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Empirical Expression for T of Tall Buildings and Minimum Sei
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cvrm
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PostPosted: Wed Nov 28, 2012 3:19 am    Post subject: Empirical Expression for T of Tall Buildings and Minimum Sei Reply with quote

Dear Colleagues:

Mr. Sarfaraj Husain reiterated the concern on the validity of the empirical Ta given in IS 1893 (Part 1) ...

The expressions for Approximate Natural Periods Ta given in the Indian seismic code IS 1893 (Part 1) were intended primarily for lower height buildings. These values have been verified (though in a very limited sense) with measurements of natural periods made on actual buildings of up to 17-20 storeys. But, it is unclear that these expressions are valid for Tall Buildings in India of the larger heights that one is discussing during the eC.

In todayís design practice in India (and elsewhere too), there is increased faith amongst designers in analytical natural periods T obtained from computer analysis. Typical low rise RC buildings in India have large amount of URM infills snugly placed between columns and beams of frame panels. Such buildings have significant contribution to lateral stiffness from unreinforced masonry (URM) infills, and hence the actual measured natural period (which is reflected by expression for Ta given in the Indian Seismic Code) is much smaller than the analytical natural period (which is known to reflect the mass reasonably well but fall short on the stiffness estimation). But, such a comparison is unavailable for Tall Buildings between analytical and empirical natural periods. The use of a variety of lighter partitions in the interior (not necessarily snug within the beams and columns of the frame panels) and of heavy URM infill walls in the exterior is being used more often in India. Adequate field data is unavailable yet in India on T of Tall Buildings with this mixed the use of URM infill walls and light partition wall.

This has implications on the design seismic lateral force of Tall Buildings. Confidence is higher, when empirical natural periods (which are based on field measurements) of Tall Buildings are used. But, the same is not true for analytical natural period, which involves significant skill and acumen to carefully include all RELIABLE stiffnesses of the building. This is where Professor Swaminathan warned us on use of stiffness contributions of URM infill walls that may not be reliable.

In closing, when the analytical modelling does not inspire confidence to the extent one needs in special structures like Tall Buildings, it seems prudent to
(1)  do due diligence in the analytical modeling of Tall Buildings, including effects of flexibility (if any) of the soil and foundation, and understanding implications of modeling on design lateral force and seismic behaviour of Tall Buildings; and
(2)  adopt a limit for at least a minimum design lateral force in the design of Tall Buildings. Setting this limit is a difficult task. Some basic research is necessary on this limit to get a feel of the implications, before embarking on a arriving at a consensus value to be adopted in design.

with warm regards...
C. V. R. Murty
..=

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Manoharbs_eq
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PostPosted: Wed Nov 28, 2012 5:44 am    Post subject: Reply with quote

Dear Sir,

The time period calculation by empirical formula is absolute, A model shall be prepared (mathematical or graphical) should include all the stiffness to its at most level. the effect of infill shall be included in model and then the time period should be accurately calculated.

A code cant just specify a formula for time period, it is a system dependent and depends on structure.so code shall allow individual to compute the time period instead of a equation.

Actually time period is one of the most important factor as resonance would cause complete collapse


Rgds
Manohar
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Yogesh.Pisal
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PostPosted: Sat Dec 01, 2012 8:05 am    Post subject: Reply with quote

Dear Manohar Sir,

I am not very much familar with the Tall building design practices. Hence, having following doubts regarding the Stiffness of Masonry infills

1. Equivalent diagonal can be used for the stiffness of masonry infill (my opinion)
2. Further, we may need to assign it as compression only element (my opinion)
3. But, modal analysis wont use to work with the compression only element.

In this case, whether it is possible to adopt present dynamic analysis methods if we wont go for scaling based on empirical time period estimations by modeling stiffness of masonry infills.

Regards,
Yogesh Pisal
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Manoharbs_eq
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PostPosted: Sat Dec 01, 2012 8:22 am    Post subject: Reply with quote

Dear Yogesh,

7.8.2 Dynamic analysis may be performed either
by the Time History Method or by the Response
Spectrum Method. However, in either method, the
design base shear ( VB) shall be compared with abase
shmr ( J?B) calculated using a fundamental period T,,
where T, is as per 7.6. Where tí~is less than ~~, all
the response quantities (for example memberforces,
displacements, storey forces, storey shears and base
reactions) shall be multiplied by ~~ / V~.

you are referring to this clause.

All the forces should be amplified to ta based eq static method, this is conservative side of code to accompany the ascertainment in loading.
we have to follow it and design the structure for enhanced forces.


regarding the diagonal strut is considered. provide the spring in diagonal passion to include the effect.


Rgds
Manohar
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Yogesh.Pisal
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PostPosted: Sat Dec 01, 2012 8:29 am    Post subject: Reply with quote

Dear Er Manohar,

I think I could not convey my message properly.

You are correct that we have to enhance the base shear eventhough we are using time history / response spectrum method.

I was just trying to highlight my opinion about your following statement in earlier post

"A model shall be prepared (mathematical or graphical) should include all the stiffness to its at most level. the effect of infill shall be included in model and then the time period should be accurately calculated.
A code cant just specify a formula for time period, it is a system dependent and depends on structure.so code shall allow individual to compute the time period instead of a equation"

Actually, when we model the stiffness of masonry infill by using compression only spring then modal analysis will not work and our conventional methods of dynamic analysis start with the modal analysis. Thus, our methods of dynamic analysis will stop working.

Hence, I feel that it is better to have wide range of empirical formulas for different buildings based on research rather than estimating the time period by modeling each and every stiffness.

Thanks & Regards,
Yogesh Pisal
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dipika_agrawal
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PostPosted: Sat Dec 01, 2012 10:41 am    Post subject: Serviceability, no factors considered for deflection calcula Reply with quote

Dear sirI have question regarding serviceability that why we don't consider factor while calculating deflection.- from Vodafone
From: "Manoharbs_eq" <forum@sefindia.org>
Date: Sat, 01 Dec 2012 13:52:09 +0530
To: <econf34289@sefindia.org>
ReplyTo: econf34289@sefindia.org
Subject: [E-CONF] Re: Empirical Expression for T of Tall Buildings and Minimum Sei

     Dear Yogesh,  7.8.2 Dynamic analysis may be performed either by the Time History Method or by the Response Spectrum Method. However, in either method, the design base shear ( VB) shall be compared with abase shmr ( J?B) calculated using a fundamental period T,, where T, is as per 7.6. Where tí~is less than ~~, all the response quantities (for example memberforces, displacements, storey forces, storey shears and base reactions) shall be multiplied by ~~ / V~.  you are referring to this clause.  All the forces should be amplified to ta based eq static method, this is conservative side of code to accompany the ascertainment in loading. we have to follow it and design the structure for enhanced forces.   regarding the diagonal strut is considered. provide the spring in diagonal passion to include the effect.   Rgds Manohar
        --

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Manoharbs_eq
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PostPosted: Sat Dec 01, 2012 1:35 pm    Post subject: Reply with quote

Please refer the attachment, we can perform the modal analysis with srings.



Rgds'
Manohar



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4-IJAEST-Volume-No-2-Issue-No-2-SEISMIC-ASSESEMENT-OF-RC-FRAME-BUILDINGS-WITH-BRICK-MASONRY-INFILLS-140-147.pdf
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 Filename:  4-IJAEST-Volume-No-2-Issue-No-2-SEISMIC-ASSESEMENT-OF-RC-FRAME-BUILDINGS-WITH-BRICK-MASONRY-INFILLS-140-147.pdf
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Yogesh.Pisal
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PostPosted: Sat Dec 01, 2012 4:52 pm    Post subject: Reply with quote

Dear Manohar sir,

I have seen push over analysis for frames with compression only struct for brick infill.

My Opinion : When we will have compression only / tension only elements or supports then the analysis will become non linear and it is not possible to carry out modal analysis for the same. Hence, generally STAAD dont use to carry out modal analysis when such elements / supports are present in the model.
I may be wrong and I will be more than happy to correct myself if I am wrong.

About the paper :
1. I think author has estimated fundamental time period using Rayleigh Method. Although he has not mentioned anything about the method which he has adopted to find out fundamental time period.
2. Author has written in the introduction that he has carried out Push over analysis and Non linear time history analysis. But, he has presented the results only for Push over analysis.
There are some other points, but those are not of interest for this discussion.

Regards,
Yogesh Pisal
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PostPosted: Mon Dec 03, 2012 4:03 am    Post subject: Reply with quote

Dear Yogesh,

Modal analysis can be performed. I am quite familiar with Etabs or SAP2000. when i started to design structures with lateral loads, especially for seismic effect i stopped using STAAD as it is time consuming , all my further work in current office and previous are using ETABS in etabs we use spring so it works.



Rgds
Manohar
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PostPosted: Mon Dec 03, 2012 4:05 am    Post subject: Reply with quote

To add upon


i calculated for a small structure by using matrix in MAT LAB and all that using time history analysis with El-centro eq and Kobe i was able to generate modes.Hence will check with Staad and come back.

Rgds
Manohar
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