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Increased Load Height for Guardrail

Question
State MO
Description Text

A safety project on a narrow two-lane road will require certain locations to be shielded with guardrail.  On this particular road, there is neither a shoulder nor any useable flat area to offset the barrier from the edge of traveled way.  A 12 in. offset to the face of the rail was chosen so as not to further constrict the width of the traveled way which is as narrow as 9 ft. in certain locations.  As shown in the attached figure, a 2H:1V inslope begins immediately at the edge of pavement, resulting in nearly 1.25  ft. of additional exposure/decreased embedment.


Since this arrangement would yield more post exposed than embedded, it is not likely to be acceptable at the 7 ft. post length as shown.


Question: Could a guardrail system with 9 ft. posts on 3 ft. 1-1/2 in. spacing, placed 24 in. down a 2H:1V slope be reasonably expected to perform at MASH TL-3?

Keywords
  • Guardrail
Other Keywords embedment
Date May 29, 2014
Attachment Decreased Embedment.bmp


Response
Response We would agree that the system shown in the attached detail would not likely perform acceptably under MASH TL-3 impact conditions based on current testing of the MGS system adjacent to slopes at both MwRSF and TTI. The main issues are the rail height, the post offset down the slope, and the overall post length/embedment. 

Current TL-3 MASH testing of the MGS system has shown acceptable safety performance when tested with:
1. Standard post spacing, 12" deep blockouts, 9 ft long posts installed with the post at the slope break point of a 2:1 slope, and a 31" rail height, as tested at MwRSF.
2. Standard post spacing, 8" deep blockouts, 8 ft long posts installed with the face of the rail at the slope break point of a 2:1 slope, and a 31" rail height, as tested at TTI.

Testing of the MGS with standard post spacing, 12" deep blockouts, 9 ft long posts installed with the post at the slope break point of a 2:1 slope, and a 27.75" rail height did not meet the MASH criteria due to vehicle instability. 

Based on these tests, there are concerns that the lower rail height, short embedment, and increased post and rail offset shown would not be acceptable due to concerns for vehicle capture, increased barrier deflections, and vehicle stability. 

The use of 9' posts at 1/2 post spacing may improve concerns with barrier deflections, but the large rail off set and the rail height would still be a concern. No testing has been done to date with the larger proposed rail offset and concerns exist regarding vehicle capture and stability. The rail height would pose similar concerns. Thus, it is difficult to recommend that configuration with respect to MASH TL-3 safety performance.

Currently the most aggressive installation available at standard post spacing would be the MGS installed with standard post spacing, 8" deep blockouts, 8 ft long posts installed with the face of the rail at the slope break point of a 2:1 slope, and a 31" rail height. Rail heights below 31" and the larger offsets could not be recommended at this time. It should be possible to  integrate this design with your existing guardrail by transitioning the guardrail from 29" to 31" top rail height over 25'-50' of guardrail as the area with the 2:1 slope is approached and employing the longer 8' posts and shorter blockouts tested in the TTI system. I have attached the TTI report on that system.
Date June 5, 2014
Attachment G-rail-on-slope.pdf


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