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Re: Response spectrum analysis  Query
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 Subject: Re: Response spectrum analysis  Query
 From: THunt(nospamat)absconsulting.com
 Date: Thu, 28 Oct 2004 07:43:57 0700
Tejas,
My personal opinion would be that you should NOT scale down a dynamic analysis. However, this is based on two very important assumptions. I.) You have an appropriate response spectrum, and 2.) You have modeled and performed your dynamic analysis correctly.
For a symmetrical/regular building without any mass, stiffness, or plan irregularities your dynamic analysis should be less than your static analysis but the code writers wanted some lower limits on low you can go since even a dynamic analysis is still not without its own potential inaccuracies.
For good reasons, the code requires a dynamic analysis for certain irregular buildings since these buildings do not fit the underlying assumptions of the static base shear equations and it is possible that you could have some weird modes causing significant increases in your building response and overall base shear. This is why you are required to do the dynamic analysis in the first place.
Since you may have a site specific response spectra that is inappropriate you are probably comparing apples to dead fish when comparing your static base shear to the dynamic base shear.
Thomas Hunt, S.E.
ABS Consulting
Tejas Ins <tejas_ins(nospamat)yahoo.com>
10/26/2004 07:24 PM


From: Tejas Ins <tejas_ins(nospamat)yahoo.com>
To: seaint(nospamat)seaint.org
Subject: Response spectrum analysis  Query
Thomas,
Thankyou for your response. I have been mulling on this issue for some time. I am at a point now, where there isn't a way to check the sitespecific response spectra generation due to contractual issues and because the original geotech has now been replaced.
In general, reading the code and the bluebook, I can see that while "it" specifically discuss increasing the responses such that base shear from a dynamic analysis is not less than the static base shear ... "it" does not discuss if the base shear from a dynamic analysis could be scaled down to the static base shear level. The "seismic design handbook by Farzad Naeim, 2nd ed." discusses in chapter 5, section 5.3.17 (UBC97 Dynamic lateral force procedure) that "if the base shear obtained from a dynamic analysis is greater than that specified by static lateral force procedure, it may be scaled down." While I have done this in the past, I am unsure on the code intent, specially because of its choice of words in section 1631.5.4.
Any thoughts / suggestions / recollections ?
Tejas Ins, P.E.
THunt(nospamat)absconsulting.com wrote:
Tejas,
I would first question your "site specific" response spectrum. Few if any geotechs know how to correctly do one. What you have is probably a "probabilistic" computer generated response spectra which if you are near a known fault is possibly wildly inaccurate. This is why the new USGS maps transition from "probabilistic " to "deterministic" values for most of California.
Thomas Hunt, S.E.
ABS Consulting
Tejas Ins <tejas_ins(nospamat)yahoo.com>
10/26/2004 01:49 PM


Hi All,
I am working on a school design project and learnt that the latest amendments (January 2004) require a site specific response spectrum analysis if the structure has any irregularity (except reentrant corner). So we got a site specific response spectrum. The response spectrum analysis gives a base shear (after dividing by R) to be much greater than the static base shear per section 1630.2
I am finding the Code language a bit confusing and would like your opinion on the CBC / UBC, where it discusses the reduction of elastic response parameters for design.
UBC/CBC section 1631.5.4: "..... with the limitation that in no case shall the elastic response parameters be reduced such that the corresponding desig n base shear is less than the Elastic Response Base Shear divided by the value of R."
My confusion is:
a. When would the above statement apply and supersede items 1, 2, 3 under section 1631.5.4
b. Why would someone scale forces, moments, displacements and then determine the "corresponding design base shear" ... rather than scale elastic response base shear to the base shear determined in accordance with section 1630.2 and then determine corresponding scaled forces, moments, displacements, etc.
c. If (Elastic Response Base Shear / R > static base shear per section 1630.2) ... what to do? Use the higher Elastic Response Base Shear / R ? OR scale down further to the Sataic base shear value ?
Thank you for your comments.
Tejas Ins, P.E.
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