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

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Bill

I believe you to be correct. The 4th Edition of the AITC Timber Construction
Manual has procedures and examples for combined bending and axial
compression. In the examples, C sub D for the check of the combined effects
of bending and compression uses the shortest duration of load, which is 1.6
for wind. The 1.6 is applied to F sub c and F sub b.

You might want to contact Brad Douglas, Director of Engineering, at the
AF&PA. 202-463-2768 to verify.


Bruce Pooley
Not a code expert, but I did work on the 4th edition of the TCM.

Timber Design
3448 South Newland Court
Lakewood, CO 80227


----- Original Message -----
From: "Bill Polhemus" <bpolhem(--nospam--at)swbell.net>
To: <aec-residential(--nospam--at)polhemus.cc>; <structx(--nospam--at)polhemus.cc>;
<seaint(--nospam--at)seaint.org>
Sent: Monday, November 13, 2000 10:29 AM
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.