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RE: QUESTION: "Duration Factor" for design of Beam-Column under 1 997 NDS

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You are right, the combination of D + S + 1/2W should have used a
CD=1.6.  However, using CD=1.15 for the D + S portion is conservative.
If you recalc F'c with CD=1.6, you will note that this number is close
to F'c using CD=1.15.  Load duration has minimal impact on F'c
(especially the more slender the column is - short columns will benefit
more from a higher CD).


Charles F. Espenlaub, III, P.E.
Martin-Espenlaub Engineering



-----Original Message-----
From: Bill Polhemus [mailto:bpolhem(--nospam--at)swbell.net]
Sent: Monday, November 13, 2000 12:30 PM
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.