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Re: AAMA TIR A9-91 v. 2005 Aluminum design manual

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According to 1986 AA Spec (1997 UBC Chapter 20 Tables 20-II-A and 20-I-D), the allowable bearing stress for 6063T6 aluminum is 24,000psi(40ksi/1.65<63 ksi/2.34). 
V. Steve Gordin, SE
Irvine CA
----- Original Message -----
From: Jim Wilson
Sent: Saturday, July 18, 2009 11:49
Subject: AAMA TIR A9-91 v. 2005 Aluminum design manual

There are differences in calculation methods between the AAMA TIR A9-91 and the Aluminum Design Manual.
For example, TIR A9-91 calculates bolt bearing in aluminum as FbDt (or FbyDt/1.65) where Fby = bearing yield strength of aluminum.  For 6063-T6, it uses 24000psi.  The 2005 Aluminum Design Manual uses the allowable bolt bearing as 2FtuDt/1.95(factor of safety) with Ftu=30000psi for 6063-T6.  The difference amounts to about 30% more capacity per the Aluminum Design Manual.
The separate 2/3 adjustment for slotted holes is consistent between documents.
Fb when determined from the Aluminum Design Manual Design Guides and referenced section 3.4.5 for 6063-T6 is 31000psi.  This also equates close to 2*30000psi/1.95, but is still far greater than 24000psi. 
My questions are:
1. Is AAMA TIR A9-91 (technically) superseded by the newer Aluminum Design Manual?  The TIR has not been updated since 1991 (the 2000addendum adds to, but doesn't update the 1991 original).  The TIR document does reference the Specification for Aluminum Structures by the AA (page 8 of TIR A9-91), but the referenced table is not in the current design manual.
2. The difference in the allowable bearing stress is substantial - where would TIR A9-91 arrive at a number like 24000psi for 6063-T6?  Has the material spec changed since 1991?  Is there a more conservative way to arrive at allowable bearing stress than the published values in the Aluminum Design Manual?
Thanks in advance,
Jim Wilson
Stroudsburg, PA