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|Description Text||We have a box culvert with about a 44 foot span and low fill over the top. A copy of the plan sheet is provided. We would like to use box beam guardrail due to blowing snow issues. To my knowledge no box beam attachment to a box culvert has ever been tested.
This appears to be an ideal case for adapting the new MGS Box Culvert Rail to a box beam rail. As you know, box beam uses the same post (S3x5.7) as the MGS Bridge and Box Culvert Rails. Box beam has been demonstrated in redirectional crash tests (mounted at 28 inches) to perform in a much more forgiving manner than conventional w-beam and may resemble redirectional tests with the taller, more robust MGS.
Would it be possible to adapt the MGS Culvert System, only using box beam (presumably still mounted at 28”)? It would appear that the post spacing might be the biggest issue to determine since I am guessing the deflection of a box beam bridge rail should probably not exceed the MGS Bridge Rail in order to assure there would be no greater wheel snagging from an impacting vehicle as it rebounds back onto the box culvert curb. I would assume that using a box beam with a 3 foot spacing would be quite a bit stiffer than the MGS Bridge Rail, but perhaps at a four foot or even the standard six foot spacing, it might replicate the stiffness of the MGS Bridge Rail. Would this require crash testing or could an adequate “paper” case be made for using such a system?
|Other Keywords||culvert, MGS Bridge Rail|
|Date||February 19, 2014|
|Attachment||Rawlins-Medicine Bow Box Culvert.pdf|
|Attachment||PF Question - Box Beam Box Culvert.docx|
We would agree that there is a significant potential to adopt box-beam guardrail to a culvert mounted system similar to what we have done with the MGS bridge rail based on the similarities of the posts and the fact that the box beam system has successfully met the MASH criteria when tested with the 2270P vehicle at TTI. However, it is not likely that we could recommend the system be used without further research and full-scale testing due to several concerns.
1. The dynamic deflection of the box beam system tested at TTI was 57.7 in. This is significantly higher than the 40-50 in. deflections that are typically observed for the MGS and the MGS bridge rail under TL-3 impacts. This additional deflection may pose some concerns for increased vehicle overhang of the culvert and potential vehicle instabilities. As you noted, reduced posts spacing could be used to address the increased deflection. However, that changes the system from what was previous crash tested and would require further study to determine the level of deflection reduction and the effect of the reduced spacing on the barrier performance. Alteration of the post spacing may also affect the need for stiffness transitions in the system, although that is unlikely.
2. The 28" height of the box beam system and the shape of the rail element may provide a different level of vehicle capture than the 31" high W-beam used in the MGS bridge rail system. The differences in the vehicle capture aren't full known, but it is possible that the box beam rail capture would not be as effective as the MGS W-beam, especially when considering the extension of the impacting vehicle over the edge of the culvert.
3. In order to get approval of a box beam culvert mounted system, small car testing with the 1100C vehicle may be required. It was required with the MGS bridge rail. We would have to contact FHWA and get feedback on if such testing was required for any proposed design.
|Date||February 20, 2014|
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