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RE: QUESTION: "Duration Factor" for design of Beam-Column under 1997 NDS

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The wording in the 91NDS was the same.  The fourth edition of Timber
Construction Manual, published in 1994 by American Institute of Timber
Construction, has an example on page 5-196 that says 1.6 is the factor to
use for checking with wind load included.  The second edition of Wood
Engineering and Construction Handbook, copyrighted 1995 by McGraw-Hill shows
a similar use of the 1.6 factor in an example on page 3.27.


-----Original Message-----
From:	Bill Polhemus [mailto:bpolhem(--nospam--at)swbell.net]
Sent:	Monday, November 13, 2000 11:30 AM
To:	aec-residential(--nospam--at)polhemus.cc; structx(--nospam--at)polhemus.cc; seaint(--nospam--at)seaint.org
Subject:	QUESTION: "Duration Factor" for design of Beam-Column under 1997
NDS

I just completed the first two modules of a seminar on "Designing with Wood"
distributed by Auburn University's Continuing Education Extension Service.
It
was really very good, and I do recommend it, expecially for those like me
that
really don't have the time to seek out C.E. seminars. The price was very
reasonable and you have the tapes and other materials to refer to later when
you
are done! You take an "exit exam" and send it to AU, and they issue you a
certificate, and also do the "bookkeeping" required by those states where
you
must have PDHs to maintain licensure (I am licensed in Alabama which is why
I
have to do this).

But I have a question about one of the design examples given in the seminar.

The example was of a stud acting as a beam column, having axial compression
load
from Dead and Snow Loads, and also lateral loading due to Wind.

The compression loading and the bending were checked separately, of course,
and
then the combined axial compression and bending was checked.

But in the latter case, the Duration factor C-sub-D was set at 1.15, which
would
be appropriate for SNOW loading or combined D+S. I would have thought that,
since 2.3.2.2 of 1997 NDS states "The load duration factor, C-sub-D, for the
shortest duration load in a combination of loads shall apply for that load
combination", the Duration Factor would be 1.6 (appropriate for Wind loading
and
combinations containing Wind, as stated in Table 2.3.2 of the 1997 NDS).

In the example given in the seminar, however, the presenter simply took the
allowables from the two separately-calculated cases ("D+S" and "W only"),
and
then for applied loads used the D + S + 1/2 W combination.

I would think you'd have to go back and recalculate F'c using the C-sub-D of
1.6.

Am I wrong, or is he? If I'm wrong, why? Some NDS code expert please
enlighten!

Regards.