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Re: 1991 NDS Values for Lumber

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In a message dated 9/10/99 3:25:07 PM Pacific Daylight Time, 
jmo_engineering(--nospam--at) writes:

<< Some jurisdictions require the use of the 1991 NDS design values on
 everything.  Others say that if it was built before 1986 you can use the
 "old" values.
 I think it would be helpful if there was a mandate made by those
 knowledgeable in the subject which indicated when the 1991 NDS values are
 required to be used in all jurisdictions.
 Any thoughts on this subject would be appreciated.
 Joseph M. Otto, P.E.
 Ireland Engineering
 Fremont, CA

The problem is that the 91 standards is based upon the results of a twenty 
year study on the quality of wood. Although the quality of the wood may be 
the same after the '91 NDS was published as it was in 1980, the old stress 
values were allowed by code prior to adoption of the '91 NDS.
In my opinion, there is sufficient room in the factor of safety (as well as 
the fact that this is a statistical average for wood) to accept the prior 
code stress when recalculating the member for revised loading or remodeling 
occuring after the adoption of the '91 NDS. 
I have faced this problem often when calculating the capacity of a roof 
member for new roofing. Often the members won't calculate due to the 
reduction in stress. However, I tend to inspect the wood and if there appears 
to be no noticible effect of the existing loads I accept the capacities 
allowed at the time the member was installed in the capacity for new roofing.

Remember that a visually or mechanically graded member may change if the 
rafter or joist is cut to fit the field conditions. The grading is based upon 
the shipping condition of the wood which considers the location and spacing 
of imperfections over the length of the member shipped. However, if you were 
to cut off both ends of the piece, you may be altering the ratio of 
imperfections to good wood and inadvertently changing the ratings.

You need to inspect and use your professional judgment when qualifying a 
piece of wood for allowable stresses. I think that consideration to the 
existing loads applied for the length of time it has been in service is a 
strong consideration for how the new applied load will affect the member.

Dennis S. Wish PE