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Vertical Adjustment of the Midwest Guardrail System

Question
State IL
Description Text

We are looking at installing some new Midwest Guardrail System to replace corroded weathering steel guardrail.  This is a very worthy safety improvement, and we want to get it accomplished as quickly as possible.  However, the location is programmed for a resurfacing project to follow within a handful of years (3 to 5 probably).  The resurfacing thickness will be 4.25 inches, and thus the height of the relatively new guardrail would be out of tolerance.

We are wondering if it would be acceptable to specify an additional bolt hole in the flange of the steel post such that the wood blockout could be raised by about 4" to match the new height of the overlay?  This would raise the top of the blockout and rail by this amount above the top of the steel post.  Also, we would be able to add some fill material around the posts, but it would be very difficult and expensive to compact this mechanically.  Perhaps we could use a dense graded aggregate that would at least develop some cohesion with moisture and natural settling into place.  Is this a reasonable method to allow for one vertical adjustment of the MGS?  Would there be any other steps we could take to make this idea successful?

Keywords
  • Guardrail
Other Keywords none
Date October 4, 2007


Response
Response

I have spoken with several of my colleagues regarding the MGS situation that you will likely encounter with the future overlay. For the standard MGS, we believe that the MGS will perform acceptably if you raise the rail height approximately 4 in. along with a similar increase in the asphalt overlay. As such, you could have an extra guardrail hole placed in the post's flange such that the rail and blockout could be raised at a later date. Note that with a longer load height above the ground line, the posts will yield at a lower magnitude of load and result is higher dynamic rail deflections.

There are additional issues to consider. Originally, the MGS R&D program was begun with 5-ft long foundation tubes. When the rail was raised, so was the slope of the anchor cable. This resulted in the foundation tubes to begin to be pulled upward and out of the ground during impacts. Further testing with 6-ft long tubes showed that this behavior was mitigated. Now, with another increase in rail height, it may be necessary to use even longer foundation tubes, such as 7 or 8-ft long tubes. Since we have used 8-ft tubes in other systems, you may want to use that length to ensure that pullout will not occur.

Finally, it has been noted that one could also use non-standard posts (slightly longer) such that the post would extend slightly above the blockout when first installed and also would use two holes in the posts. The extended post would be a reminder that this guardrail was intended for a future overlay.

In summary, we feel that your proposed MGS applications would be acceptable when incorporating the primary consideration noted above.

Date October 5, 2007


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