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Guardrail – Slope Question

Question
State KS
Description Text

I wanted to pass a guardrail-slope question past you.  KDOT uses 6"x8"x6'6" wood posts or W6x9x6'6" steel post for guardrail. Typically, we would use a 10:1 platform that is a minimum of 4' from the face of the rail to the slope break line.   Below are details of our wood and steel post with the 4' minimum platform. Would it be acceptable on a guardrail site to allow a 3:1 slope graded from the back of post (no platform behind post) with our 6'6" posts at normal 6'3" post spacing or would it be preferable to use half post spacing (any nesting?) ?   I recall your study grading a 2:1 slope with 7' posts on half post spacing.  I believe you did not nest the rail either.  Anyway, please give me your thoughts.   I appreciate it! 

Keywords
  • Guardrail
Other Keywords none
Date December 9, 2008
Attachment KDOT wood post.png
Attachment KDOT steel post.png


Response
Response

As you noted, MwRSF has conducted two studies on the placement of strong-post, W-beam guardrail systems near fill slopes.

 

The first study involved the development and testing of metric height, W-beam rail (706 mm = 27-3/4") supported by W6x9 by 7' long steel posts spaced 37.5" on center. The center of each post was placed at the slope breakpoint for a 2:1 fill slope.

 

The second study involved the development and testing of the MGS, W-beam rail (787 mm = 31") supported by W6x9 by 9' long steel posts spaced 75" on center. The center of each post was placed at the slope breakpoint for a 2:1 fill slope.

 

Following this research and upon receiving requests for guidance on the placement of guardrail near slopes, Dean and Bob developed additional guidance for the two designs and for varying fill slopes and fill distances behind posts. This guidance was noted in MwRSF's prior Pooled Fund consulting summaries as well as in the 2007 discussions on MGS implementation. It is as follows:

 

MwRSF (10-29-2007 Email to MGS Implementation Routing List): Recently, the Mn DOT requested guidance for placement of standard and MGS guardrail adjacent to slopes of various configurations. In response to this request and using available crash test data as well as engineering judgment, Dr. Dean Sicking and Mr. Bob Bielenberg prepared the preliminary guidance, subject to refinement in the future. It is as follows:

 

For standard W-beam guardrail:

  1. Standard W-beam guardrail placed adjacent to any slope with 2' of level soil behind the posts is acceptable.
  2. For w-beam guardrail placed 1'-2' adjacent to a 6:1 or flatter slope, standard 6' W6x9 posts at standard spacing are recommended.
  3. For w-beam guardrail placed 1'-2' adjacent to a 3:1 to 6:1 slope, 7' W6x9 posts at standard spacing are recommended.
  4. For w-beam guardrail placed less than 1' adjacent to a 3:1 or steeper slope, 7' W6x9 posts at half spacing are recommended.

 

For MGS guardrail:

  1. Standard MGS guardrail placed adjacent to any slope with 2' of level soil behind the posts is acceptable.
  2. For MGS guardrail placed 1'-2' adjacent to a 6:1 or flatter slope, standard 6' W6x9 posts at standard spacing are recommended.
  3. For MGS guardrail placed 1'-2' adjacent to a 3:1 to 6:1 slope, 7' W6x9 posts at standard spacing are recommended.
  4. For MGS guardrail placed less than 1' adjacent to a 3:1 or steeper slope, 9' W6x9 posts at standard spacing are recommended.

 

Based on your inquiry, the KsDOT provides approximately 29" of fill behind the wood posts and 31" of fill behind the steel posts. For both KsDOT configurations, more than 2' of fill is provided behind the steel and wood posts, thus resulting in guidance that any slope could be used beyond the 24" of generally level terrain. This recommendation is based on the use of 6' long posts in standard W-beam and MGS systems. The use of 6'-6" posts would provide increased post-soil forces over those provided with the 6' long posts. In addition, the safety performance of the KsDOT W-beam guardrail systems using 6'-6" post lengths would be nearly identical for systems installed in level terrain as well as the terrain described in your email. Finally, the two guardrail systems shown below could utilize 6' post lengths instead of the current length of 6'-6" in standard installations with sufficient compacted soil fill is placed behind the posts.

Date December 11, 2008


Response
Response

I am not certain if I told the whole story about the post. I think my situation is close to #3.

3. For w-beam guardrail placed 1'-2' adjacent to a 3:1 to 6:1 slope, 7' W6x9 posts at standard spacing are recommended.


See the attached detail for our scenario. We want to avoid different post lengths from our typical 6'6" post in case these posts/guardrail get hit and the maintenance personnel replaces them with our typical 6' 6" posts. Since we are slightly different than your breakdown of recommendations, would the 6'6" shown in the detail with the 3:1 slope be adequate at normal post spacing?

Date December 16, 2008
Attachment KDOT posts on slope.jpg


Response
Response

Based on your new detail that was provided on 12-16-2008, it appears as though you have a 3:1 fill slope starting at the center of the post. This situation is different from what I assumed according to your previous email which showed a greater region of somewhat level terrain behind the posts. Your situation now appears to be closer to case 4 below for standard W-beam guardrail. For case 4, we have recommended the use of 7-ft long posts at a half-post spacing. However, as noted in the previous email, we tested standard height w-beam rail at the break point of a 2:1 slope with ½ post spacing and 7' long posts. For your installation, you are requesting to use 6'-6" long posts at ½ post spacing at the break point of a 3:1 slope. Based on our understanding of soil behavior with respect to embedment depths and slopes, we believe that this is this installation will have similar stiffness to the 7' posts at ½ post spacing on the 2:1 slope. Thus, we would recommend this configuration.


Please note that case 4 utilized metric height guardrail.

Date December 17, 2008


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