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Re: 1991 NDS Values for Lumber

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In a message dated 9/13/1999 4:44:39 PM Pacific Daylight Time, 
73527.1356(--nospam--at) writes:

<< I would recommend that *all* analysis of older wood structures be based on 
 the allowable stresses that resulted from the full size, in-grade tests, 
 i.e., the values in the 1991 NDS and later.
 As for equivalent grades, Construction Grade was the commonly used grade 
 prior to 1970 and is equivalent to today's No. 1 Grade.
 A. Roger Turk, P.E.(Structural)
 Tucson, Arizona >>

I would tend to agree with you - the final determination should be based on a 
visual inspection of the lumber that was used at the time of construction. I 
received some private email to set me straight, but I think that there is one 
issue that may have been misinterpreted from my comments.

I would not recommend, nor did I intend to recommend that old stress values 
be evaluated using new NDS formula's. This will not work as the newer 
stresses, although appearing lower in value are adjusted using multipliers 
specific to the new code provisions.

In the back of my mind, while answering the questions, was the common delimma 
of calculating the capacity of a roof rafter or joist to carry a new load 
such as a heavier roof tile. Historically, I found that often a rafter will 
not calculate for the load that was originally placed on it when considering 
the post '91 NDS criteria. However, when calculated using the original stress 
design methods, the joist worked.

One thing to keep in mind is that some programs used to calculate wood 
members are written based on the current NDS standards. It is not possible to 
simply input an older stress value and expect accurate results. I remember 
saving older versions of Enercalc just so I could analize older wood members 
using the design criteria in place at the time the structure was constructed 
(I was too lazy to write my own templates).  I would caution users of current 
software and who are trying to evaluate an older structure to be cautious of 
the results.

Thanks for the warning Roger - your caution is well noted.