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Re: computer modeling of diaphragms

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Nii,

You may find it helpful, as you model the concrete building with flexible
diaphragms, to take a look at Division 95 of the Los Angeles City Building
Code, and some seminar notes that I'll describe below.

Division 95 of the Los Angeles City Building Code deals with earthquake
hazard reduction in existing concrete buildings, and concrete frame
buildings with masonry infill.  It prescribes four different analysis
methods that, depending on the characteristics of the building, vary from
relatively simple, to a three-dimensional response spectrum analysis.
Flexible diaphragms are accounted for by assigning to the mass of the
vertical shear resisting element (wall or frame) being analyzed, an
additional mass that is coupled to the vertical shear resisting elements by
the adjacent diaphragms.  The diaphragm is assumed to couple to the vertical
shear resisting elements a mass that is limited by the yield capacity of the
diaphragm edge adjacent to the element.  Working stress capacities of
flexible diaphragms are given in other sections of the Los Angeles City
Building Code -- the working stress capacities are generally related to
strength values by a factor of 4.  Once the coupled mass is included in the
mass of the vertical shear resisting element, it is not included in any
other way in the model of the building.

Papers that may be used as commentaries to Division 95 were discussed in a
seminar titled "Structural Considerations of Building Conservation  Toward
the 21st Century" presented in January 1996 by the Structural Engineers
Association of Southern CA.  I think copies are available from the SEAOC web
page.  Papers by John Kariotis and by Jim Hill are applicable to Division 95
and are included in the seminar notes.  A paper by Dick Phillips, also on
Division 95, is missing from my copy of the notes.

The portion of the Los Angeles City Building Code that includes Division 95
is available from ICBO on their web page ( www.icbo.org ) as item No.
133A99.

I hope this helps

Nels Roselund
Structural Engineer
South San Gabriel, CA
njineer(--nospam--at)att.net


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