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1997 UBC 2320.2 & 1605.2

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Section 2320.2/Design of Portions - essentially states that
"nonconventional" structural elements are to be designed in accordance with
Section 1605.2

Section 1605.2 requires a "rational analysis in accordance with well
established principles of mechanics ... etc."

Here's the problem:  two story wood frame residence with braced wall panels
in the 2nd story that are not continuous through the 1st story to the
foundation (i.e. Unusual shape per 2320.5.4.1).  The braced wall panels
above (2 - 4' long plywood panels) terminate on a 5.125 x 16.5 glu-lam beam
over the Garage.  The lateral resisting element used below is a single
cantilever steel tube column (attached to the glu-lam).  The steel column
being considered as a "nonconventional" structural element (it's not wood,
and it's not a braced wall panel).

What is the so-called "rational" force that this cantilever column should be
designed for?

1. Determine the greater of the tributary wind load and seismic load to the
cantilever column.  For seismic, using R = 2.2 for this cantilever steel
column only.  Let's ignore the rigid vs. flexible wood diaphragm issue for
this discussion, OR

2. Determine the number of braced wall panels, in the 1st story, that
"would" be required to meet Section 2320.11.3 (say 2 - 4' long plywood
panels) ... calculate the total allowable load that those 2 panels could
resist ... then design the cantilever steel column for that load.

Obviously, there may be a huge difference between approach #1 & #2.
Personally I have a problem with approach #2.

I'd like to hear other opinions and share them with the design engineer.
You can send responses direct to me as well.

Steven T. Hiner, SE

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