a minimum length of 25 ft was recommended between the upstream end of the asymmetrical W-beam to thrie beam transition section (i.e., post m on your Index 400) and the start of the flared section (i.e. bend between flare and tangent sections). No flaring of the actual transition was recommended.

Flaring of the transition itself was not recommended due to limited knowledge regarding the use of flared approach guardrail transitions and their safety performance. At this time, no research or full-scale crash testing of flared approach guardrail transitions has been conducted under the NCHRP Report No. 350 or MASH evaluation criteria. However, previous testing of flared guardrail systems and transitions suggests several concerns related to flared transitions. First, flaring of the transition would tend to increase the effective impact angle on the barrier, which would raise the potential for vehicle snag on both the posts and the end of the rigid parapet. Similarly, an increase in the loads imparted to the barrier would be expected due to the higher effective impact angle and an associated increase in the potential for barrier pocketing. Loads on the rail elements in approach transitions are already increased due to limited barrier deflection, and there is potential for increased flare rates to raise the loads in the rail elements to the point of rail rupture. Full-scale crash testing of the MGS upstream stiffness transition with the 1100C vehicle indicated that wedging of the vehicle under the asymmetrical W-to-thrie beam transition section and snag on the posts in the system resulted in relatively rapid vehicle deceleration and yawing. While the deceleration did not raise occupant risk issues to critical levels in those tests, there may be potential for increased occupant risk values as the flare rate increases for the critical small car impact. Finally, the use of flared transitions may increase the potential for vehicle instability due to the increased impact angle, increased vehicle snag, and the increased potential for pocketing.

In addition, nesting of the W-beam that you show in Index 430 would not be necessary. The reason for the nesting of the W-beam rail in the system with a curb was to stiffen the rail just upstream of the asymmetrical transition section in order to prevent rail rupture. With a double sided system, we believe that the additional stiffness would not be necessary.

Let me know if you need further information.
Date March 18, 2014


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