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----- Original Message -----
Sent: Tuesday, May 29, 2001 5:52 PM
Subject: AITC TCM vs NDS

The AITC Timber Construction Manual is a design aide updated every 9 to 10 years.  It incorporates the provisions of the National Design Specification at the time of publication, but the NDS changes every three to five years.  Some wood engineering principles are revised in each new edition of the NDS so it should be used if there is a conflict in design methodology (equations).
Design values for glued laminated timber are developed by AITC and incorporated into the NDS each time it is published.  In addition, AITC maintains NER-466 and updates design values every two years (if necessary).  The NDS supplement should be used to obtain solid sawn lumber values, but AITC 117-xx Design should be used for glued laminated timber values if they are different than those published in the NDS.  AITC issues supplements to AITC 117 which reflect approved NER values.
Thanks for the question.
Mike Caldwell, PE
Executive Vice President