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Re: seaint Digest for 5 Apr 1999

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> From: BPCE(--nospam--at)aol.com
> Subject: Column K factor.
> To: Seaint(--nospam--at)seaint.org
> 
> NEED OPINIONS:
> We designed a tilt-up building with metal deck for roof  (no zonolite or
> structural concrete fill). The building is approximately 200'x200'. Framing
> consists of OWSJ supported on WF steel girders.. The girders cantilever about
> 4 ' over columns with drop-in beams at alternate bays.Connection is with
> shear plates. Girders have full height stiffners on both side of web at
> column supports and bolted with 4 bolts to the cap plate. OWSJ have slip
> connection at bot. chord to allow for free deflection of the joists. The
> columns are spaced at 40' on center, both direction,  and are HSS 6x6x0.5 X
> 22' tall. The framing, including columns were designed using RAMSTEEL
> programme (over kill) with eccentricites included in in both axis.This is a
> VA non-essential facility in West LA. Building and is all complete and ready
> for TI improvements.
> SO FAR SO GOOD. We now have a Very Senior VA engineer from Washington (shall
> remain name less for now) telling us that the columns should be designed per
> AISC Table C-2.1 column (f) (Page 5-135) with K=2. i.e with pin at ground -
> Rotation fixed and translation free at top.
> We have designed the columns as pin-pin with K=1. We, very strongly, do not
> agree with his assesment but because of his high position we cannot convince
> VA that our design is adequate.
> We thought this would make an intresting topic for discussion.
> Thanks to all, in advance.
> Kulwant Brah

In bracing, as in life, everything is relative. I assume that your
primary bracing system is based on a (nearly) rigid roof diaphragm
distributing loads to perimeter bracing or shear walls. In this case,
unless you can show otherwise, your interior posts may actually be
stressed significantly in biaxial bending due to the semi-rigid moment
connection(s) at the top.

To win your case (e.g. to use K=1) you will have to prove that your roof
diaphragm is MUCH stiffer than the inverted cantilever (both directions)
and that your OWSJ tie connection never goes into bearing - this is
essentially a self-defeating condition; if it bears, you have a moment
connection but if it doesn't bear, then why does it exist ...

Your first analysis establishes the lower bounds for the post stresses
and an upper bound for your bracing stress and deflections. For the
upper bound on the post stresses you should reanalyze the building
assuming rigid moment connections at the top. If the post stress
difference is very small, then you have a good case for using K=1.

Note, that K is a convenience for estimating sway, P-delta or other
secondary effects in primary analysis. If you explicitly consider these
secondary effects then you can use K=1. RamSteel cannot do this
automagically, but it is possible to do it with notional loads.

I am curious about the "eccentricities in both directions" that you have
included ...

-- 
Paul Ransom, P. Eng.
Civil/Structural/Project
Burlington, Ontario, Canada
<ad026(--nospam--at)hwcn.org> <http://www.hwcn.org/~ad026/civil.html>