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I have a question relating to an existing question in the Q/A, number 1779. Feel free to append to if desire. We have a situation in construction right now (hence the urgency) where they’re asking to use double blocks (two 8-in blocks) for posts 1, 2, 3 of the AGT. We use the heavier W6X15 posts for #1,#2,#3 for our AGT (see our standard attached) instead of the W6x8.5/9.

I would like to know what concerns there are if all three posts (W6x15 posts) each had to used 16” blockout depths (two 8-in blocks). What occurred was the bridge approach panel was poured wide and requires the three posts to be setback. All other posts are not affected. If the situation only affected two posts base on 1779 I would be willing to okay it, however the situation I have impacts a 3^rd post and not sure what the risks would be even though it is in a region we’re not expecting as much rotation.

Thanks,

In the past, we have considered the use of deeper blockouts in limited cases dependent on system in question. We have used 16” deep blockouts in certain systems, but we have not used 24” deep blockouts in system due to concerns that the additional blockout depth may begin to affect the way the guardrail post is loaded and may increase the potential for later-torsion buckling of the post rather than the desired post loading modes of strong axis bending and rotation of the post through the soil. As such, we have limited these extended blockouts to a single post in a run of guardrail in order to deal with obstacles or other issues. We currently have testing planned in the pooled fund to evaluate 24” blockouts continuously in the MGS.

We have allowed deeper blockouts in approach guardrail transitions in the past. The concern for altering the post loading is less prevalent for the transition posts as they tend to be closer spaced and deflect less, which lowers the concern for buckling of the post.

Thus, we believe that it would be acceptable to use large blockouts for post nos. 1-3 shown in your detail without adversely affecting performance due to the special circumstance you are faced with. The 16” depth is only 4” more than the 12” deep blockouts used for the MGS, and the use of ½ post spacing and the limited deflection in that area would help prevent concerns for altering the post loading as noted about. However, for general installations we would recommend using the tested configuration as the use of the deeper blockouts has not been formally investigated or tested.

Let me know if you need anything else.

Thanks