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Re: ICBO Seminar for 1997 UBC Earthquake Regulations

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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. 

Another example, a 20 ft. by 40 ft. building similar to the first example with
two cantileverd steel columns on the front 20 ft. side, a 20 ft. long  plywood
shear wall on the interior wall 20 ft behind the front wall and another 20 ft
long  shear wall at the rear, 40 ft behind the front wall.   The side walls
are solid 40 ft. long plywood shear walls. I'll distrubute the loads by
tributary width, but I'll double the loads on my front wall(equivalentg to
Rw=3) and  multiply the interior wall load by 1.5 (the equivalent of the
interior wall carrying the building lateral load from the front to the
interior).

My point is USE YOUR JUDGEMENT. Keeping in mind the intent or interpretation
of a code requirement, do not just follow specific code section application
instructions blindly. Try to imagine how building lateral loads act on the
building based on  physical and structural characteristics of the building and
lateral loading conditions.

Just my opinion.

Ernie Natividad