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Box culvert post and blockout

State AZ
Description Text

I have recently installed an anchored metal base plate post to the top of a concrete box culvert. We used 12 inch mondo blockouts in lieu of 8 inch.  The reason I installed 12 inch was because the system was changed to MGS which utilizes 12 inch blockouts so I continued the guardrail runs over the box culverts this way. The owner is FHWA utilizing Arizona DOT drawings which show an 8 inch blockout.  They have not approved my 12 inch blockouts and want us to install 8 inch unless we can get a verifiable response that 12 inch blockouts are crash worthy. Can you please help me out with this?

Attached herewith are the details. The rail height is 31 inches. The position of the post from headwall is different at each location but between 2' minimum and 6'.  There is a combination of through bolts and embeds depending on if the base plate fell on an interior or edge wall. All the anchors are 7/8" diameter with the embeds placed 6" deep utilizing simpson ET HP epoxy.  If you need more information I'll get back to you right away.

  • Guardrail
Other Keywords Culvert, Blockout
Date June 25, 2021
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The system that you sent details for appears to be a culvert mounted strong post guardrail system that was developed and tested at TTI and based off work we had done previously under NCHRP 350. The report for the TTI research is linked below.


The TTI system was full-scale crash tested to MASH TL-3 with 8” deep timber blockouts. We have conducted most of out testing on the MGS with 12” deep blockouts, including a similar culvert mounted guardrail system (


Our current experience with blockout depths and 31” guardrail systems has been that the 8” and 12” blockouts have performed similarly. The 12” blockouts provide some benefit in terms of reduced vehicle snag on the posts and maintaining the rail height as the barrier deflects. In the case of this system, we don’t see an issue with using the deeper blockouts for this system.


You may want to contact the researchers at TTI and see if they would concur with that conclusion as they designed and tested this version of the culvert mounted guardrail. William Williams was the author of that research and is very helpful. His contact information is listed below.


William Williams, P.E.

Associate Research Engineer

3135 TAMUS

Texas A&M University System

College Station, Texas 77843-3135

(979) 317-2707 (office)

(979) 575-5200 (cell)


Please let us know if you have further questions.

Date June 28, 2021

I have contacted Mr. Williams but he is very busy and has not gotten back to us yet. This is time sensitive with safety. I understand 12 inch blockouts are approved and you have asked me to ask Mr. Williams to concur. I understand there is no central approval authority. I have copied Mr. Cooper of Arizona DOT and Ms. Quintana of FHWA on this email as well so we can all be in the loop. Please let us know any other appropriate steps to take to be approved of 12 inch blockouts for this project. Your assistance is greatly appreciated.

Date June 29, 2021

Just to be clear, myself nor Midwest Roadside Safety Facility can provide approval on any system. We are a research facility that also conducts crash testing. We are happy to provide engineering support and guidance on systems that we have developed here.


With respect to the system you noted previously, we do not believe that the use of 12” blockouts would be an issue, but we have not tested it in that configuration and we did not design the system. We are basing that comment on previous testing of similar barrier systems with 12” and 8” deep blockouts.


If you require a system that for installation on culverts that has been crash tested with 12” blockouts, we have a similar system that was developed and tested in cooperation with the Wisconsin DOT. It has met both MASH crash tests for TL-3 with 12” blockouts. See the report below.



Date June 30, 2021