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Could you please provide your thoughts if the attached IDOT Standard “Steel Plate Beam Guardrail” Type B (37 ½” closed post spacing) shown on page 1, (Elevation view is also provided below), would need to be updated based on TTI Report: 610211-01, REV 1, dated October 2021 (MASH CRASH TESTING AND EVALUATION OF THE MGS WITH REDUCED POST SPACING), (Fig. 6.1 is also shown below), where a couple of recommendations they have suggested:

• 10” tall and long block-out for the half post spacing guardrail


• The rail is NOT bolted to the post and block-outs at all locations

Thanks in advance for any suggestions you will provide.

We originally evaluated the MGS with ¼ post spacing under NCHRP 350. We did not evaluate reduce post spacing under MASH previously.

TTI’s pooled fund undertook the effort to evaluate the MGS with reduced post spacing and 8” blockouts to MASH TL-3. During that testing, the ¼ post spacing worked acceptably, but they observed a rail rupture on the ½ post spacing system when testing with the pickup. Post test analysis conducted at TTI indicated that the blockouts in the system created a stress concentrator that helped initiate the rail rupture. As such, they incorporated shorter blockouts, reran the ½ post spacing test and it subsequently met MASH TL-3. I believe that system had posts bolted to the rail at all locations. I believe they may have left some posts unbolted in related transition testing, but it may be worthwhile to confirm that with TTI.

https://www.roadsidepooledfund.org/wp-content/uploads/2023/06/TRNo610211-01-Final-REV-1-Signed.pdf

We have tested other systems with ½ post spacing did not exhibit the fracture observed in the TTI testing. This included testing of a culvert mounted, strong post MGS and an MGS adjacent to a 2:1 slope with 7’ posts at ½ post spacing. These all used standard 12” MGS blockouts. Thus, it is not entirely clear why the TTI test failed.

https://mwrsf.unl.edu/researchhub/files/Report499/TRP-03-452-24.pdf

https://mwrsf.unl.edu/researchhub/files/Report409/TRP-03-383-20-R1.pdf

That being said the TTI system is currently the only ½ post spacing version of the MGS that has been successfully evaluated to MASH. As such, it may be worth consideration to using the shorter blockout evaluated at TTI. One could also consider only using the ¼ post spacing option that was tested with standard blockouts as this did meet MASH TL-3.

Let me know if that helps or if you have further questions.

Thanks!