Logged in as: Public User

Guardrai Post alignment and which post bolt hole to utilze

Description Text We have been approached on several occasions lately by various stakeholders as to which is the correct post bolt hole to utilize when bolting standard guardrail? I believe the issue is coming up more frequently due to standard DOT drawings showing all post bolts installed consistently using one post bolt hole or the other and newer DOT inspection forces not having the history of guardrail installations as some of their older (and perhaps now retired) predecessors and counterparts.

Typically most states show 2 post bolt holes – one in each flange – of the guardrail post. Typically the blocks utilized with guardrail panels and guardrail posts are designed to allow bolting to either flange of the guardrail post. Further, the slots in the guardrail are ¾”H x 2-1/2”W and of course a 5/8”D bolt goes through the entire assembly. The slots being as wide as they are would appear to suggest that either post flange bolt hole could be utilized – the one that best aligns for instance.

I’d be interested in learning if MwRSF can provide any information as to which post bolt hole should be used (or must be used) to bolt standard W-beam guardrail to the guardrail post? Does it matter? Is there any testing conducted where the bolting of the panel to the post varied on the MGS to use as an example?

Since field conditions of MGS may not be as consistent as testing laboratory installations and the spacing from post-to-post might vary slightly (typical I have seen is a tolerance of +/- 1”) would MwRSF suggest that the appropriate guardrail post hole to utilize would be the hole that best aligns with the slot in the guardrail?

Any information on this subject that you might be able to provide would be appreciated. Thanks - Greg

  • Guardrail
Other Keywords none
Date October 4, 2018


This issue has been brought up in the past to us as well, and we also have encountered it in testing.


We generally conduct guardrail testing with the bolt on the upstream side of the post for consistency. That said, we have run tests with it on the downstream side as well. We don’t believe that the effect is significant either way.


When you impact a guardrail system, there is lateral loading of the post and longitudinal loading of the post. The lateral loading of the post and compression of the blockout would not really be affected by the post bolt location. With the longitudinal loading, there is some torsion applied to the post through the blockout and post bolt due to tension in the rail. However, the torsion is applied to the post in different directions upstream and downstream of the impact. Thus, changing the post bolt location would not really change the loading of the posts.

Thus, we have generally recommended that states select a consistent bolt location for more uniform installations, but we don’t believe it is required. Guardrail with different connections or terminals may have other considerations, but for the MGS, we would believe this is true.



Date October 5, 2018
Attachment guardrail loads.jpg

Contact Us:
130 Whittier Research Center
2200 Vine Street
Lincoln, NE 68583-0853
(402) 472-0965
Email: mwrsf@unl.edu
The information contained on the Midwest Roadside Safety Facility (MwRSF) website is subject to change without prior notice. The University of Nebraska and the MwRSF is not responsible or liable, directly or indirectly, for any damage or loss caused or alleged to be caused by or in connection with the use or misuse of or reliance upon any such content, goods, or services available on this site.