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Long Span Guardrail Using Thrie-Beam

Question
State
Description Text I was asked a question by a Wisconsin contractor who is being asked by the Wisconsin resident engineer on a project to use thrie-beam in what they are referring to as a long span installation. The job has both w-beam (31”) and thrie-beam installations and on one of the thrie-beam runs they encountered a concrete footer of one of the bridge piers. (I believe it is from a railroad bridge.) They are estimating the span to get by this concrete footer is between 13’ to 20’. As I understand the contractor, the resident engineer wants him to install this similarly as that of the w-beam long span MGS, except using thrie-beam. The contractor is questioning this and is looking for some guidance as to whether this is the proper type installation or is there another alternative. I can see what the resident engineer is wanting to do and does seem to make some sense, but like the contractor I too cannot find anything to really support this in an installation using thrie-beam. Can you offer some guidance and advice on proper installation in such a situation.
Keywords
  • Guardrail
Other Keywords Long Span
Date May 27, 2014


Response
Response

The W-beam long span and MGS long span systems can accommodate 13 to 20 ft obstructions below grade. However, the dynamic deflections and working widths for these two systems are larger than observed for the baseline guardrail systems without posts removed. You note the use of a 31” W-beam guardrail as well as thrie beam guardrail. We have not yet developed long-span systems for use with thrie beam. However, there exists a potential for this type of system to meet impact safety standards. Improved performance would likely be achieved with the use of similar post embedment depth to MGS (40”), deep blockouts (12” versus 8”), and shortened thrie beam blockouts similar to those used in bullnose system as well as the newer thrie beam approach guardrail transitions. The guardrail height may need to be slightly higher than 31” and as used in some of the prior successful thrie beam crash tests under 350. With these considerations, there would be a more reasonable chance that a thrie beam long-span system would meet AASHTO MASH or NCHRP 350. Of course, this specific system has not been crash tested, and crash testing is the best method for evaluating crashworthiness.

 

These are my preliminary thoughts on this topic. I will scan through our Pooled Fund Consulting Website to see if we have provided similar guidance in the past.

Date May 27, 2014


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