Appendix 3



Trail Shelter Structural Analysis
AHJ Engineers
Janaury 2007


From: "Chris Holladay" (cholladay@ahjengineers.com)
Date: January 3, 2007 12:05:52 PM MST
To: (flwarch@ctcweb.net)
Cc: "Keith Jones" (kjones@ahjengineers.com)
Subject: USFS Tail Maintenance Shelter - Washington

Fred,

I had a chance to review the shelters with the following assumptions:

1)      The bending stress for the Douglas fir used in the analysis is Fb=1200 psi.

2)      The bending stress for the Western Cedar used in the analysis is Fb=700 psi.

a.      Both of these bending stresses were the worst case bending stress found in the 1966 American Institute of Timber Construction Manual.

3)      Based on the ASCE 7-02, a snow density of 30 pcf as well as a 5 psf of rain on snow surcharge was used in the analysis.

4)      Neither the foundation, nor the lateral resisting system was review since you were only looking for the roof snow load and snow depth.


If a Douglas fir roof system exists, the roof can support up to 4'-3" of snow with the exception of one beam. The log beam that spans 14' -0" without any knee braces can only support 9-inches of snow. For this particular beam to support up to 4'-3" of snow the beam will either require a center post during the winter or knee brace would need to be added similarly to the front beam.

a)      If you want the Douglas fir roof to support 21'-0" of snow then 2 posts would have to be place under each of the major beams; including the center log truss. These posts would have to be equally spaced under each beam. Except for the 6"-diameter rafters can only support 7'-4" of snow.

If a Western Cedar roof system exists, the roof can support up to 2'-3" of snow with the exception of one beam. The log beam that spans 14'-0" without any knee brace can only support 1-inch of snow. For this beam to support up to 2'-3" of snow the beam will either require a center post during the winter or knee brace would need to be added similarly to the front beam.


a)      If you want the Western Cedar roof to support 12'-6" of snow then 2 posts would have to be place under each of the major beams; including the center log truss. These posts would have to be equally spaced under each beam. Except for the 6"-diameter rafters can only support 4'-0" of snow.

If you have any questions please let me know.

Chris Holladay

AHJ Engineers, P.C.


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