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MGS Long Span - TRP-03-187-07

Question
State WY
Description Text

Two questions:

 

Question1 1 - Slope Requirements Question:

 

The first test conducted under this study was in a configuration shown on page 15 of the report with the headwall of the box culvert 1 foot behind the back of guardrail post line.  No specific slope criteria was specified in this drawing.

 

The second test conducted was with the headwall of the box culvert flush with the back of post line.  In this drawing on page 65, it shows a 3:1 slope 2 feet behind the back of post line for a distance of six post spaces upstream and downstream of the installation.

 

Was the 3:1 slope a requirement for the installation to work properly, or would any slope at the slope break line (located 2 ft. behind the back of post line) be acceptable?  e.g. 2:1 or steeper, and/or flatter than a 3:1?

 

There is no mention of this in the summary, conclusions and recommendations, so perhaps this was only to document the installation as it was tested?

 

Question 2 - CRT Details:

 

In preparing our standard plans for the MGS, we noticed Road Systems is has created a "universal CRT post" as shown in the attachment.  This way they can use the same post for w-beam SKT's as well as MGS SKT's.  The hole locations are midway between what is used for conventional w-beam an that specified for the MGS as detailed in Midwest's report (and also attached).  Is it O.K. to use this "universal" CRT post for the long span as well?  Every chance we can get to reduce the total number of stockpiled components really helps our maintenance folks as well as fabricators.

Keywords
  • Guardrail
Other Keywords MGS, Long Span, Culvert
Date December 12, 2012
Attachment Universal CRT Post.docx


Response
Response

Question 1

With regards to the slope used in the second MGS long span guardrail test, a 3:1 slope was started 610 mm (24.0 in.) behind the back face of the guardrail posts of the MGS long-span, and the wingwalls were modified to match the soil slope. The choice of the slope profile was based on choosing the flattest slope of the typical culvert installations submitted by the sponsoring states. The choice of the flattest slope maximized the potential for vehicle interaction with the wingwalls of the culvert during the impact event.

Based on this, it is reasonable to assume that steeper slopes would be acceptable as long as sufficient offset was provide to the slope point in order to provide proper soil resistive forces for the posts. We have recommended a 2' offset distance in the past from the back of the post to the slope break point. For areas where you cannot get the required 2' offset we have provided additional guidance on post lengths. See the link below.

http://mwrsf-qa.unl.edu/view.php?id=322

At this time, we do not have data to determine the effect of flatter slopes with the long span. Flatter slopes than 3:1 may be acceptable, but there is concern that vehicle snag on the downstream wingwall would increase significantly as the slope flattened. Thus, at this time it may not be possible to recommend flatter slopes with the MGS long span without further study and analysis.

Question 2

We would not recommend switching to the RSI CRT post. The RSI design would place the CRT holes approximately 2" higher that the CRT posts used in the design and testing of the MGS long span. This would put the top CRT hole slightly above groundline.  There are a couple of concerns with respect to the change in the location of the CRT hole. First, the change may adversely affect the loading and fracture of the hole and change the breakaway forces and energies of the post. Second , the higher hole would increase the amount of post sticking above groundline after then post fractures and potentially increase vehicle interaction/tripping/snag on the fractured lower post section. The effect of these concerns on the performance of the long span is difficult to quantify. Thus, we cannot recommend changing the posts.

Date December 13, 2012


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