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Nested 12 gauge rails is typically considered stronger than 10 gauge rail as there is more section for the nested rail sections. However, it is difficult to directly compare the two because the nested rails don’t have a totally effective shear transfer between the nested panels (think of a web between the flanges of an I-section post). Looking at their plastic moment capacities, 12 gauge thrie beam rail has a capacity of 153.6 kip-in. 10 gauge thrie beam has a capacity of 195.6 kip-in. If we assume that plastic hinging of nested 12 gauge requires bending of tow 12 gauge thrie sections, then the total moment capacity is 307.2 kin-in. this would mean that nested 12 gauge thrie beam has 57% more bending capacity than 10 gauge. Note that this is not a perfect analysis, but it does suggest that there is more bending capacity for nested thrie beam.

In terms the performance of both options in approach guardrail transitions, we previously evaluated the use of 10 gauge thrie beam in an AGT in the attached report under NCHRP 350. The 10 gauge rail performed adequately in that scenario. Thus, under NCHRP 350, we historically recommended that 10 gauge rail could be used interchangeably for nested 12 gauge in thrie beam AGTs. For MASH TL-3, we have not seen any issues within our current transition testing to suggest that the 10 gauge rail option would be an issue, but that has not been fully validated through crash testing.

To date, MASH testing of thrie beam AGTs with nested 12 gauge rail have performed acceptably under MASH TL-3 conditions, but no 10 gauge thrie beam AGT’s have been tested to MASH TL-3. While, the loading is higher under MASH TL-3, we have not observed specific behaviors in our AGT testing to date that would suggest that 10 gauge thrie beam does not have sufficient capacity to perform adequately as well. This biggest issue would likely be reduced bending section and a corresponding increase in deflections, which may increase pocketing and vehicle snag. If single 10-gauge rails are desired as a replacement for nested 12-gauge thrie beam in MASH AGTs, it may be beneficial to further investigate some of the more critical impact scenarios and systems with computer simulation and/or dynamic testing.