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Seismic- IBC 2000

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Questions regarding "rho" as defined in IBC Section 1617.2.2: The definition
of Ai refers to the "floor area ... of the diaphragm level immediately above
the story". 

1. What if the level above the story is a "roof diaphragm" rather than a
"floor"? Is Ai the area of the "roof" or zero?

2. What if the structure is a single story rigid frame system without a
"diaphragm"? Is Ai zero and rho = 1.00? 

3. Does it make sense for rho to be less for a rigid frame without a
diaphragm than a structure with a diaphragm? How much redundancy does a
single rigid bent have? 

It would be helpful if the code better defined how Ai is to be applied in
such situations. 

Section 1617.1.1 could also be clarified to make it clearer that the second
term in Equation 16-28 is intended to be the vertical seismic effects, i.e.
by defining "D" as "the effect of dead load due to gravity". (One could
interpret the "effect" of dead load as being a lateral effect during an
earthquake. Something in the second term should be clearly defined as being
a vertical effect, just as Qe is defined as being "the effect of horizontal
seismic forces".) 


William C. Sherman, PE
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)cdm.com

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