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Re: ICBO Seminar for 1997 UBC Earthquake Regulations

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Dennis, my friend, you have confused me on a few issues. First of all, it is
(was) very clear in the 1994 UBC regarding the definition of a flexible vs.
rigid diaphragm (see section 1628.5). If the diaphragm deflection is greater
than 2 times the deflection of the lateral resisting elements, then it is a
flexible diaphragm. Otherwise, it is a rigid diaphragm. Unfortunately, I
don't believe academia has given us the tools to calculate the deflection of
an unblocked (or partially blocked) wood diaphragm, but it would be hard to
imagine that a 20'-0" x 40'-0" wood diaphragm would deflect more than
2*0.005h. If I am correct in this assumption, the diaphragm is behaving more
like a rigid diaphragm than a flexible one.

Second, if the hypothetical structure has two "flagpole" columns at the
garage entry and a full shear wall at the rear, I would suspect that the
flagpoles at the front are less rigid than the shear wall at the rear.

Outside of the structural mechanics, I agree with the potential soft story
problem. Maybe this can be (should be?) compensated for by the 3Rw/8 feature
of the code (or the ubiquitous Omega sub naught in the 1997 UBC).

My $0.02
Bill Allen

-----Original Message-----
From: Dennis S. Wish <wish(--nospam--at)>
To: seaoc(--nospam--at) <seaoc(--nospam--at)>
Date: Tuesday, April 21, 1998 8:48 AM
Subject: RE: ICBO Seminar for 1997 UBC Earthquake Regulations

>If the diaphragm were rigid, I would very much agree with you, however, the
>structure is far too ductile to consider that because the elements on the
>front of the garage are made stiffer than the back that they should take a
>greater proportion of the total base shear.
>The soft story ordinance in many cities (Santa Monica for one) is based
>the fact that a box with one open side and a flexible diaphragm is not
>structurally stable because the stiffness of the other three sides
>compensates for the weak front. In fact, this has been the cause of many
>failures in Los Angeles after Northridge and the need to stiffen the
>resisting elements in the soft-story. This is only done where there is a
>living unit above since the effects are enhanced by the mass above.
>My opinion for wood framed structures with stiff shear elements such as
>steel (whether a moment frame or pendulum) is that the load from the
>diaphragm to the stiff shear element is not increased - especially due to
>the distance from the reactions - only the deflection is controlled and the
>element can deflect only as much as the magnitude of the load will allow.
>This also becomes a function of the diaphragm's capacity - which should
>if exceeded or if the total lateral load is transferred into the stiffer
>Based upon your explanation, we would need to assume that no wood diaphragm
>is flexible and provide stiffness calculations for all structures to
>compensate for potential failures based upon rigidity failures of internal
>shearwalls since they should not work based upon a proportional
>of loading. Yet, historically, buildings designed by proportional
>distribution have done well - in fact, any building with a soft-story has
>done well when the open front has been laterally secure, regardless of
>element. Only buildings with "flexible" diaphragms designed by rotational
>methods has been the concern.
>Ernie, I don't mind changing my opinion if I have something conclusive to
>prove it otherwise. Have any models been tested with this criteria?
>-----Original Message-----
>From: ErnieNSE [mailto:ErnieNSE(--nospam--at)]
>Sent: Tuesday, April 21, 1998 4:49 AM
>To: seaoc(--nospam--at)
>Subject: Re: ICBO Seminar for 1997 UBC Earthquake Regulations
>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
>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 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
>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 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,
>dimensions, and other factors that affect lateral load distribution, I use
>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
>at the front.
>Another example, a 20 ft. by 40 ft. building similar to the first example
>two cantileverd steel columns on the front 20 ft. side, a 20 ft. long
>shear wall on the interior wall 20 ft behind the front wall and another 20
>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
>My point is USE YOUR JUDGEMENT. Keeping in mind the intent or
>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
>lateral loading conditions.
>Just my opinion.
>Ernie Natividad