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.
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