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ODOT has a standard detail for mounting MGS on box culverts (pasted below), and it comes from the TTI study “MASH TEST 3-11 OF THE W-BEAM GUARDRAIL ON LOW-FILL BOX CULVERT”. As far as I can tell, the report does not indicate a minimum length for this system.

The working width during the crash test on this system was 49.6”, and there were 16 anchored posts at the standard 6.25’ spacing.

~12” headwall + 18” offset + 6” post + 12” blockout + the w-beam thickness = ~50”.

Typically, the span of the box culvert dictates the number of anchored posts, and we generally only have 1-3 anchored posts mounted on the box. Would you anticipate that the reduced length of the rigid/anchored system would impact the performance/working width?  Should there be a minimum # of anchored posts?

A designer asked this morning if they should have half-post spacing since they are not meeting 60” MGS working width with this detail, which made me question if that min. 18” should be extended so that the total working width is equivalent to MGS to compensate for the inevitably fewer posts. Any insight is appreciated.

A few comments on the email below.

1. I don’t believe there would be a minimum installation length for the culvert mounted post type system. The posts in those system were intended to have similar stiffness and deflection to standard posts. Thus, the length of the culvert mounted posts should significantly affect the performance.
2. In terms of working width, we have developed and tested several types of guardrail over culvert systems. This includes culvert mounted strong posts like those shown in your email, weak post culvert mounted guardrail, and long-span guardrail systems. In those tests, we have seen vehicle extension past the edge of the culvert headwall with acceptable performance. Thus, in the case of your culvert mounted system below, we don’t see much concern for deflections/working widths that are slightly larger than the distance to the culvert.
3. If you want reduced deflections, we have evaluated a similar system at half post spacing. The details of that can be found in this report - https://mwrsf.unl.edu/researchhub/files/Report409/TRP-03-383-20-R1.pdf

Let me know if that helps or if you need anything else.