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QUESTION: What is the difference between this equation used by the NSCP from the standard IBC equations?
Is V measured in kips?
V =  Z I C W / Rw
Z=seismic zone factor
I= Occupancy importance
C= base shear coefficient
W=total dead load
S=site coefficient
Rw= moment resisting frame

ANSWER: Dear Franco

Please write me back the IBC relationship. The difference is basically On method of design, working stress ( in this relationship) and ultimate design ( I guess IBC follow this one). Then usually you see a factor 1.4 to 1.6 difference between them.

The units is dependent of unit of W in both codes.


[an error occurred while processing this directive]---------- FOLLOW-UP ----------

QUESTION: IBC standard equation for base shear:

V = Cs W
W = effective seismic weight
Cs = Seismic response coefficient

Cs= Sds/(R*Ie)
R= response modification factor
Ie= importance factor

for T<=Tl (period and long transitional period)

see page 166

Dear Franco

As I stated in my last letter , The difference is in R and Rw factor. In IBC you have smaller R which lead to ultimate design forces but in the some codes they use Rw which is bigger in quantity and lead to allowable stress design. The difference between R and Rw is almost  the factor of safety say 1.5 to 2

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Babak Esmailzadeh Hakimi


I can answer questions in Structural Engineering, Earthquake Engineering, Project Management and Project Control topics!


I am experienced in the area of earthquake engineering in electrical power industry, i.e. Lifeline Earthquake Engineering. My focus has been on structural seismic design of power installations. Recently I have got involved in project management in the category of constructional engineering.

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Bs degree in Civil Engineering from Ferdowsi University ,1988, Mashhad Iran, Ms degree in Earthquake Engineering, Sharif University of Technology, 1993, Tehran, Iran

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