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RE: ICBO Seminar for 1997 UBC Earthquake Regulations

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Bill, if you introduce a spring constant in the center of the beam, wouldn't
that be the same as introducing a flexible support? Wouldn't you then expect
some reaction at the middle spring support?
I've been out of school a long time so I'm really rusty on this one. My
assumption was that the wood diaphragm and the steel beam deflect and are
simply supported. The idea of deflection is used only as a control for the
stiffness of the member. I realized that the rules of statics governs in
determinant systems, but I don't see how a roof diaphragm spanning between
two supports of different rigidity is anything other than determinant.
What do others think on this issue?
Also - does this change when the diaphragms converge on an intermediate
shear element at different elevations? Is the shear transfer from high roof
to low through a sheathed cripple wall assumed as stiff as the element
Bill, I've stopped arguing this one since I am only interested in
understanding how the code requirement accepting the lower R value for the
complete structure originated. Was this consideration intended for wood
construction with a wood diaphragm?


-----Original Message-----
From: Bill Allen, S.E. [mailto:billallen(--nospam--at)]
Sent: Wednesday, April 29, 1998 4:46 PM
To: seaoc(--nospam--at)
Subject: Re: ICBO Seminar for 1997 UBC Earthquake Regulations

Of course, if your structure is statically determinant (simple beam with two
supports), the reactions will be the same regardless of the relative
stiffness of the beam and supports. Although most of us model wood
diaphragms as "flexible" diaphragms, considering they simply span between
shear resisting elements, this is also a statically determinant structure.
If, in the hypothetical structure you described below, you had a spring
support at the middle of the beam, the reactions would certainly be
dependent on the relative stiffnesses of the beam and springs. Similarly, if
the wood diaphragm were considered "rigid" or otherwise analyzed considering
the relative stiffnesses of the diaphragms and shear walls this would also
be true.

Bill Allen

-----Original Message-----
From: Dennis S. Wish <wish(--nospam--at)>
To: seaoc(--nospam--at) <seaoc(--nospam--at)>
Date: Wednesday, April 29, 1998 2:34 PM
Subject: RE: ICBO Seminar for 1997 UBC Earthquake Regulations

>Jump in Stan, don't wait. I think that there are a lot of valid
>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
>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
>as the reactions was equal to both supports based upon a uniformly
>distributed load. The next model pinned the left reaction but replaced the
>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
>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?
>Dennis Wish PE
>-----Original Message-----
>From: Stan J [mailto:hawneng(--nospam--at)]
>Sent: Wednesday, April 29, 1998 1:10 PM
>To: seaoc(--nospam--at)
>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
>> 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
>> shear walls on three sides and cantilevered steel columns on one side and
>> plywood roof diaphragm. Assuming flexible diaphragm, we distribute the
>> loads by tributary areas without regards to wall rigidities. Half the
>> load in one direction goes to the front cantilevered columns and the
>> 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
>> 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
>> an example, but on similar buildings of this type, I use judgement. My
>> is that the roof diaphragm is not 100% flexible and does not distribute
>> lateral loads by tributary width without regard to relative wall
>> Depending on how rigid the rear wall is compared to the front wall,
>> dimensions, and other factors that affect lateral load distribution, I
>> building lateral load to the front and 80% to 100% building lateral load
>> the rear. Now, using  Rw=3 at the front, I'll use 100% building lateral
>> 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!  :)