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RE: QUESTION: "Duration Factor" for design of Beam-Column under 1 997 NDS[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: RE: QUESTION: "Duration Factor" for design of Beam-Column under 1 997 NDS
- From: Charles Espenlaub <Cespenlaub(--nospam--at)martinaia.com>
- Date: Mon, 13 Nov 2000 13:32:18 -0500
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 184.108.40.206 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.
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