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An issue has come up concerning the size of wood blockouts used with the MGS for IDOT and the Illinois Tollway.  We have a contractor that has placed 2-piece wood blockouts that measure 11.5" from back of rail to front face of steel post.  The standard clearly shows that this dimension should be 12".

The contractor is throwing around nominal versus actual and construction tolerances.  What is your opinion on this?  I would assume that it would test ok, but how much wiggle room is there in the dimensions?

It is my opinion that they should be replaced with the correct size, but the contractor is obviously resisting.

Recently, I have received many calls and emails on this topic in Illinois from the DOT, guardrail installers, fabricators, etc. From what I have noted to all of them, MwRSF addressed this issue in 2007 during an email discussion regarding the implementation of the MGS. In that email discussion, MwRSF noted the following:

October 19, 2007

"Therefore, it would make sense to specify a timber blockout with the full 12-in. depth or two blocks " one 6x8 and the other 6x4. MwRSF researchers also believe that the reduced depth of 11 ¼ and 11 5/8 in., as determined for fabricated and single rough-sawn blocks, would provide acceptable performance within the MGS. However, crash test results with reduced-depth blocks in the MGS are not available at this time. A reduction of ¾ or 3/8 of an inch in blockout depth may fall within the noise level in performance and may not allow us to discern much difference if multiple tests were performed. In any event, we feel that the 12-in. blockout depth provides the safest alternative of the three depth options (i.e., 12, 11 5/8, and 11 ¼)."

From my recent discussion and email correspondence with ILDOT, it now appears that the DOT will be contacting fabricators in order to obtain input before determining the acceptable tolerance on blockout dimensions. Although it is desired to use a 12-in. offset, it is also important to request a product that is economically reasonable. The ILDOT is beginning this investigative effort now.

In terms of your comments and questions noted below, it is correct to say that the blockout dimensions may vary depending on whether the blocks are supplied at full sawn, rough sawn, or dressed. However, it still would be preferable to utilize the full, 12-in. lateral offset purely from a safety performance perspective. If the MGS has been installed with a 11.5-in. blockout, I would not be inclined to swap out those existing blocks with deeper 12-in. blocks.

On another note, I am aware of a plastic block manufacturer having its routed block crash tested with the MGS. I believe that the block was 12-3/8-in. with a 3/8-in route on the post side, thus resulting in a true 12-in. lateral offset.

The Texas A&M Transportation Institute has performed full-scale crash tests of the MGS with 8-in. deep blockouts resulting in successful performance to MASH TL-3 impact conditions. Likewise, MwRSF successfully full-scale crash tested a design of the MGS without blockouts in combination with a mechanically-stabilized earth wall: https://mwrsf.unl.edu/reportResult.php?reportId=26. With these approved configurations, it is likely acceptable to utilize blockouts with a 1/2" variation in depth, as long as the depths are consistent between consecutive posts.