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When it comes to approach guardrail transitions, is nested 12-gauge thrie-beam considered equivalent to a single 10-gauge thrie-beam (and vice-versa)?  How about nested w-beam?

Nested 12-gauge thrie beam would provide greater overall bending and tensile strength than that provided by a single 10-gauge thrie beam. Most crash tests on thrie beam bridge railing and approach guardrail transition systems likely have utilized nested 12-gauge thrie when additional strength was needed or desired. However, MwRSF has conducted limited crash testing on thrie beam bridge railing and approach guardrail transitions where only one single 10-gauge thrie beam was utilized. Historically, many of us have been comfortable with allowing both thrie beam alternatives (i.e., nested 12-gauge T and single 10-gauge T) in situations where additional strength has been desired. Some of the complaints often pertain to the need to stock and differentiate between 10 and 12 gauge thrie beam sections. However, others may not necessarily hold the same opinion.

With regards to W-beam sections, nested 12-gauge beams would once again be stronger than a single 10-gauge beam in terms of bending and tensile capacity. To date, MwRSF has not conducted any research on strong-post W-beam guardrail systems where rupture concerns were fixed with a single 10-gauge rail instead of nested 12-gauge rails. However, MwRSF had proposed this option as one of many solutions for the original rupture observed in testing the Nebraska W-beam guardrail over a 4" tall concrete curb. In the end, two nested 12-gauge rails were used and provided successful performance. I would suspect that a single 10-gauge W-beam may also have worked to mitigate rupture concerns.

If single 10-gauge rails are desired as a replacement for all systems which use nested 12-gauge rails, it may be necessary to further investigate some of the more critical impact scenarios and systems with computer simulation and/or dynamic testing.

Please let me know if you have any further questions or comments on this matter. Thanks!