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Shorten MGS Post Length to Bridge Guardrail over Two Large Pipes

State IN
Description Text INDOT has come across a location where we have twin 10ft span x 7ft rise pipe arches on a 45 degree skew.  The cover over these pipes is 2.5 ft so a standard 6 ft MGS post cannot be driven over the pipes.  We do know that there are some weak post socket designs that will be completed soon.  However we would like to ask the following question in the anticipation that our maintenance staff may question why we want to introduce a new type of post to stock in our inventory.  Our question is, would it be an option to reduce the post length to 5 ft and space the posts at 3'-1.5", assuming a working width of 5.0 ft (similar to the 6'-3" post spacing)?  There would 2 ft (10:1 max.) of embankment behind the back of the posts.  We do understand that reduced post spacing has not been MASH tested but based on other tests would it be possible to give some guidance. Thank you
  • Guardrail
Other Keywords Post Length
Date September 19, 2018
Attachment Plan View of Twin Pipes to be Bridged with Guardrail.pdf


That’s a tough problem that doesn’t have a lot of well defined an tested solutions. The situation you have appears too large for a long span system, and you appear to have resistance to the weak post version of the MGS. 


As you noted, one approach would be to use half post spacing with reduced (28”) embedment. That is a reasonable approach. The concern is that we have very little data with respect to post with embedment less than 36”. There is concern that a post with such a limited embedment may rotate too easily or pull out of the ground and greatly reduce the lateral resistance of the system. Thus it is hard to recommend that approach without further research to quantify the post response at the reduced embedment. 


The only other solution I can envision is related to work we completed on the MGS attached to a culvert headwall 9” below grade. This system used strong posts bolted to the culvert slab at 1/2 post spacing. Thus, we could do something similar here by building a slab footing below grade and using this type of post and attachment. One would have to design the slab to resist the post moment, but it should be achievable. 


I can send you the details on the post and connection if you would like. 




Date September 20, 2018


Thank you for the review and suggestions.  Given we have a little time, we may try to persuade others to try the weak post system.  We have looked at mostly using the weak post system with a concrete cylinder connected to the top of a culvert.  I see there is also a slab option that may be available, see picture below.  Would it be reasonable to place concert as deep the steel tube?  Also, do I remember correctly there is an option to have just a concrete cylinder that is not attached to a culvert?  I know this report is coming soon, so if I need to wait I will.  Thanks again.

Date September 21, 2018
Attachment IND_Obs.jpg


There are two weak post options available for attachment to culverts. We did not test them as standalone options. One we as concrete cylinder that was anchored to the culvert slab and the other was a steel socket. NDOT asked us to try an unreinforced concrete slab, but that did not perform acceptably.


In order to use a slab type system for weak posts, it would have to be larger and reinforced such that it could develop the post capacity.

This could be a 18” deep reinforced slab or an approximately 12”x24” reinforced vertical wall that has the sockets for the post built in it.


Does that make sense?




Date September 24, 2018
Attachment 12-5TopSocket_R10.pdf
Attachment 36SocketCulvert_R3.pdf
Attachment 36TopSocket_R7.pdf

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