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Triple Blockouts

State OH
Description Text We have a guardrail replacement project that is wrapping up on Interstate 70 in Columbus.  Because of an inlet, the contractor has used triple 12" blockouts at several approach locations to a structure.  The inlet is in poor condition and the project engineer is concerned that the structure is too weak to properly support the load of an impact.  From past questions I see that a triple 8" blockout can be used under certain conditions.  Is a triple 12" blockout ever an option?  If so, can it be used at a transition section such as this?  Could some combination of triple blockout and steel post attached to the inlet be used?  Is the only option in this case to remove the old inlet and use steel posts attached to the top of the new inlet?  Thanks!
  • Approach Guardrail Transitions
  • Guardrail
Other Keywords none
Date December 1, 2015
Attachment old guardrail.JPG
Attachment IMG_4322.JPG
Attachment IMG_4323.JPG
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Attachment IMG_4467.JPG

Response In the past, we have recommended no more than one triple 8" blockout installation very 50' for guardrail installations. This is based on concerns that the ability of the triple blockout to transmit load to the post would be compromised for large deflections. With regards to transitions, we have used a similar rationale and have limited the installation of triple blockouts to a single post in the transition at limited locations. For your installation shown, we believe that the number of consecutive triple blockouts is likely too many. 

The MGS utilizes 12-in.deep blocks for standard applications as well as for special applications. For example, the MGS long span design utilizes one 12-in. wood block with three CRT posts instead of two stacked 12-in. deep blocks. For the MGS, it would seem reasonable that the use of two 12-in. deep stacked blocks could be accommodated at a few locations as well, thus also resulting in a rail offset of 24 in. However, it is uncertain as to whether the use of two 12-in. deep blocks may be too excessive when used continuously with the MGS.

Thus, based on previous testing of systems with deep or extended blockouts and an analysis of the contact lengths of typical MGS testing, MwRSF would recommend the following:

1. Double standard blockouts or combinations of blockouts up to 16-in. deep may be used continuously in a guardrail system.
2. Triple standard blockouts or combinations of blockouts up to 24-in. deep should be limited to one in any 75 ft of guardrail.

There is currently a problem statement in the Year 27 Pooled Fund Program to address this issue specifically, "Additional Options for Post Conflicts within the Approach Guardrail Transition". 

With respect to attachment to the top of the inlet, that would depend on the connection to the inlet and the relative stiffness of that post configuration. One would also need to be sure that the inlet attachment had sufficient structural capacity as the short, stiff post may overload the top of the inlet. 

Date January 25, 2016

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