View Q&A

I hope all is well. I have a couple of questions regarding the MGS to Thrie-Beam Stiffness Transition with 4” Tall Curb, as described in FHWA letter B-263 and MwRSF Report TRP-03-291-14. It is my understanding that MwRSF used wood blockouts during the development and testing of this transition. Has MwRSF issued any professional opinions or done further testing or evaluations with the option of using composite blockouts in lieu of wood blockouts within the transition limits? If not, is this something that could be addressed with a professional opinion, or do you believe further testing or evaluations are necessary to make this determination? Please note that Mondo makes and sells a composite blockout (see attached drawing) with comparable dimensions to wood blockout Part a2 used by MwRSF in posts 10 – 15 of this transition (refer to image below), although Mondo has clearly stated this type of offset block has never been subjected to any crash testing.

In summary, I’m interested in your opinion regarding the possible use of composite blockouts in lieu of wood blockouts within the transition limits.

Thank you.

When considering alternative blockouts, we typically consider both geometry and strength of the block.  Alternative blockouts should have the same geometry as the as-tested system to ensure similar performance.  In other words, the block should have the same height and depth (lateral distance) as the original as-tested blockout.  Widths may vary a little, but the blockout width should also be similar to the original blockout.  Secondly, the alternative blockout should have a comparable strength to the original as-tested system.  Most blockouts, including timber blockouts) retain their shape under impact loading, so any alternative blcokouts should have enough strength to resist compression deformations during impacts.  Additionally, the blockout should have enough strength to retain the bolt.  Some plastic blockouts have thin walled bolt holes and thin side walls.  We have seen this type of plastic blockout prematurely tear free from the bolt due to failure of these thin walls.  One example: snow being plowed from the roadway impacting a guardrail system and knocking the blockouts from the attachment bolts.  Thus, strength considerations include both resistance to compression as well as being able to contain the bolt and prevent premature rupture.

If these geometry and strength criteria are satisfied, We would consider the alternative blockout equivalent to the original as-tested blockout.

Thanks for the information and quick response. Do you think the strength of the block could be ascertained by doing a simple comparison to other blocks with a similar geometry and materials (e.g., if an 8” deep Mondo block for use with thrie-beam guardrail is considered crashworthy, then a 12” deep Mondo block with similar geometry and materials would also be considered crashworthy since it offers additional strength due to the additional 4” of blockout depth), or do you think blockout strength would have to be determined through FEA or some sort of testing (e.g., pendulum testing, bogie vehicle testing, full-scale crash testing, etc.)?

If the two blocks in question were made of the same materials and had the same cross section, It would seem reasonable to assume they had equivalent strength.

I have another question related to the offset blocks used on the W6x15 posts within the transition limits. During development and testing, MwRSF used 12” deep offset blocks on the W6x8.5/9 posts, but used 8” deep offset blocks on the W6x15 posts. I couldn’t find anything in the reports explaining why this was done. What was the reasoning for using offset blocks with different depths? Do you foresee any issues with using 12” deep offset blocks (instead of 8” deep blocks) on the W6x15 posts in order to use offset blocks with a consistent 12” depth throughout the transition limits? This would simplify construction of the transition.

12” deep blockouts in the upstream transition region were used to offset the rail from the post and mitigate snag in this sensitive area where the rail transitions shape and the posts transition to a tighter spacing. Even still, there was a significant amount of snag on those posts during testing. Thus, MwRSF has maintained that 12” blockouts should be used in the upstream transition area (on the W6x8.5 posts).  8” deep blockouts are common on the downstream regions of transitions where the thrie beam and larger posts often limit deflections and vehicle snag potential. 

For consistency, the use of 12” deep blocks throughout the entire transition is an acceptable alternative to the as-tested configuration, as this would only further limit snag potential on the downstream end. However, 8” deep blockouts are not recommended to be use in the upstream transition region without further evaluation due to concerns for excessive vehicle snag.