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We have a situation where we want to place guardrail on a 1V:8H slope which extends a few feet behind the guardrail, before breaking off on a 1V:3H slope. Would a high tension cable guardrail such as Trinity's TL-3 system perform adequately in this situation? The standard system deflects up to around 9 feet (16 foot post spacing), so an impacting vehicle would be traversing the 1V:3H slope during redirection. I believe this deflection would not extend beyond the 1V:3H slope. Please see the attached drawing.
|Date||May 6, 2011|
We looked over your installation and compared it with the testing we conducted with the MGS on 8:1 slope and our current cable testing. In that testing, we found that with a 5 ft offset, the MGS (with a 31 in. top rail height) just captured and contained the 2270P vehicle. We don't have similar cable testing on 8:1 slopes to compare with, but we know that the cable can redirect vehicles on the 4:1 slope as long as the vehicle is effectively captured. Based on this comparison we would recommend that the Trinity system be installed with the 2 ft offset rather than the larger 5 ft offset. The decreased offset should compensate for the cable heights of the Trinity system. In addition, offset should allow for capture of the vehicle and redirection even with the 1:3 slope behind the system. We believe that if the Trinity system captures the vehicle then the system will redirect the vehicle even if it intrudes on the 1:3 slope to some extent.
This short offset recommendation is based on the fact that the Trinity system has not been tested on approach slopes, and thus it is difficult to make recommendations regarding its performance in those situations. In our experience, it is necessary to capture the front corner and rear corner of the vehicle with a cable system on slope to capture and redirect the vehicle. Therefore, the shorter 2 ft offset will gives more confidence that the system will perform safely.
|Date||May 9, 2011|
130 Whittier Research Center
2200 Vine Street
Lincoln, NE 68583-0853
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