[SEFI] Static vs Dynamic Base Shear

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avinash...@yahoo.com

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Aug 17, 2007, 8:14:34 AM8/17/07

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Dear Mr. Arafath,
As Mr Vipul has pointed out, the enhancement of dynamic base shear to the static base shear is inherently attributed to the low values of period obtained from dynamic analysis resulting from a "bare-frame" model.. The time period evaluated based on the model wherein we dont have the stiffening elements like the partition walls etc would generally be on the lower bound, which results in lower base shear than the actual.. The empirical value of time period given by the code is for regular structures and based on the actual measured values.. So even though you are getting high value for dynamic base shear when compared to static base shear, you may still have to enhance it, rather than reducing it, to account for the modelling discrepancies..
Thanks

Avinash S.
Structural Engineer
Arif & Bintoak
(Consulting Architects & Engineers)
P.O Box 3670
Dubai, UAE
Ph: +9714 3353342 (Off)
Mob: +97150 2426438 (Personal)

----- Original Message ----
From: "gen...@sefindia.org" <gen...@sefindia.org>
To: avinash...@yahoo.com
Sent: Sunday, August 12, 2007 10:38:41 PM
Subject: [SEFI] Daily Digest Mon Aug 13 00:00:04 2007

----------------------------------------------------------
From: araf...@yahoo.com
Subject: Re: Base shear scaling.
Date: 12/08/07
Time: 09:00:20
----------------------------------------------------------
Dear Nipul Patel,
Thanks for your replay,
But please note that, the clause cleary mentioned that,"Where Vb is less than Vb1", But in my case Vb is more than Vb1, . in this case all response quantities like (for example member force,displacements,storey forces,storey shears and base reactions) is more than what we calculated by static method,ie Vb1.
my question is do we have to scale down the response quantities??. the code mentioned case is just scale up.

in my case, i have a 30 storied building , which is having a 15 degree slope, (leaning building). all column and core walls are sloping 15 degree towards horizontal.

let me give you some more clue.
Generally for vertical cantilever building (90 degree), the maximum seismic base shear is 8 to 10 % of weight (DL + % Live load).
But for horizonatl cantilever building ( 0 dergree )seismic base shear is 100 % weight (Dl + % live load,)
since my case inbetween neither 90 degree nore 0 degree.this is the reason, we are getting dynamic base shear is more than static base shear.

Please discuss in detail.

Thanks and Regards.
Arafath

gen...@sefindia.org wrote:
----------------------------------------------------------
From: nil_...@yahoo.com
Subject: Re: [SEFI] Base shear scaling
Date: 11/08/07
Time: 13:03:29
----------------------------------------------------------
Hi dear friend,

refer

IS 1893, Clause 7.8.2

"Dynamic analysis may be performed either by the Time History Method or by the Respose Sspectrum Method.However,in either method,the design base shear (vb) shall be compared with a base shear(vb1) calculated using a fundamental period Ta,Where Ta is as per clause 7.6.Where Vb is less than Vb1,all the response quantities(for example member force,displacements,storey forces,storey shears and base reactions) shall be multiplied by vb1/vb."

Need not to scale the base shear,scale the forces accoriding to that ratio.

nipul patel
design engineer,
VMS,Ahd.

araf...@yahoo.com wrote:
Dear SEFIans,
Could you please clarify , if base shear calculated as per UBC
spectra ( Dynamic ) is more than static base shear, do i have to scale down
the dynamic base shear to match with static base shear.
For example.
UBC response spectra base shear is 25,000 kN
and
UBC static base shear is 18,000 kN
do i have to scale down dynamic base shear to 18,000 kN with a
scale factor of 0.72 ??..
or,
shall i proced with 25,000 kN as my base shear for further analysis
and design.
Please discuss in detail.
Thanks
***************************************************************************************************

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----------------------------------------------------------
From: ahuja...@rediffmail.com
Subject: Re: [SEFI] Re: Base shear scaling.
Date: 12/08/07
Time: 10:59:35
----------------------------------------------------------
Dear Mr. Arafath

The code basically recognizes that the response spectrum method is more accurate than the static equivalent method especially for irregular structures. So it encourages you to perform a response spectrum analysis for all cases though mandates it some. But in computer analysis as the time period is generally based on a bare frame structure (without the effect of infil walls etc) it puts a cap on the time period--that is emperically observed. In most cases the code specified time period would be lesser than calculated in the run. However in your case there is probably an inherent stiffening on account of the tilt (like a brace)--which is probably why you are getting a shorter period than the emperically observed results (that are based on upright buildings). You should be extra careful in accepting these results as they still do not reflect the stiffening due to infil walls (unless you have modelled those as well)--you could still be non-conservative to
that extent. The questi!
on of reducing your results to match with the code defined period does not arise.

best regards

Vipul Ahuja

On Sun, 12 Aug 2007 araf...@yahoo.com wrote :
>Dear Nipul Patel,
> Thanks for your replay,
> But please note that, the clause cleary mentioned that,"Where Vb is less than Vb1", But in my case Vb is more than Vb1, . in this case all response quantities like (for example member force,displacements,storey forces,storey shears and base reactions) is more than what we calculated by static method,ie Vb1.
> my question is do we have to scale down the response quantities??. the code mentioned case is just scale up.
>
> in my case, i have a 30 storied building , which is having a 15 degree slope, (leaning building). all column and core walls are sloping 15 degree towards horizontal.
>
> let me give you some more clue.
> Generally for vertical cantilever building (90 degree), the maximum seismic base shear is 8 to 10 % of weight (DL + % Live load).
> But for horizonatl cantilever building ( 0 dergree )seismic base shear is 100 % weight (Dl + % live load,)
> since my case inbetween neither 90 degree nore 0 degree.this is the reason, we are getting dynamic base shear is more than static base shear.
>
> Please discuss in detail.
>
> Thanks and Regards.
> Arafath
>
>
>gen...@sefindia.org wrote:
> ----------------------------------------------------------
> From: nil_...@yahoo.com
>Subject: Re: [SEFI] Base shear scaling
>Date: 11/08/07
>Time: 13:03:29
>----------------------------------------------------------
>Hi dear friend,
>
>refer
>
>IS 1893, Clause 7.8.2
>
>"Dynamic analysis may be performed either by the Time History Method or by the Respose Sspectrum Method.However,in either method,the design base shear (vb) shall be compared with a base shear(vb1) calculated using a fundamental period Ta,Where Ta is as per clause 7.6.Where Vb is less than Vb1,all the response quantities(for example member force,displacements,storey forces,storey shears and base reactions) shall be multiplied by vb1/vb."
>
>Need not to scale the base shear,scale the forces accoriding to that ratio.
>
>nipul patel
>design engineer,
>VMS,Ahd.
>
>araf...@yahoo.com wrote:
>Dear SEFIans,
>Could you please clarify , if base shear calculated as per UBC
>spectra ( Dynamic ) is more than static base shear, do i have to scale down
>the dynamic base shear to match with static base shear.
>For example.
>UBC response spectra base shear is 25,000 kN
>and
>UBC static base shear is 18,000 kN
>do i have to scale down dynamic base shear to 18,000 kN with a
>scale factor of 0.72 ??..
>or,
>shall i proced with 25,000 kN as my base shear for further analysis
>and design.
>Please discuss in detail.
>Thanks
>***************************************************************************************************
>
>
>
>
>---------------------------------
>Be a better Heartthrob. Get better relationship answers from someone who knows.
>Yahoo! Answers - Check it out.
>
>
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