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As per our conversation at the Pooled Fund annual meeting, it was advised that the MGS standard steel post spacing (6’-3”) can be installed in the attached location, where there is 4’-6” from the edge of shoulder to the bridge pier. In the case that it is desired to follow a min working width of 60 inches, a half post spacing MGS can be proposed. I believe there is not a transition from the half post spacing to the standard post spacing MGS. However, is there a minimum reduced post spacing section needed before adding the standard MGS, on the west side of the pier?  

On the east side of the pier, can a guardrail end terminal be connected to the half post spacing MGS?

If we are recommending the use of the standard MGS, how can be this supported if the working width is not being met? In other words, can the design be justified based on the dynamic deflection as an alternative reference?

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Continuing from my previous email, planning is proposing to set the face of guardrail 1 ft off the edge of shoulder to mitigate potential snowplow damage. With the guardrail width of 1.76 ft, this would result in approximately 1.73 ft clearance between the face of the pier and the back of the guardrail.

We would appreciate any guidance on this.

Sorry for the delay in replying. My email backlog from the pooled fund meeting and the TF13 meeting was deep.

If the space behind the barrier system has been reduced, you will likely need to go with some form of reduced deflection MGS system.

As I read the email below, it appears that you originally had 4.5 feet of space between the edge of shoulder and the pier, and the rail has been offset 1’ from the edge of shoulder. That leaves you with 42” of space between the pier and the front face of the guardrail.

TTI has previously tested the MGS with ½ and ¼ post spacing options under MASH (https://www.roadsidepooledfund.org/wp-content/uploads/2025/12/TRNo610211-01-Final-REV-2.pdf). In that testing they used 8” blockouts and the blockouts were shortened for the ½ post spacing option. The ½ post spacing system had a working width of 43.1” and a dynamic deflection of 25.6”, while the ¼ post spacing system had a working width of 37.1” and a dynamic deflection of 19.5”.

The ½ post spacing option had a working width just over the 42” of space you have allotted. This is likely the post rotation back during the test accounting for the working width. The dynamic rail deflection and associated vehicle position was considerably less at 25.6”. Thus there is potential to use the ½ post spacing option in this application.

If you wish to be more conservative, the ¼ post spacing will meet the working width.

Another option would be to utilize a reduced deflection version of the MGS developed in NCHRP 22-38. This system uses a continuous tube in lieu of blockouts and ½ post spacing. This system had a TL-3 dynamic deflection of 10.7” and a working width of 28.8. This system is detailed in NCHRP Report 1100.

We cannot recommend direct attachment of MGS end terminals to a reduced post spacing system. We have typically recommended at least one 12.5’ rail section of standard post spacing between the reduced spacing system and the maximum stroke length of the terminal observed in test no. 3-31.

Thanks!