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Tolerance on the Length of Wood Guardrail Posts

State WY
Description Text Have you ever encountered any established tolerances on the length of wood guardrail posts?
  • Guardrail
Other Keywords Wood Post
Date March 8, 2018


I don’t believe that I have ever seen established tolerances for wood post lengths, but I have some thoughts.


We generally use a relationship to describe the changes in post-soil forces that varies with the square of the ratio of the embedment depths of the post.


F2 = F1(d2/d1)2


So, if we increase embedment depth from 40” to 43”, we expect the post-soil forces to increase by a factor of 1.156.


The concern with wood post with increased lengths would be that increased length could significantly increase the post-soil forces and lead to fracture of the wood post rather than rotation through the soil. Fracture of the post can lead to pocketing, increased rail loads and fracture, and vehicle instability similar to a system with an omitted post.


Based on these concerns, we would recommend that the post length tolerance for wood posts be around plus or minus 2”. For a 40” post embedment, this limits the variation in post soil forces to around 10%. Variation more than this may start to adversely affect barrier performance.


Let me know if you need anything else.

Date March 8, 2018

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