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RE: ICBO Seminar for 1997 UBC Earthquake Regulations

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Jump in Stan, don't wait. I think that there are a lot of valid professional
opinions on this but that the solution is far from reached. Yes, the code
appears to be specific about the use of Rw over the entire structure, but I
believe the committees decision to do this is based upon the majority
opinion of a group, like ours, who debate what will occur without really
having the proof.
If someone would set me straight on this - I would assume that the lateral
design would be similar to modeling a beam with either two spring supports
of different damping ability, or of the same with one pinned support (the
wood shearwall) and a spring or dampened support at the other end. Has
anyone done this analysis?

I modeled a steel beam 20 feet long (I assumed that the steel is not rigid
since it is subject to deflection the same as a diaphragm 20 feet deep and I
felt this was a close approximation) and pinned one end. At the other, I
applied only a spring and varied it's stiffness. The statics did not changed
as the reactions was equal to both supports based upon a uniformly
distributed load. The next model pinned the left reaction but replaced the y
restraint with a spring. I left the right side as a roller with the y
restraint replaced with a spring. Again, the reactions are equivalent and
the degree of deflection is dependent upon the stiffness of the spring.

So my question remains. If we assume that the diaphragm is relatively rigid
the reactions will be the same regardless of the stiffness of the individual
elements. Why, then must we assume that both sides need to be designed with
the worst case R factor?. It appears that you are only interested in
assuring that the steel pendulum column does not deflect any more than
allowable story drift. Please someone, explain this to me.
This is, as far as I can tell, still a static design.

Can anyone respond to this who worked on this code, such as Ron Hamburger?

Thanks
Dennis Wish PE

-----Original Message-----
From: Stan J [mailto:hawneng(--nospam--at)wac.com]
Sent: Wednesday, April 29, 1998 1:10 PM
To: seaoc(--nospam--at)seaoc.org
Subject: Re: ICBO Seminar for 1997 UBC Earthquake Regulations


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!  :)