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Re: ICBO Seminar for 1997 UBC Earthquake Regulations

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ErnieNSE wrote:
> Regarding the use of Rw=3 on one wall line only where the cantilever column
> occurs, I'll be carefull about this. We have to use our judgement.
> For example, a 20 ft. by 20 ft. wood framed garage building with solid plywood
> shear walls on three sides and cantilevered steel columns on one side and
> plywood roof diaphragm. Assuming flexible diaphragm, we distribute the lateral
> loads by tributary areas without regards to wall rigidities. Half the lateral
> load in one direction goes to the front cantilevered columns and the other
> half goes to the solid plywood shear wall at the rear. Using Rw=3 for the
> front wall only is the equivalent of doubling the lateral load at the front(or
> using 100% of the building lateral load at the front wall)) causing the
> columns to be stiffer due to the bigger load.
> This is not the usual way I design this type of building. The garage was just
> an example, but on similar buildings of this type, I use judgement. My feeling
> is that the roof diaphragm is not 100% flexible and does not distribute the
> lateral loads by tributary width without regard to relative wall rigidity.
> Depending on how rigid the rear wall is compared to the front wall, building
> dimensions, and other factors that affect lateral load distribution, I use 50%
> building lateral load to the front and 80% to 100% building lateral load to
> the rear. Now, using  Rw=3 at the front, I'll use 100% building lateral load
> at the front.

A broad note:

One should consider, however, that the Rw=3 for
cantilevered columns is not a simple matter of
stiffnesses.  Rw allows us to reduce full force
level to an equivalent force for allowable stress
based design.  Rw tries to take into account how
well any particular type of system can withstand
damage (which is expected to occur in the design
event) and still maintain lifesafety.  A
cantilevered column system is not expected to be
able to take as much damage or absorb as much
energy before it gives up the ghost. Thus it gets
a low Rw.

Of course, when you are dealing with a 20'x20'
building, you could probably justify doing a
torsional distribution to the three main walls
(100% to the back and the other two walls take out
the torsion), then the cantilevered column can
become icing on the cake.

Stan Johnson, P.E., I don't think I should try to
address the rest of the discussion, I've waded in
too deep already!  :)