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Re: Response spectrum analysis - Query

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Using elastic linear dynamic analysis, scaling (up and down) to the code prescribed values (80%, 90%, 100% of static base shear) has always been considered correct. The reason is that the dynamic analysis using spectra (site specific or not) and/or time history is meant to give you a better distribution of the base shear - by considering the effects of all important modes not just the first mode as in static analysis - and not the actual magnitude of the base shear. Also since you are using a more refined distribution method, codes allow you to reduce the static base shear in some regular structures.

Reza Dashti P.Eng
Vancouver, BC

From: Tejas Ins <tejas_ins(--nospam--at)>
Reply-To: <seaint(--nospam--at)>
To: seaint(--nospam--at)
Subject: Re: Response spectrum analysis - Query
Date: Wed, 27 Oct 2004 10:16:33 -0700 (PDT)

Good luck on the big one !
I agree on the logic for scaling down elastic response parameters per 1631.5.4 spcially with all the SRSS output numbers ... but I haven't found this specifically addressed in the code. What has been your experience on scaling down to static baseshear, if any with the plan-check / peer-reviews?

Tejas Ins, P.E.

David Topete <davetopete(--nospam--at)> wrote:
I think the intent of the code (UBC/CBC) to scale down Elastic Response parameters (per 1631.5.4) is because a Dynamic Analysis is intended to provide a more "realistic" response spectra versus one produced using the Static Force procedure. This is according to S.K. Ghosh. I recently took a review course to prepare for the CA SE exam (wish me luck...) and Dr. Ghosh, in one of his MANY publications, asserts that this is the intent of scaling down. I would agree though with Thomas that rarely can a geotech correctly generate a "site-specific" response spectra.
Hope that helps.
David Topete, PE

Tejas Ins <tejas_ins(--nospam--at)> wrote:
From:  Tejas Ins <tejas_ins(--nospam--at)>
To: seaint(--nospam--at)
Subject: Response spectrum analysis - Query

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 site-specific response spectra generation due to contractual issues and because the original geotech has now been replaced.

In general, reading the code and the blue-book, 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 (UBC-97 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(--nospam--at) wrote:


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(--nospam--at)>
10/26/2004 01:49 PM Please respond to

seaint(--nospam--at) cc
Response spectrum analysis - Query

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 design 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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