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Re: UBC seismic importance factor

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>From: "Stephen K. Harris" <SKH(--nospam--at)>
>Organization:  EQE International
>To: seaoc(--nospam--at)
>Date:          Fri, 21 Jul 1995 09:11:01 PST
>Subject:       Re: UBC seismic importance factor
>Priority: normal
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>Message-ID: <6515E1220E(--nospam--at)>
>>> Date:          Thu, 20 Jul 95 21:49 PDT
>>> From:          seaoc(--nospam--at) (SEAOC)
>>>Subject:       UBC seismic importance factor
>>>From: "Bill Sherman" <SHERMANWC(--nospam--at)>
>>>To: seaoc(--nospam--at)
>>>Subject: UBC seismic importance factor
>> >
>> >For discussion purposes, what is the purpose/basis of the importance factor
>> >in the UBC formula for seismic base shear?  While clearly it is intended to 
>> >enhance the seismic performance of critical facilities, is it intended to 
>> >increase the magnitude of seismic event a building can resist?  Or to
>> >the effective "return period" of seismic event to be resisted?  Or to
>> >the effective "R-value" so as to reduce damage for a given magnitude
>The "I" factor is intended to decrease the amount of expected damage, 
>i.e., to increase the likelihood of post-earthquake functionality.  
>The return period remains the same, as does the ground motion, but 
>the expected performance is changed.  You could think of this as a 
>reduction in the "R" value, although this obfuscates the real meaning 
>of the factors.  
>> > 
>> >The answer has applicability when critical facilities are to be designed
>> >site specific seismic data outside normal code requirements.  When
>> >ground accelerations and return periods are reported in a geotechnical 
>> >investigation for a given site, I've heard various interpretations of
what to 
>> >do with the importance factor, when the facility to be designed is
defined by 
>> >the client to be a "critical facility".   
>> > 
>> >Should the site specific ground acceleration be increased by the importance 
>> >factor to increase the effective return period and/or magnitude of seismic 
>> >resistance, based on its definition as a critical facility?  
>>>Should the site 
>> >specific ground acceleration be compared with the code defined zone factor 
>> >multiplied by the importance factor, and the higher of the two values
>> >Or should the importance factor be ignored and ground acceleration and 
>> >R-values be selected by engineering judgement for the given facility? 
>> > 
>> >Is a site specific ground acceleration, based on a 10 percent
probability of 
>> >being exceeded in 50 years, directly comparable to the UBC zone factor?
>Yes.  The Z factor, when applied to the UBC response spectrum, is 
>intended to represent a ground motion with a 10 percent exceedance 
>probability in 50 years.  If you have a site-specific spectrum based 
>on this same return period, you should substitute it for the UBC 
>spectrum and keep the "I" factor.

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