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.
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