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Time period-Tall Building Design
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sukanta.adhikari
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PostPosted: Wed Nov 21, 2012 11:50 am    Post subject: Time period-Tall Building Design Reply with quote

Dear all,

In continuation to discussion of tall structure...I want to know is there any limitation on time period of structure for tall structures.

Regards,
S Adhikari
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suhastayade
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PostPosted: Wed Nov 21, 2012 12:15 pm    Post subject: Time period-Tall Building Design Reply with quote

Dear All,

I also want to understand the limits of Time period with respective height of buildings. Also range of time period for tall building and other structures


Thanks & regards,

Suhas Tayade


On Wed, Nov 21, 2012 at 5:20 PM, sukanta.adhikari <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           Dear all,

In continuation to discussion of tall structure...I want to know is there any limitation on time period of structure for tall structures.

Regards,
S Adhikari
     



     


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gautam chattopadhyay
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PostPosted: Wed Nov 21, 2012 2:23 pm    Post subject: Time period-Tall Building Design Reply with quote

No, there is no restriction. In fact based on Zero Period Accelaration we compute the storey forces.less the time period more is the spectral acceleration. Hence if you consider a very low T your Sa/g will be more and design will be on the conservative side.

On Wed, Nov 21, 2012 at 5:20 PM, sukanta.adhikari <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           Dear all,

In continuation to discussion of tall structure...I want to know is there any limitation on time period of structure for tall structures.

Regards,
S Adhikari
     



     


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deviationz
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PostPosted: Thu Nov 22, 2012 8:47 am    Post subject: Reply with quote

There are no limitations, but the American Codes have limitations on minimum base shear based on a building with long time period (I think T = 10s). Beyond that the response spectrum flattens out meaning that any building with higher time periods will still have to be designed for base shears for T=10s.

In most regions, taller buildings will more often than not end up being driven by wind loads rather than seismic.
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V V SAPRE
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PostPosted: Sat Nov 24, 2012 10:57 am    Post subject: Time period-Tall Building Design Reply with quote

Ok


From: deviationz [mailto:forum@sefindia.org]
Sent: 22 November 2012 14:18
To: econf34289@sefindia.org
Subject: [E-CONF] Re: Time period-Tall Building Design



There are no limitations, but the American Codes have limitations on minimum base shear based on a building with long time period (I think T = 10s). Beyond that the response spectrum flattens out meaning that any building with higher time periods will still have to be designed for base shears for T=10s.

In most regions, taller buildings will more often than not end up being driven by wind loads rather than seismic.

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u.mukesh
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PostPosted: Mon Nov 26, 2012 8:33 am    Post subject: Time period-Tall Building Design Reply with quote

Hello allIn tall buildings time period will be on higher side; due to which it will
attract less forces.

So is there any limitation of "T" on higher side; so that buildings dont attract
very less earthquake forces?


Regards
Mukesh

On Thu, Nov 22, 2012 at 12:00 PM, gautam chattopadhyay <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           No, there is no restriction. In fact based on Zero Period Accelaration we compute the storey forces.less the time period more is the spectral acceleration. Hence if you consider a very low T your Sa/g will be more and design will be on the conservative side.

On Wed, Nov 21, 2012 at 5:20 PM, sukanta.adhikari forum@sefindia.org (forum@sefindia.org))> wrote:
      :                             Dear all,

In continuation to discussion of tall structure...I want to know is there any limitation on time period of structure for tall structures.

Regards,
S Adhikari
     



     
     
     



     



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gautam chattopadhyay
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PostPosted: Mon Nov 26, 2012 1:33 pm    Post subject: Time period-Tall Building Design Reply with quote

Please refer to IS 1893 (latest) i faintly remember there is some limiting value of T. I am not very sure about the clause.

On Mon, Nov 26, 2012 at 4:59 PM, u.mukesh <forum@sefindia.org (forum@sefindia.org)> wrote:
[quote]            Hello allIn tall buildings time period will be on higher side; due to which it will
attract less forces.

So is there any limitation of "T" on higher side; so that buildings dont attract
very less earthquake forces?


Regards
Mukesh

On Thu, Nov 22, 2012 at 12:00 PM, gautam chattopadhyay forum@sefindia.org (forum@sefindia.org))> wrote:
      --auto removed--

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B.V.Harsoda
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PostPosted: Mon Nov 26, 2012 10:15 pm    Post subject: Reply with quote

Papers:-
An Accurate Estimation of the Fundamental Period of Regular Tall Buildings
http://www.iitk.ac.in/nicee/wcee/article/8_vol4_427.pdf

Regards,
Er. B. V. Harsoda
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krishnan at its.caltec...
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PostPosted: Tue Nov 27, 2012 3:18 am    Post subject: Time period-Tall Building Design Reply with quote

I am not sure I am interpreting Er. Mukesh's comment correctly.
Nevertheless, I am going to rephrase it into an argument that is often
made for going for greater flexibility in tall buildings-- "greater
flexibility (i.e., longer fundamental period T for a given mass
distribution) implies it will attract smaller seismic forces based on
the 1/T drop in the design spectrum for longer period structures; I
can get away with a leaner structure that has to resist smaller
earthquake forces".

Why is this not a good argument for creating very flexible structures?

There are two very important reasons.

(i) Displacement demand: Let us consider two cantilevered SDOF
structures, each with mass M (weight P), one with lateral stiffness K
and the other with lateral stiffness K/4 (flexible).

Period of stiff structure/Period of flex structure = T_stiff/T_flex = 1/2

Let equivalent static lateral force for stiff structure = V
Then equivalent static lateral force for flexible structure = V * T_stiff/T_flex = V/2

Let lateral displacement in the two structures be denoted by u_stiff and u_flex.

P-Delta effects not considered: u_stiff = V/K; u_flex = V/2/K/4 = 2V/K;
P-Delta effects considered: u_stiff = V/(K-Mg/H); u_flex = V/2/(K/4-Mg/H) = 2V/(K-4Mg/H);
(http://www.earthquakespectra.org/doi/abs/10.1193/1.1585429)

You can see how quickly the displacements can get out of hand
(especially with P-Delta included). Can you build in enough ductility
into your structural system (not just in your materials) for it to be
able to undergo multiple cycles of the increased displacement
amplitude without losing stability?

(ii) Dynamics: The more flexible longer period structures are more
susceptible to long-period long-duration ground motion. Such ground
motion is produced by large magnitude earthquakes (moderate
earthquakes can also produce long-period pulse-like ground motion in
the near-source region) and get amplified in deep sedimentary basins
and/or soft soil deposits. Unfortunately, there are not enough number
of records from such earthquakes collected in basins around the world.
As a result, the global dataset on which all design spectra are based
upon is deficient in long-period motion. Ground motion from
relatively small magnitudes tend to have much less long-period content
and the 1/T dropoff is reasonable for such conditions. The demands in
the longer-period regime from these rare large events may be much
greater than what the code spectra across the globe seem to suggest.
The 1985 Mexico city earthquake demonstrated this; more recently, the
Christchurch event reinforced this (although this was not a large
event, the longer period ground motions were greatly amplified by the
soft soils). As engineers we must decide, whether to protect our
structures against such rare events or bet against the occurrence of
such events during the lifetime of the building.

Swaminathan Krishnan
California Institute of Technology
http://krishnan.caltech.edu

On 11/26/2012 03:29 AM, u.mukesh wrote:
Quote:
    Hello allIn tall buildings time period will be on higher side; due to which it will
attract less forces.

So is there any limitation of "T" on higher side; so that buildings dont attract
very less earthquake forces?


Regards
Mukesh




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sarfaraj.husain
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PostPosted: Tue Nov 27, 2012 6:27 am    Post subject: Time period-Tall Building Design Reply with quote

dear all

consider a tall building of height h =300m

time pd. T =0.075*300^0.75 =5.4 secs

how to get Sa/g ..........as per 1893-2002 all formula valid for T<= 4.00 secs...

if it is correct ...please clear my doubt...

sarfraj


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