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Re: Construction method for Second Story Additions

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At 09:32 12/3/97 -0800, Norb Volny wrote:
> .... You'll also find that
>the older grading rules and published model codes show that the allowable
>stresses in many types of lumber available 40 to 60 years ago were higher
>than they are now. The lumber available and used then was of a definitely
>higher quality than that used now.

[Bill Cain]  I agree that the lumber in older structures was generally of
much better quality than now, but...

Allowable stresses for a given quality of lumber have also been reduced
over time recognizing failures that have taken place, particularly in
members (such as collar ties and trusses) subject to tension loading or
bending members subject to a large portion of their capacity (e.g., widely
spaced rafters with many layers (too many to meet the code in some
instances) of roofing material added).  It is important to use quality of
the old lumber from observation and/or formal grading along with current
knowledge and methods to evaluate what stress should be used rather than
relying on the allowable stresses from old grading rules and codes.
Cummulative duration of load is a significant factor that also must be
considered.
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BILL CAIN, SE
OAKLAND, CA
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