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RE: Partially Tapered Column Analysis (CODE CHECK)

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Paul (and anyone else following this thread),

Actually the 'partially tapered' column is part of a frame that has a
stepped column with a fixed base on the other side (for a crane rail).
The other crane rail column has long since been removed when the crane
came out, leaving this funky looking frame.

What I really meant was not really the analysis of the section, but
rather the code check.  I can get from my program the appropriate fa and
fb which take the taper into account.  My question is more about how to
calculate Fa and Fb and the interaction equation.  I've used the tables
in the back of the ASD 9th edition to figure out the k factor for the
tapered column, but because the column does not have a consistent taper
full height, I am a little concerned about using those tables.  But just
a little.  I was wondering whether someone knew of a design guide that
covered it or something.

Thanks for your help,
Brian 

-----Original Message-----
From: Paul Ransom [mailto:ad026(--nospam--at)hwcn.org] 
Sent: Thursday, January 12, 2006 10:54 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: Partially Tapered Column Analysis

> From: "Brian S Bossley" <BSBossley(--nospam--at)venturaengineering.com>

> I am looking at an analyzing an existing building that consists of
> pre-manufactured rigid frames with a 'partially tapered' column on the
> one side.  What I mean by "partially tapered" is that the column is
> tapered from 0' to 30' up and then above that it takes on a uniform
> depth from 30' to 45'.  I've seen these all over before, but I've
never
> analyzed one until now.

This sounds like the column was designed for a crane bracket or maybe a
clearance issue. This is quite common.

> But here's my question: If I look at this column in a piecewise
fashion,
> what do I use for K of the two segments. It seems like the K for the
> bottom section would have to be larger than 2.0 because the WHOLE
column
> should have a K of 2.0.

k=2 is worst case, despite the changes along the length.
Since there is flexibility at the haunch, the actual k<2.
Conservatively, same k for the full length.

> Alternately, If I look at this as one big tapered member 45' tall with
a
> K=2.0, is it really fair to use the KL/r at the base of the taper?  I

It is common to use the section properties at the section being
assessed. It is conservative to use the minimum properties between
lateral brace points. This requires you to assess a LOT of sections for
each load case - custom software helps.

I recall that there is a commercial product for analysis of tapered
member frames for about $5000. Probably ASD 9th, only. Be cautious.

-- 
R. Paul Ransom, P. Eng.
Civil/Structural/Project/International
Burlington, Ontario, Canada
<mailto:ado26(--nospam--at)hwcn.org> <http://www.hwcn.org/~ad026/civil.html>

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