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# RE: NDS Equation 3.3-6 (CL)

• To: <seaint(--nospam--at)seaint.org>
• Subject: RE: NDS Equation 3.3-6 (CL)
• From: "Bill Allen" <T.W.Allen(--nospam--at)cox.net>
• Date: Mon, 5 Apr 2004 23:13:15 -0700

```Roger (and everyone else who may be interested),

I've looked more into NDS equation 3.3-6 and have realized that, since Cd is
used in Fb*, it's an integral part of CL. Doing a little trial and error, it
appears that, as Lu increases, The effects of Cd diminish. In other words,
for high values of Lu, Fb' with Cd = 1.25 approaches Fb1 with Cd = 1.

I understand the point you (Roger) make about lateral stability being more
of a function of stiffness than strength, but most of the formulae dealing
with lateral stability (apparently except for this one) are independent of
load duration effects. My previous example of a steel WF "flagpole" is one
example. Another is LRFD. While I don't profess to know much about limit
states, I'm fairly certain that load duration has nothing to do with the
left side of the equation phi*R_sub_n >= R_sub_u.

I guess it's a quandary I'll have to deal with for a while.

Sigh...

T. William (Bill) Allen, S.E. (CA #2607)
ALLEN DESIGNS (http://www.AllenDesigns.com)
San Juan Capistrano, CA

:-----Original Message-----
:From: Roger Turk [mailto:73527.1356(--nospam--at)compuserve.com]
:Sent: Friday, April 02, 2004 10:29 AM
:To: seaint(--nospam--at)seaint.org
:Subject: RE: NDS Equation 3.3-6 (CL)
:
:Bill,
:
:When the column design requirements were based on short, medium and long
:columns, Cd could only be applied to the short and medium columns.  When,
:in
:their infinite wisdom, AF&PA rewrote the column formula into a single
:formula, they said that they (somehow) included Cd in the short and medium
:portions of the formula, but did not include it in the long portion of the
:formula.  This is an accomplishment that truly deserves recognition.  I was
:not aware that they extended this accomplishment to lateral stability which
: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 = Cd should not be applied.)
:
:A. Roger Turk, P.E.(Structural)
:Tucson, Arizona
:
: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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