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ODOT has a standard drawing for the socketed weak post that is side mounted to the headwall for box culverts, derived from “SAFETY PERFORMANCE EVALUATION OF WEAK-POST, W-BEAM GUARDRAIL ATTACHED TO CULVERT”. This design uses half-post spacing but performs similarly to MGS at standard post spacing, and typically we transition directly to standard MGS. But we have a situation where the culvert wing wall is within 5’ of the face of rail, therefore we are planning to install half-post spacing MGS attaching directly to the side mounted socketed weak posts.

• Should we be concerned about the risk of pocketing with going from a system with a ~5’ working width to a system with a 3’-6” working width or can it be assumed that it would perform similarly to when we connect standard post-spacing to half-post spacing in a regular run of guardrail?


• Since we only have 2’ from the headwall to where the face of the rail is going to be, we are concerned that there is not going to be adequate space for post rotation. Therefore, I was wondering if non-blocked MGS at half-post spacing would be acceptable? The report (MwRSF TRP-03-262-12) for non-blocked MGS does not recommend using the design with reduced post spacing without further research, but we now know the original half-post spacing test failed due to the full sized blockout initiating a tear in the rail. What are your thoughts on non-blocked MGS at half-post spacing?

The weak-post MGS installation in the sockets is based on the MGS bridge rail system that was developed and tested through the pooled fund. This system uses S3x5.7 posts at half-post spacing to produce very similar behavior to the standard MGS at full- post spacing.

https://mwrsf.unl.edu/researchhub/files/Report53/TRP-03-226-10.pdf

For example, the standard MGS had a dynamic deflection of 43.9” and a working width of 48.6” under MASH TL-3 testing. The MGS bridge rail with the weak posts had a dynamic deflection of 48.9” and a working width of 53.2” under MASH TL-3 testing.

In the MGS bridge rail research, we determined that a transition was not required between the bridge rail system and the standard MGS due to their similar deflection characteristics.

In your installation, the wing wall prohibits the use of blocked out MGS as the posts would be close to the wing wall which would prohibit post rotation (you current show about 2’ behind the face of the rail).

The best option would likely be the non-blocked MGS system at standard post spacing. This would provide for adequate room behind the posts for rotation in the soil while providing the closest stiffness to the weak-post MGS at half-post spacing. The non blocked MGS had a dynamic deflection of 34.1” and a working width of 43.2” under MASH TL-3 testing. While this is a somewhat stiffer response than the MGS bridge rail, transitioning from the non-blocked MGS to the weak-post would not be an issues as you are going from a stiffer to a less stiff system. Reverse direction impacts from the weak-post system to the non-blocked system would be more of a concern due to an increase in barrier stiffness, but we would not expect a serious transition stiffness issue in that direction either.

Using half post spacing MGS would pose minimal benefit when transitioning from standard MGS to the weak post system and would pose a greater stiffness differential in the reverse impact direction. Also, that system is not recommended for reduced post spacing at this time due to concerns with increased small car snag on the posts and associated velocity drops. Thus, we would not recommend the reduced post spacing at this time.

Let me know if that addressed your questions or if you want to discuss further.

Thanks!