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Tolerances for Timber Posts

State WY
Description Text What should the tolerances be for timber posts and block outs? For example, the standard barrier guide shows in the drawings a 6"x8" posts as a 6" x 7 7/8" with a tolerence in the direction parallel to the bolt of 1/4 inch, presumably applied to 7 7/8" dimension. No tolerence is indicated for the width dimension of 6". Does this mean it must be 6 inches exactly? Likewise, what should be the tolerences for an 8" round post?
  • Guardrail
Other Keywords Timber, Post, Tolerance
Date May 31, 2013


First off, I believe the drawings for a timber post should read 6" x 8".  Back in the older guide (1979) the dimensions were listed as 6"x8".  During the metric conversions into the 1994 Guide, the section was converted to a 150mm x 200 mm post through a soft conversion and rounding.  Now, in going back to US Customary Units, someone has converted the 200 mm to 7.87", or 7 7/8".  Thus, two conversions (and rounding twice) has resulted in the 7 7/8" dimension.  I high doubt that this 1/8" difference is going to cause issues, but I thought that the 7 7/8" dimension was a bit silly when the rest of the dimensions for the post are in whole integers.


As far as the tolerance issues, a tolerance of 1/4" was specified for the depth of the post because it has a far greater impact on post bending strength than the width of the post does.  Thus, this tolerance was set to hold the strength of the post and prevent premature fracture.  A larger tolerance window may be acceptable for the post width.  However, you don't want to alter the width too far from 6" as this will result in a change to the post-soil interaction forces.  Some states have adopted the 1/4" tolerance for both the depth and the width of a timber post.  You may choose to do this as well, or loosen the tolerance as you see fit.

Concerning round timber post, the 1/4" tolerance can be used for the post diameter in the same manner as the rectangular post depth.  It should be noted that under sized diameters are far more critical than oversized diameters because of the possibility of fracture.  Thus, timber posts with diameters greater than the upper tolerance window are far less of a concern than posts with smaller diameters.  I believe the MGS round timber post report (TRP-03-179-07) describes this in more detail. 

Date June 3, 2013

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