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RE: 1997 UBC 2320.2 & 1605.2

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"The braced wall panels above (2 - 4' long plywood panels) terminate on a
5.125 x 16.5 glu-lam beam
over the Garage. "

It is an unusual shape when upper level exterior braced wall lines are
setback from lower level braced wall lines unless you meet one of the
2305.4.1. If the floor sheathing and floor framing are discontinuous between
the upper exterior braced wall line and the lower braced wall line it is
definitely an 'unusual shape'.

Unusually shaped buildings need to have the entire system designed per
2320.5.4.  You can't use 2320.2 'design of portions' for 'unusually shaped
buildings' [can in the IBC but not IRC].  Design of portions is intended for
"braced wall lines" in 'usual shaped buildings' that do not have the right
percentage of bracing or that do not have 'braced wall panels' starting
within 8 feet of the beginning of the braced grid line, ect...


"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."

Locally we require the designer to determine the code required seismic
force, how much is transmitted to the "non-conventional" element, and to
design the element for that force.  There is no way on God's green earth
that I would let someone base a moment frames design force on the capacity
of two 48 inch long gypsum board shear walls.


There are many things about the conventional construction provisions that
defy logic.  It is based on "where are the bodies philosophy" not sound
engineering.  I would not design my house with it but this is America and it
is OK to make a profit at  the expense of people that get liberal arts
degrees or that work at Taco Bell.




-----Original Message-----
From: Steve Hiner [mailto:shiner(--nospam--at)folsom.ca.us]
Sent: Friday, February 08, 2002 11:04 AM
To: 'seaint(--nospam--at)seaint.org'
Subject: 1997 UBC 2320.2 & 1605.2


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.

Regards,
Steven T. Hiner, SE
shiner(--nospam--at)folsom.ca.us

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