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Alternate blockout material

State OH
Description Text ODOT is using our old Type 5 guardrail standard GR.3.4 as a transition from deep beam bridge guardrail to the  MGS barrier.  The GR 3.4 drawing permits wood blockouts only.  Would alternate materials be acceptable? 
  • Approach Guardrail Transitions
Other Keywords none
Date May 21, 2018
Attachment MGS transition to bridge railing.pdf
Attachment DBR to MGS transition.pdf

Response Multiple alternative material blockouts have shown to perform similarly to wood blockouts (both in testing and in practice).  As such, MwRSF does not have concerns with using blockouts made from other materials as long as the are the same size as the wood blockout it is replacing, and the alternative blockout has the strength to keep its shape during impact events.

There have been multiple proprietary blockouts made from plastics and/or composite materials that have been shown to have adequate strength to perform similarly to wood blcokouts.  Additionally, steel tube blockouts could also be utilized as alternatives to wood blockouts (steel tube blockouts are regularly used in some thrie-beam transition designs).

Conversely, steel I-shaped blockouts have caused issues in the past.  With only a thin web between the front and back flanges, steel I-shaped blockouts have been shown to bend and fold over during impacts, effectively reducing the offset distance between rail and post. As a result, steel I-shaped blockouts can cause increased snag and increased risk of rail tearing due to rail contact with the post flange edges.  Thus, the use of steel I-shaped blockouts is not recommended without further evaluation.
Date May 22, 2018

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