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RE: NDS Equation 3.3-6 (CL)[Subject Prev][Subject Next][Thread Prev][Thread Next]
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- Subject: RE: NDS Equation 3.3-6 (CL)
- From: AWC Info <AWCInfo(--nospam--at)afandpa.org>
- Date: Tue, 13 Apr 2004 15:31:34 -0400
Title: RE: NDS Equation 3.3-6 (CL)
Sorry I didn't get to this last week, I've been on travel and Grand Jury duty the past couple weeks and am playing catchup.
I'll try to deal with this in a progressive fashion. All references are from the 2001 NDS or 1997 NDS Commentary. Btw, the 1997 NDS Commentary is now available online in PDF format at http://www.awc.org/Standards/ndscommentary.html
1) Load duration does not apply to compression perp (Fcperp) or modulus of elasticity (E) - NDS 18.104.22.168
2) The load duration factor for the shortest load duration in a combination of loads shall apply for that load combination - NDS 22.214.171.124 and NDS Appendix B(there's a good example of this in the 1997 NDS Commentary 126.96.36.199.) So if wind is part of the load combination, then use C_D=1.6. However, you still need to check the dead load only combination with C_D=0.9, and other load combos with the appropriate load duration factor, to see what controls.
3) Since load duration does not apply to E, a member subject to buckling should be analyzed for the critical load combination after the critical buckling design value has been calculated. (Again, there's a good example of this in the NDS Commentary 188.8.131.52) In other words, you can't divide the load combination by the load duration factor to see which one controls if you are designing a buckling critical member.
4) The column (and beam) stability equations (NDS 3.7.1 and 3.3.3) can be thought of as interaction equations. For short columns, the equation gives results that converge on the compression design value adjusted by all applicable adjustment factors including the load duration factor, Fc*. So as C_P approaches 1.0, Fc' approaches Fc*. For long columns the equation gives results that converge on the Euler value, F_cE. Note that E is not adjusted for load duration in the Euler equation. For intermediate columns, the equation basically provides an interaction value between pure compression and pure buckling. There are a couple of good graphics of this in the NDS Commentary 184.108.40.206.
John "Buddy" Showalter, P.E.
Director, Technical Media
AF&PA/American Wood Council
1111 19th Street, NW, Suite 800
Washington, DC 20036
The American Wood Council (AWC) is the wood products division of the American Forest & Paper Association (AF&PA). AWC develops internationally recognized standards for wood design and construction. Its efforts with building codes and standards, engineering and research, and technology transfer ensure proper application for engineered and traditional wood products.
>The guidance provided herein is not a formal interpretation of any AF&PA standard. Interpretations of AF&PA standards are only available through a formal process outlined in AF&PA's standards development procedures.
From: "Bill Allen" <T.W.Allen(--nospam--at)cox.net>
Subject: RE: NDS Equation 3.3-6 (CL)
Well, Cd doesn't apply to short columns when they are end bearing on a =
and Fcperp governs the design.
You wouldn't multiply the allowable stress by Cd=3D0.9 if the load was =
If you don't buy Cd with regards to unbraced wood beams, what about =
beams? If I had a cantilevered flagpole supporting a sign subjected to =
loads and the WF flagpole had a reduced allowable stress due to unbraced
length (8th Ed. AISC F1.3), are you saying I can't multiply the =
stress by 1.33?
I'm sure there are many, many other examples where the capacity of a =
(using published code values) produce a reduced allowable stress which =
then multiplied by some form of a load duration factor. Yeah, yeah, I =
the "1.33" is disappearing from steel design, but it's not gone yet!
T. William (Bill) Allen, S.E. (CA #2607)
ALLEN DESIGNS (http://www.AllenDesigns.com)
San Juan Capistrano, CA
:From: Roger Turk [mailto:73527.1356(--nospam--at)compuserve.com]
:Sent: Friday, April 02, 2004 10:29 AM
:Subject: RE: NDS Equation 3.3-6 (CL)
:When the column design requirements were based on short, medium and =
:columns, Cd could only be applied to the short and medium columns. =
:their infinite wisdom, AF&PA rewrote the column formula into a single
:formula, they said that they (somehow) included Cd in the short and =
:portions of the formula, but did not include it in the long portion of =
:formula. This is an accomplishment that truly deserves recognition. I =
:not aware that they extended this accomplishment to lateral stability =
:never was stress controlled.
:I don't have my NDS at hand right now, but will be looking forward to a
:response from AWC. (No response =3D Cd should not be applied.)
:A. Roger Turk, P.E.(Structural)
:Bill Allen wrote:
:. > Roger-
:. > What about columns (when Fcperp doesn't govern)?
:. > In either case, NDS allows the use of Cd.
:. > T. William (Bill) Allen, S.E. (CA #2607)
:. > ALLEN DESIGNS (http://www.AllenDesigns.com)
:. > San Juan Capistrano, CA
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