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MGS Installed on 2:1 Slopes

Question
State WI
Description Text

I've been working on developing WisDOT's standard detail drawings for the MGS system and I have a question. I was reviewing MwRSF's crash test report, "Development and Evaluation of the Midwest Guardrail Systems (MGS) Placed Adjacent to a 2:1 fill Slope" (TRP-03-185-10).

 

In this report, MGS with standard post spacing, 6' 4" post embedment was tested at two rail heights (27 ¾" and 31"). The 31" height passed MASH, but the 27 ¾" rail height  failed MASH.

 

What would be the lower height limit for MGS on a 2:1 slope? What  modifications should a designer do a MGS system with standard post spacing and 6'4" of embedment that has had the 31" rail mounting height lessened by overlays (e.g. install a normal length post at the midspan)?

Keywords
  • Guardrail
Other Keywords none
Date October 1, 2010


Response
Response Initially, the development and crash testing program began with the plan to demonstrate satisfactory safety performance for the MGS installed on 2:1 fill slopes using the same lower height tolerance obtained for standard MGS. After the first test failed MASH test designation 3-11, we chose to then re-test the modified barrier system at the same nominal height used for the standard MGS. Although we believe that the MGS installed on 2:1 fill slopes would perform in an acceptable manner at heights below 31 in. (such as for 29 or 30 in.), we are unable recommend these lower height tolerances without first demonstrating acceptable safety performance through the use of full-scale vehicle crash testing.


For situations where pavement overlays are placed adjacent to the MGS installed on 2:1 fill slopes, one possible option would include raising the guardrail post and rail height by an amount approximately equal to the thickness of the adjacent pavement overlay.


We have discussed options for your prior question regarding overlays next to MGS for 2:1 fill slopes. Our options are listed below.


(1) Utilize two bolt holes in the 9-ft long steel posts placed at full-post spacing. If an overlay causing the rail height to drop 2 to 3 in., the W-beam rail and blockout can be mounted to each post using the upper bolt hole in order to retain 31-in. top rail height.
(2) Implement Option 1 plus also install 7-ft long posts at half-post spacings. This conservative option may provide most safety.
(3) Install MGS at 33 in. near 2:1 fill slopes and taper barrier ends to 31 in. after extending beyond 2:1 sloped region. After overlay is placed, the rail height would be about 31 in.


A distant 4th option is noted below but it carries more risk of vehicle override with lower rail height.
(4) Install 7-ft long posts at half-post spacings and between existing 9-ft long posts. Leave rail height at 28 to 29 in. after overlay placed.
Date October 8, 2010


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