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Empirical Expression for T of Tall Buildings and Minimum Sei
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Yogesh.Pisal
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PostPosted: Mon Dec 03, 2012 8:06 am    Post subject: Reply with quote

Dear Manohar,

I agree with your views.

For industrial structures, we have very good manual "Guidelines for Seismic Evaluation and Design of Petrochemical Facilities" by ASCE which covers empirical formulas for different industrial strucutres.

Let us check whether such formulas are available for tall buildings also in other country codes.

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Yogesh Pisal
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thirumalaichettiar
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PostPosted: Tue Dec 04, 2012 2:09 am    Post subject: Reply with quote

Eurocode 8 does not encourage designers to profit from the beneficial effects of masonry infills by reducing the seismic action effects for which the structure is designed.

If there is structural connection between the masonry infill & the surrounding frame( by shear connectors, other ties, belts or posts), the building is considered/designed as a confined masonry building not as a concrete structure with masonry infills.


As per the above provision the time period considered for infill in IS 1893-2002 seems to be unsafe means of designing structures against Earthquake.

Comments from Moderators and Experts please.

T.RangaRajan.
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gautam chattopadhyay
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PostPosted: Tue Dec 04, 2012 2:46 am    Post subject: Empirical Expression for T of Tall Buildings and Minimum Sei Reply with quote

I feel commercial wisdom of some engineers brought all nonissues to the surface. The controversy whether brick or CRM can be treated as shear wall is nearly 30 years old now and in fact resolved. Certain commercially wise engineers like to maintain the issue alive to please their clients who are normally semi literate builders. Phrase mongering and gimmicks are affecting proper engineering. Predicting fundamental time period of a multystoreyed building by looking at the building is a howler of the past when people did not know much about dynamic analysis and also there was no computer to solve eigen value problem of a dynamic matrix of a ten storeyed building. Such pseudo experts caused many disasters, many buildings collapsed, people died yet these pseudo experts remained untouched. If one seriously wants to conduct
dynamic analysis must learn dynamic analysis first.

On Tue, Dec 4, 2012 at 7:39 AM, thirumalaichettiar <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           Eurocode 8 does not encourage designers to profit from the beneficial effects of masonry infills by reducing the semic action effects for which the structure is designed.

If there is structural connection between the masoears old nownry infill & the surrounding frame( be y shear connectors, other ties, belts or posts), the building is considered/designed as a confined masonry building not as a concrete structure with masonry infills.[/color:5b168c14eb]

As per the above provision the time period considered for infill in IS 1893-2002 seems to be unsafe means of designing structures against Earthquake.

Comments from Moderators and Experts please.

T.RangaRajan.[/color:5b168c14eb]
     



     


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Manoharbs_eq
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PostPosted: Tue Dec 04, 2012 4:10 am    Post subject: Reply with quote

Dear Gutam sir,

That was my original post discussion, every structure should be analysed and designed to its own behavior not by empirical equations.


I strongly suggest that structures should be modeled and  analysed to its original behavior and state.


Rgds
Manohar
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Yogesh.Pisal
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PostPosted: Tue Dec 04, 2012 4:32 am    Post subject: Reply with quote

Dear Rangarajan sir,

I have following opinion

1. We do not intend to take benefit from the stiffness of the masonry infills.

2. We just want to keep minimum cut-off to the seismic percentage.

3. Further, as per IS1893 part 1 provisions we are not allowed to increase base shear as per empirical expressions. Hence, in any case base shear / time period estimates can not be UNSAFE as they are not the one which we should follow when the time period estimates from modal analysis governs.

4. There are many shortcumings of the modelings (One which I have posted in earlier posts in this thread). Hence, it is justified to fix up minimum seismic percentage. So that there will not be any underestimation of base shear in any case.

Now about IS1893 Part 1

1. I too feel that empirical time period estimates are not sufficient to cover all type of buildings. Further draft code to IS1893 Part 4 clearly specifies that empirical time period estimates as mentioned in Part 1 shall not be used for industrial bulidings.

2. I think same shall be the case for tall buildings also.

3. But, as there are chances of underestimation of seismic time period (due to inaccuracy in modeling - specially lowering of time period due to brick infills) I feel that code should either mention minimum seismic percentage or give some empirical expressions which will put limit on minimum seismic percentage.

4. Dynamic analysis shall be mandatory to have uniform distribution of base shear.

5. Code has not asked to take benefit of brick infill as shear wall


Dear Gautam sir,

Empirical expression does not ask to consider brick infill as shear wall. Their purpose is to have minimum seismic percentage. I dont feel that peoples have died because of these empirical expressions. That may be due to the misinterpretation about their purpose.

Further, I agree with the fact that expression specified in IS1893 Part 1 can not be used for tall buildings / industrial structure (in short for non regular structures). Hence, we are emphasizing on such wide range of formulas based on research.

Regards,
Yogesh Pisal
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Yogesh.Pisal
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PostPosted: Tue Dec 04, 2012 4:36 am    Post subject: Reply with quote

Dear Manohar,

You are correct. Every structure shall be modeled and designed for its actual behaviour.

My point is that we can not guarantee 100% perfection in modeling. Hence, it will be a good idea to assign minimum cut off to the base shear. Further, this has to be based on actual research.

I really dont know whether these expressions will be available in near further or not.

Regards,
Yogesh Pisal
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gautam chattopadhyay
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PostPosted: Tue Dec 04, 2012 10:21 am    Post subject: Empirical Expression for T of Tall Buildings and Minimum Sei Reply with quote

When you are convinced that the empirical formula is not for tall structures why are you wasting time by asking about its authenticity? A multisorey building is always a MDOF system and hence have roots of characteristic equation of the dynamic matrix. Such frequency cannot be expressed bu a formula or as a multiple of the number of storeyes. The formula did not kill people but engineers who used the formula beacause of their ignorance killed people. More responsible are those who introduced such empirical expression for time period of a multistorey building.

On Tue, Dec 4, 2012 at 10:02 AM, Yogesh.Pisal <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           Dear Rangarajan sir,

I have following opinion

1. We do not intend to take benefit from the stiffness of the masonry infills.

2. We just want to keep minimum cut-off to the seismic percentage.

3. Further, as per IS1893 part 1 provisions we are not allowed to increase base shear as per empirical expressions. Hence, in any case base shear / time period estimates can not be UNSAFE as they are not the one which we should follow when the time period estimates from modal analysis governs.

4. There are many shortcumings of the modelings (One which I have posted in earlier posts in this thread). Hence, it is justified to fix up minimum seismic percentage. So that there will not be any underestimation of base shear in any case.

Now about IS1893 Part 1

1. I too feel that empirical time period estimates are not sufficient to cover all type of buildings. Further draft code to IS1893 Part 4 clearly specifies that empirical time period estimates as mentioned in Part 1 shall not be used for industrial bulidings.

2. I think same shall be the case for tall buildings also.

3. But, as there are chances of underestimation of seismic time period (due to inaccuracy in modeling - specially lowering of time period due to brick infills) I feel that code should either mention minimum seismic percentage or give some empirical expressions which will put limit on minimum seismic percentage.

4. Dynamic analysis shall be mandatory to have uniform distribution of base shear.

5. Code has not asked to take benefit of brick infill as shear wall


Dear Gautam sir,

Empirical expression does not ask to consider brick infill as shear wall. Their purpose is to have minimum seismic percentage. I dont feel that peoples have died because of these empirical expressions. That may be due to the misinterpretation about their purpose.

Further, I agree with the fact that expression specified in IS1893 Part 1 can not be used for tall buildings / industrial structure (in short for non regular structures). Hence, we are emphasizing on such wide range of formulas based on research.

Regards,
Yogesh Pisal
     



     


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Yogesh.Pisal
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PostPosted: Tue Dec 04, 2012 11:12 am    Post subject: Reply with quote

Dear Gautam sir,

I was not expecting such a false language from senior person like you

"When you are convinced that the empirical formula is not for tall structures why are you wasting time by asking about its authenticity?"



This is a open forum and everybody has the right to put his thought. Further, many code committee members are the part of this conference. Hence, my intention is just to highlight this issue. I think nothing is wrong in this.


I think I have to keep posting until my doubts are clear. I am waiting for the reply by other wise SEFIans.

Regards,
Yogesh Pisal
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cckeshav
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PostPosted: Tue Dec 04, 2012 12:04 pm    Post subject: Empirical expressions in IS1893 for Seismic Period Reply with quote

Dear SEFIans:

There have been quite a lot of discussions on this aspect in this conference. Still there seems to be some confusion regarding provision of infill walls.

Firstly, the additional stiffness provided by infill walls also means that these walls draw some amount of seismic shear. However, they may not have enough strength to sustain these shears and may fail. During such a failure considerable inelastic energy is absorbed leading to damping.. which is actually good for the structure. However, the problem here is the damage caused by these failed walls which can be substantial. I believe in Latur, single storey structures with stone roofs caused many deaths.

Secondly, the period given by the code is not necessarily scientific. Further, the clause introduced in IS1893-2002 of enhancing the seismic base shear computed by a dynamic analysis to that of the approximate one worked out using empirical equations makes dynamic analysis almost redundant on most structures. As such, structural engineers who were using dynamic analysis earlier stopped using the same. It looks like this clause penalises dyamic analysis unjustly. My suggestion is that similar to ASCE 7 an increase in time period above that given by the approximate empirical period given for structures with infill walls can be allowed subject to the condition that a proper dynamic analysis considering all relevant stiffnesses is performed and the resulting period is more than that of the period mentioned above. Thus, more designers are encouraged to use dynamic analysis.

Thirdly, for tall structures, wherein a very large period is computed, a limit on minimum base shear can be considered for each seismic zone.

C. Channakeshava

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Yogesh.Pisal
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PostPosted: Tue Dec 04, 2012 12:36 pm    Post subject: Reply with quote

Dear Channakeshava Sir,

Thanks for the mail. I too have the opinion that there shall be some minimum limit on the percent base shear to be considered for design.

This limit can be in any form e.g. Limit on higher time period or limit on percent base shear etc. Finally safety shall be given prime importance.

Regards,
Yogesh Pisal
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