To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
Subject: RE: effective length factor for sway moment
From: "Haan, Scott M." <HaanSM(--nospam--at)ci.anchorage.ak.us>
Date: Fri, 6 Apr 2001 10:05:22 -0800
I can understand saying using P-Delta instead of K if there are drift
limits. It used to be written in the UBC that way, but the UBC required the
building to inside drift limits and it said all joints need to be continuous
How is an approximate P-Delta analysis done for a structure analyzed by hand
as a partially restrained wind-portal-frame? Are there drift limits for
storage racks? What is a typical deflection for storage rack moment frame
under seismic load, 12"? 16"?
I have seen a moment amplification for storage racks
M=Mmax*[Cm=.85]*1/(1-Pdesign/Peuler). Is the Peuler=PI^2*E*I/(K*L)^2
applicable for with K=1 when you are not doing a P-Delta analysis, you are
not limiting drift, using a wind frame analysis, and an approximate portal
What I have seen is some people really just want K=1, so there can be
smaller columns, without any other limitations. The bottom line is that the
IBC does not amend the AISC Seismic Provisions to allow specifically allow
K=1 in sway frames carte-blanch. Also nowhere in the code does it say K=1
carte blanch if a P-Delta check is employed.
Scott M Haan
Plan Review Engineer
Building Safety Division http://www.muni.org/building
Development Services Department
Municipality of Anchorage
From: Jim Todd [mailto:JIMT(--nospam--at)performainc.com]
Sent: Friday, April 06, 2001 9:14 AM
Subject: Re: effective length factor for sway moment
Once again, as others have mentioned, K factors (not just their nomograms)
were introduced in the "pre computer" days. We really have much better
tools now, and should use them.
And I also used a slide rule which now sits in my desk awaiting a battery
failure in my calculator. However, I have retired my K factor nomogram
completely. I don't even use it for preliminary checking anymore.
Peter Higgins, SE
What method do you use for K-factor determination? Something from Yura?