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RE: 1991 NDS Values for Lumber

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Neil:

My understanding of the code leads to the following:

Lets use the example of a 24' long 4x14 DF#1 purlin.  The allowable moment
(neglecting repetitive use and noting that the moisture content will not
exceed 19%) are as follows:

The "old" values
S(fb)(Cf)(LDF) = 102.41in^3(1500 psi)(12/13.25)^1/9(1.25)(1
kip/1000#)(1ft./12in.) = 15.84 kip-ft.

The 1991 NDS leads to the following:
S(fb)(Cf)(LDF) = 102.41in^3(1000psi)(1.0)(1.25)(1/12000)= 10.67 kip-ft.

Are these the design moments that should be used?

Joe

-----Original Message-----
From: nmoore(--nospam--at)innercite.com [mailto:nmoore(--nospam--at)innercite.com]
Sent: Monday, September 13, 1999 4:38 PM
To: seaint(--nospam--at)seaint.org; Seaint@Seaint. Org
Subject: Re: 1991 NDS Values for Lumber


Joe:

If  I understand you correctly, you are just plugging in the new NDS values
into the older code formulas. Or are you using the 1991 NDS formulas and
all of the adjustment factors that have to be considered?

Neil Moore, S.E.


At 03:22 PM 09/10/1999 -0700, From the desk of Joe Otto wrote:
>Everybody knows that the 1991 NDS published values for lumber which were
>significantly less than previously allowed.  I don't have any problems with
>this.  However, it has put a wrench into analyzing existing roofs.
>
>Calculations on existing roofs using the 1991 NDS values often don't work
>for the existing loads let alone any additional loads.
>
>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
>
>
>
>
>