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I’m guessing someone else has already asked before and the answer’s out there somewhere, but I’m curious to know if a 30 inch mounting height (to top of thrie-beam) for AGTs is too low.  We have a situation where a bridge deck is being overlaid.  Currently, the AGT has a 32 inch mounting height.  The mounting height of the AGT will drop to 30 inches relative to the new bridge deck surface after the overlay. 

The bridge rail itself is 34 inches tall, so after the overlay it will be 32 inches relative to the new deck.  However, to reestablish the 32 inch mounting height for the AGT, we would need to drill new holes 2 inches above the current holes.  We’ll see this quite often in the future, so I figured I better look into this now.

Your question is directly related to an on-going project that we have with NDOT.  They are also overlaying bridges and approach slabs, but they are using a 3” overlay which drops their AGT height from 31” to 28”.  In previous crash testing on the upstream end of AGTs, a guardrail height of 27.75” resulted in rollover of the pickup truck, while 31” guardrail safely redirected the truck.  From this data point, we have recommended that NDOT retrofit their AGTs to bring the height up to 31” to maintain TL-3 crashworthiness.  They decided to take the approach of removing the AGTs and reinstalling them with a specialized attachment piece that utilized the same bolts/holes in the bridge rail buttress.  That way the concrete bridge rail did not have to be altered, just the adjacent guardrail. That project has not been completed, but If you want more information I can send you some sketches of the attachment piece.

Your AGT height dropping to 30” leaves you in the gray area.  It is only 1” lower than nominal, so it may satisfy TL-3.  Then again, it may not.  We just don’t have any research or testing on AGT heights between 28” and 31”.  Sorry for the vague non-answer to your question.  I guess that means it was a good question because we don’t have an answer to it.  You may consider submitting the investigation of minimum AGT heights as a problem statement for next year’s Pooled Fund.  I know more than one DOT would be interested in this.