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Blockout adjustment for Thrie-Beam

State MO
Description Text

We had a question raised in regards to a low-fill culvert where the post are bolted.  Instead of digging up and replacing the post, the question was asked if the same post could be used with an adjusted blockout to meet the minimum standard height.  The guardrail would be thrie-beam though.  I know this is something that can be done with w-beam in cases, so it makes me think it might be able to be accomplished, but I thought I would check first.


Any help is appreciated. Thanks!

  • Guardrail
Other Keywords Culvert, Thrie Beam, Blockout
Date June 22, 2021
Attachment THRIE1a_system1_new_0_1.pdf

I looked at the configuration you sent. I would have some concerns that the large section (W8x25) posts used over the culvert may pose a significant snag hazard for vehicles.


There has been previous research done for standard W-beam guardrail suggesting that you can raise the blockouts for older W-beam guardrail to the current31” rail height standard by raising the blockouts. Blockout extension has also been done on previous AGT systems in some cases.


The concern here is that the culvert mounted posts have a very large section compared to typical guardrail and AGT posts. Additionally, they are anchored to the culvert which will prevent their displacement laterally. This may result in vehicle snag that produces unsafe vehicle decelerations and/or occupant compartment deformations. Use of thrie beam may improve things to some degree but would not completely alleviate the concern. Thus, while raising the blockout and corresponding rail height seems feasible, we have concerns that the large section posts may not provide a crashworthy solution.


You could raise the blockouts on the approach guardrail on each side of the culvert without an issue.


We do have some options for this type of installation, but they may require some extra effort.


If the culvert is short enough, you can use the MGS long span guardrail system. This allows one to omit posts over the culvert.


We also have a strong post mounted MGS system that can be installed over culverts and a weak post version that may fit your needs. See below.


Strong Post


Weak post



Date June 25, 2021
Attachment TM602371-P1-P4-1.pdf
Attachment TMNo605311-P1-P9-Final-1.pdf
Attachment TRNo-608421-1-Finalv2.pdf