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How should one define min stiffness requirement of tall buil
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alpa_sheth
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PostPosted: Mon Nov 19, 2012 6:08 pm    Post subject: How should one define min stiffness requirement of tall buil Reply with quote

Dear Prof. Swaminathan, 

You raise a very imp point re. stiffness - 
Typically, we seem to link stiffness with max allowable deformation under the design seismic loads of  say h/500 and  up to elastoplastic inter story limit of h/120 for shear walls (say h/100 or frame-shear walls and so on) under such loads. 
What has been seen in the recent New Zealand, Tohoku, Chile earthquakes is that the spectral accelerations experienced were many times more than design accelerations and conventional theories did not work. Hence defining stiffness in relation to allowable deformations in a design earthquake is problematic to me because then it no longer remains an absolute requirement but a relative one- relative to selected design earthquake. 
I have seen in India some projects having a fundamental period of as high as 9 seconds for a building of just 220 to 230 m and the argument put forward  is that the building is  within allowable deformation/drift  values so why should there be any objection. 
So my question is- should we have some other way of defining stiffness- either as you say ratio of area of the shear walls/vertical elements to total floor area or max fundamental period of a building with relation to its height or ....
I'd like to hear from others regarding this issue as I think we seem to be building way too flexible buildings in India.


regards,
Alpa 












On Mon, Nov 19, 2012 at 1:00 PM, krishnan_caltech <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           Dear SEFIans,

I would like to welcome all members of the Structural Engineers Forum of India to this much-anticipated e-conference on Tall Building Design and Construction. The big headline from the 2010 M8.8 Maule earthquake was that of the 3000+ tall buildings (>10 stories) in Chile, 80 buildings were damaged and only one collapsed. Given the size of the event, there is a broad consensus in the global engineering community that this outcome is more than satisfactory as far as tall building performance is concerned. Much of the credit has been attributed to the building code revisions undertaken after the 1985 M7.8 Valparaiso earthquake that caused extensive damage. With few exceptions, the seismic provisions of the American Concrete Institute’s ACI-318 building code for structural concrete were adopted for the design of new reinforced concrete structures in Chile (as in India, the material of choice for tall buildings in Chile has been reinforced concrete). Damage was mostly limited to concrete crushing and spalling, and reinforcing bar buckling and fracture at the ends of thin shear walls. The shear wall boundary element detailing provisions in ACI-318 were omitted from the revised Chilean code and these thin walls lacked the required extent of confinement reinforcement. Engineers have concluded that thicker shear walls incorporating boundary elements would have prevented most of the observed damage. On the face of it, this seems to be a ratification of the ACI-318 seismic provisions and suggests that such a code could be adopted for tall building design the world over. While this may be the best course of action, this in itself may not be sufficient to produce the Chilean outcome. The ratio of wall area to floor area in Chilean tall buildings is far greater (2%-4%) than that in US tall buildings (1%-1.5%) despite the fact that they are designed nominally to the same provisions. The Chilean tall buildings are thus much stiffer than their US counterparts. The stiff Chilean tall buildings have been tested to a certain extent in this earthquake, whereas the flexible US tall buildings are yet to be tested. According to data collected by Prof. Moroni of the University of Chile, Chilean tall buildings have been getting progressively more flexible over the last 5 decades. During the same period, the damage rate in these buildings has been increasing with increasing flexibility (ref.: Prof. Jack Moehle’s EERI/PEER presentation). These observations raise important philosophical questions in the Indian context. Should Indian tall buildings go the stiff way of Chile or the flexible way of the US? How would one achieve this through the building code? I would like to see this question of “flexible vs stiff” vociferously debated in this conference. I would also like to understand and learn about the state of ductile detailing in Indian tall buildings. Do engineers and contractors recognize the importance of ductility detailing for earthquake resistance? How do we codify, implement, and regulate this? How do we educate all the stakeholders on the critical need for seismic detailing and get their “buy-in”? I hope to find answers to all these questions in this conference.

The Chilean example has also raised other important questions for India. The peak ground velocity in Concepcion was quite strong at 67cm/s; however, the peak ground displacement was only 21 cm. The duration of significant ground motion was quite long at 88 s, but not as much as the 1960 Chile and the 1964 Alaska earthquakes. Had the displacements and durations been greater, the outcome may have been quite different. Do we know what ground shaking would result in Delhi from a great earthquake on the Himalayan Frontal Thrust? Do we have models that can predict ground motion from such an event? I hope to find out answers to these questions in this conference. I hope similar seismological questions can be addressed for other large metropolitan cities in the northern belt and the western front including, but not limited to, Mumbai, Ahmedabad, Kolkata, Surat, Pune, Jaipur, Vadodara, and Allahabad.

I am also deeply concerned about the buildings on stilts that seem to be ubiquitous (at least in the south where I have traveled extensively in the last decade). We engineers have to come up with creative solutions to address the architectural drivers of such glaring seismic vulnerabilities without compromising structural (seismic) integrity. I hope to see creative solutions for such problems outlined in this conference. Tall first and second stories, termination and/or offsetting of gravity/lateral force resisting elements such as columns and walls, also fall in this category. I hope to see case histories of buildings with such idiosyncrasies and the novel approaches that SEFI engineers have undertaken to tackle such architecturally driven situations.

Last, but certainly not the least, environmental scientists warn us that global warming is going to result in more intense and violent storms and hurricanes in the coming decades. Our coastal cities are going to be most affected. The latest storms in the eastern US (New Jersey) and in eastern India (Chennai) are harbingers of storms to come. Are our tall buildings prepared to face these “Frankenstorms”? How do we deal with the moving target of wind hazard and our ever-increasing density of tall buildings in our mega-cities? During the lifetime of our building how will the wind loading patterns change with the rapidly changing landscape. How do we anticipate these changes at the design stage and build in contingencies into our designs, without being overly conservative and jacking up the costs? Seems like a pretty stiff challenge to me. I hope to hear your considered and deep thoughts on these “gray-area” questions.

In closing, I am looking forward to two weeks of exciting, rejuvenating, informative and enlightening debates and discussions on tall building design and construction in India. I sincerely hope that the SEFI community takes full advantage of this wonderful opportunity to share and elevate the state of this art, made possible by the remarkable vision and efforts of my co-moderators, Er. Alpha Sheth and Prof. C. V. R. Murty.

Swaminathan Krishnan, Co-Moderator
California Institute of Technology
http://krishnan.caltech.edu
     



     



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sarfaraj.husain
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PostPosted: Tue Nov 20, 2012 10:27 am    Post subject: How should one define min stiffness requirement of tall buil Reply with quote

though steel buildings are not much encouraged in india........after which height it is better to go for steel building ???


sarfraj...



From: "alpa_sheth" <forum@sefindia.org>
To: econf34289@sefindia.org,  
Date: 11/19/12 11:58 PM
Subject: [E-CONF] Re: How should one define min stiffness requirement of tall buildings?



Dear Prof. Swaminathan,

You raise a very imp point re. stiffness -
Typically, we seem to link stiffness with max allowable deformation under the design seismic loads of say h/500 and up to elastoplastic inter story limit of h/120 for shear walls (say h/100 or frame-shear walls and so on) under such loads.
What has been seen in the recent New Zealand, Tohoku, Chile earthquakes is that the spectral accelerations experienced were many times more than design accelerations and conventional theories did not work. Hence defining stiffness in relation to allowable deformations in a design earthquake is problematic to me because then it no longer remains an absolute requirement but a relative one- relative to selected design earthquake.
I have seen in India some projects having a fundamental period of as high as 9 seconds for a building of just 220 to 230 m and the argument put forward is that the building is within allowable deformation/drift values so why should there be any objection.
So my question is- should we have some other way of defining stiffness- either as you say ratio of area of the shear walls/vertical elements to total floor area or max fundamental period of a building with relation to its height or ....
I'd like to hear from others regarding this issue as I think we seem to be building way too flexible buildings in India.


regards,
Alpa












On Mon, Nov 19, 2012 at 1:00 PM, krishnan_caltech forum@sefindia.org)> wrote:   :  Dear SEFIans,

I would like to welcome all members of the Structural Engineers Forum of India to this much-anticipated e-conference on Tall Building Design and Construction. The big headline from the 2010 M8.8 Maule earthquake was that of the 3000+ tall buildings (>10 stories) in Chile, 80 buildings were damaged and only one collapsed. Given the size of the event, there is a broad consensus in the global engineering community that this outcome is more than satisfactory as far as tall building performance is concerned. Much of the credit has been attributed to the building code revisions undertaken after the 1985 M7.8 Valparaiso earthquake that caused extensive damage. With few exceptions, the seismic provisions of the American Concrete Institute’s ACI-318 building code for structural concrete were adopted for the design of new reinforced concrete structures in Chile (as in India, the material of choice for tall buildings in Chile has been reinforced concrete). Damage was mostly limited to concrete crushing and spalling, and reinforcing bar buckling and fracture at the ends of thin shear walls. The shear wall boundary element detailing provisions in ACI-318 were omitted from the revised Chilean code and these thin walls lacked the required extent of confinement reinforcement. Engineers have concluded that thicker shear walls incorporating boundary elements would have prevented most of the observed damage. On the face of it, this seems to be a ratification of the ACI-318 seismic provisions and suggests that such a code could be adopted for tall building design the world over. While this may be the best course of action, this in itself may not be sufficient to produce the Chilean outcome. The ratio of wall area to floor area in Chilean tall buildings is far greater (2%-4%) than that in US tall buildings (1%-1.5%) despite the fact that they are designed nominally to the same provisions. The Chilean tall buildings are thus much stiffer than their US counterparts. The stiff Chilean tall buildings have been tested to a certain extent in this earthquake, whereas the flexible US tall buildings are yet to be tested. According to data collected by Prof. Moroni of the University of Chile, Chilean tall buildings have been getting progressively more flexible over the last 5 decades. During the same period, the damage rate in these buildings has been increasing with increasing flexibility (ref.: Prof. Jack Moehle’s EERI/PEER presentation). These observations raise important philosophical questions in the Indian context. Should Indian tall buildings go the stiff way of Chile or the flexible way of the US? How would one achieve this through the building code? I would like to see this question of “flexible vs stiff” vociferously debated in this conference. I would also like to understand and learn about the state of ductile detailing in Indian tall buildings. Do engineers and contractors recognize the importance of ductility detailing for earthquake resistance? How do we codify, implement, and regulate this? How do we educate all the stakeholders on the critical need for seismic detailing and get their “buy-in”? I hope to find answers to all these questions in this conference.

The Chilean example has also raised other important questions for India. The peak ground velocity in Concepcion was quite strong at 67cm/s; however, the peak ground displacement was only 21 cm. The duration of significant ground motion was quite long at 88 s, but not as much as the 1960 Chile and the 1964 Alaska earthquakes. Had the displacements and durations been greater, the outcome may have been quite different. Do we know what ground shaking would result in Delhi from a great earthquake on the Himalayan Frontal Thrust? Do we have models that can predict ground motion from such an event? I hope to find out answers to these questions in this conference. I hope similar seismological questions can be addressed for other large metropolitan cities in the northern belt and the western front including, but not limited to, Mumbai, Ahmedabad, Kolkata, Surat, Pune, Jaipur, Vadodara, and Allahabad.

I am also deeply concerned about the buildings on stilts that seem to be ubiquitous (at least in the south where I have traveled extensively in the last decade). We engineers have to come up with creative solutions to address the architectural drivers of such glaring seismic vulnerabilities without compromising structural (seismic) integrity. I hope to see creative solutions for such problems outlined in this conference. Tall first and second stories, termination and/or offsetting of gravity/lateral force resisting elements such as columns and walls, also fall in this category. I hope to see case histories of buildings with such idiosyncrasies and the novel approaches that SEFI engineers have undertaken to tackle such architecturally driven situations.

Last, but certainly not the least, environmental scientists warn us that global warming is going to result in more intense and violent storms and hurricanes in the coming decades. Our coastal cities are going to be most affected. The latest storms in the eastern US (New Jersey) and in eastern India (Chennai) are harbingers of storms to come. Are our tall buildings prepared to face these “Frankenstorms”? How do we deal with the moving target of wind hazard and our ever-increasing density of tall buildings in our mega-cities? During the lifetime of our building how will the wind loading patterns change with the rapidly changing landscape. How do we anticipate these changes at the design stage and build in contingencies into our designs, without being overly conservative and jacking up the costs? Seems like a pretty stiff challenge to me. I hope to hear your considered and deep thoughts on these “gray-area” questions.

In closing, I am looking forward to two weeks of exciting, rejuvenating, informative and enlightening debates and discussions on tall building design and construction in India. I sincerely hope that the SEFI community takes full advantage of this wonderful opportunity to share and elevate the state of this art, made possible by the remarkable vision and efforts of my co-moderators, Er. Alpha Sheth and Prof. C. V. R. Murty.

Swaminathan Krishnan, Co-Moderator
California Institute of Technology
http://krishnan.caltech.edu












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vikas.pai
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PostPosted: Tue Nov 20, 2012 10:45 am    Post subject: Tall Buildings Stiffness Reply with quote

Hi Alpa,<xml><o></o>
<o> </o>
Based on my nuclear, heavy civil and bridge sector experience, I would say that the natural frequency of the structure should be the defining parameter. Actually the lateral deflections are derived from the natural frequency calculations and should be treated as thumb rule. But for all buildings that do not follow general rules of aspect ratio, 'soft storey' and/or deemed to attract higher importance than normal, controlling natural frequency within limits shall be the only way of satisfying basic engineering principles. Codes are just the guidelines and not 'bible'. Engineering judgement of what would work in the conditions given are of paramount importance and hence would need to be formed on sound engineering principles.<o></o>


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PostPosted: Tue Nov 20, 2012 3:21 pm    Post subject: Reply with quote

Reference to the above I would like to quote the following from

SP240: Performance-Based Design of Concrete Building for Wind Loads

Chapter 2: The Nature of Wind Loads and Dynamic Response by D. Boggs and J. Dragovich

The ASCE Standard [ASCE 7-05] classifies a structure as dynamically sensitive, or “flexible” if f0 < 1 Hz, otherwise it is considered to be “rigid.” The classification used by the ASCE Standard is widely accepted as a reasonable boundary between dynamic and rigid behavior. Consequently, this paper is focused on tall and medium-height buildings  for which f0 < 1 Hz, i.e. flexible buildings.

Approximate rules for determining f0 exist and will be briefly described. Where a building is designed for reasonable drift ratio, one widely used approximate formula is < 1 Hz, i.e. flexible buildings. Approximate rules for determining f0 exist and will be briefly described. Where a building is designed for reasonable drift ratio, one widely used approximate formula is
f0 = 46/H (H = building height in m)
  = 150/H (H in ft)                           (1)
which appears to agree well with measured values of many buildings worldwide. However, calculated frequencies of buildings in the U.S. appear to be better described by
75/H < f0 < 100/H (H in ft)         (2)
or, alternately as
75 <f0H < 100 (H in ft)             (2b)
Equation (2b) can be inferred directly from Fig. 2, where the y-axis is the product of f0 H. Note that if one considers a 10-foot story height, the upper bound of 100/H in equation (2a) is equivalent to the approximate period formula of T = 0.1N, used in many seismic building codes, where T is the period of vibration and N is the number of stories. The above equations imply that the calculated frequencies are typically less than the measured frequencies, thus conservatively classifying structures as flexible based on calculation. Because of this, it is recommended that equation (2a) be used as a preliminary indicator of f0, and a possible flexible condition. If the structure is flexible, it is essential that the designer determine the natural frequency and mode shapes of the first few modes of vibration accurately. This is typically accomplished using eigenvalue analysis routines in commercial three-dimensional finite element computer programs.

T.RangaRajan.
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sarfaraj.husain
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PostPosted: Wed Nov 21, 2012 4:29 am    Post subject: How should one define min stiffness requirement of tall buil Reply with quote

though steel buildings are not much encouraged in india........after which height it is better to go for steel building ???


sarfraj...



From: "alpa_sheth" <forum@sefindia.org>
To: econf34289@sefindia.org,  
Date: 11/19/12 11:58 PM
Subject: [E-CONF] Re: How should one define min stiffness requirement of tall buildings?



Dear Prof. Swaminathan,

You raise a very imp point re. stiffness -
Typically, we seem to link stiffness with max allowable deformation under the design seismic loads of say h/500 and up to elastoplastic inter story limit of h/120 for shear walls (say h/100 or frame-shear walls and so on) under such loads.
What has been seen in the recent New Zealand, Tohoku, Chile earthquakes is that the spectral accelerations experienced were many times more than design accelerations and conventional theories did not work. Hence defining stiffness in relation to allowable deformations in a design earthquake is problematic to me because then it no longer remains an absolute requirement but a relative one- relative to selected design earthquake.
I have seen in India some projects having a fundamental period of as high as 9 seconds for a building of just 220 to 230 m and the argument put forward is that the building is within allowable deformation/drift values so why should there be any objection.
So my question is- should we have some other way of defining stiffness- either as you say ratio of area of the shear walls/vertical elements to total floor area or max fundamental period of a building with relation to its height or ....
I'd like to hear from others regarding this issue as I think we seem to be building way too flexible buildings in India.


regards,
Alpa












On Mon, Nov 19, 2012 at 1:00 PM, krishnan_caltech forum@sefindia.org)> wrote:   :  Dear SEFIans,

I would like to welcome all members of the Structural Engineers Forum of India to this much-anticipated e-conference on Tall Building Design and Construction. The big headline from the 2010 M8.8 Maule earthquake was that of the 3000+ tall buildings (>10 stories) in Chile, 80 buildings were damaged and only one collapsed. Given the size of the event, there is a broad consensus in the global engineering community that this outcome is more than satisfactory as far as tall building performance is concerned. Much of the credit has been attributed to the building code revisions undertaken after the 1985 M7.8 Valparaiso earthquake that caused extensive damage. With few exceptions, the seismic provisions of the American Concrete Institute’s ACI-318 building code for structural concrete were adopted for the design of new reinforced concrete structures in Chile (as in India, the material of choice for tall buildings in Chile has been reinforced concrete). Damage was mostly limited to concrete crushing and spalling, and reinforcing bar buckling and fracture at the ends of thin shear walls. The shear wall boundary element detailing provisions in ACI-318 were omitted from the revised Chilean code and these thin walls lacked the required extent of confinement reinforcement. Engineers have concluded that thicker shear walls incorporating boundary elements would have prevented most of the observed damage. On the face of it, this seems to be a ratification of the ACI-318 seismic provisions and suggests that such a code could be adopted for tall building design the world over. While this may be the best course of action, this in itself may not be sufficient to produce the Chilean outcome. The ratio of wall area to floor area in Chilean tall buildings is far greater (2%-4%) than that in US tall buildings (1%-1.5%) despite the fact that they are designed nominally to the same provisions. The Chilean tall buildings are thus much stiffer than their US counterparts. The stiff Chilean tall buildings have been tested to a certain extent in this earthquake, whereas the flexible US tall buildings are yet to be tested. According to data collected by Prof. Moroni of the University of Chile, Chilean tall buildings have been getting progressively more flexible over the last 5 decades. During the same period, the damage rate in these buildings has been increasing with increasing flexibility (ref.: Prof. Jack Moehle’s EERI/PEER presentation). These observations raise important philosophical questions in the Indian context. Should Indian tall buildings go the stiff way of Chile or the flexible way of the US? How would one achieve this through the building code? I would like to see this question of “flexible vs stiff” vociferously debated in this conference. I would also like to understand and learn about the state of ductile detailing in Indian tall buildings. Do engineers and contractors recognize the importance of ductility detailing for earthquake resistance? How do we codify, implement, and regulate this? How do we educate all the stakeholders on the critical need for seismic detailing and get their “buy-in”? I hope to find answers to all these questions in this conference.

The Chilean example has also raised other important questions for India. The peak ground velocity in Concepcion was quite strong at 67cm/s; however, the peak ground displacement was only 21 cm. The duration of significant ground motion was quite long at 88 s, but not as much as the 1960 Chile and the 1964 Alaska earthquakes. Had the displacements and durations been greater, the outcome may have been quite different. Do we know what ground shaking would result in Delhi from a great earthquake on the Himalayan Frontal Thrust? Do we have models that can predict ground motion from such an event? I hope to find out answers to these questions in this conference. I hope similar seismological questions can be addressed for other large metropolitan cities in the northern belt and the western front including, but not limited to, Mumbai, Ahmedabad, Kolkata, Surat, Pune, Jaipur, Vadodara, and Allahabad.

I am also deeply concerned about the buildings on stilts that seem to be ubiquitous (at least in the south where I have traveled extensively in the last decade). We engineers have to come up with creative solutions to address the architectural drivers of such glaring seismic vulnerabilities without compromising structural (seismic) integrity. I hope to see creative solutions for such problems outlined in this conference. Tall first and second stories, termination and/or offsetting of gravity/lateral force resisting elements such as columns and walls, also fall in this category. I hope to see case histories of buildings with such idiosyncrasies and the novel approaches that SEFI engineers have undertaken to tackle such architecturally driven situations.

Last, but certainly not the least, environmental scientists warn us that global warming is going to result in more intense and violent storms and hurricanes in the coming decades. Our coastal cities are going to be most affected. The latest storms in the eastern US (New Jersey) and in eastern India (Chennai) are harbingers of storms to come. Are our tall buildings prepared to face these “Frankenstorms”? How do we deal with the moving target of wind hazard and our ever-increasing density of tall buildings in our mega-cities? During the lifetime of our building how will the wind loading patterns change with the rapidly changing landscape. How do we anticipate these changes at the design stage and build in contingencies into our designs, without being overly conservative and jacking up the costs? Seems like a pretty stiff challenge to me. I hope to hear your considered and deep thoughts on these “gray-area” questions.

In closing, I am looking forward to two weeks of exciting, rejuvenating, informative and enlightening debates and discussions on tall building design and construction in India. I sincerely hope that the SEFI community takes full advantage of this wonderful opportunity to share and elevate the state of this art, made possible by the remarkable vision and efforts of my co-moderators, Er. Alpha Sheth and Prof. C. V. R. Murty.

Swaminathan Krishnan, Co-Moderator
California Institute of Technology
http://krishnan.caltech.edu












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ykalamkar
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PostPosted: Wed Nov 21, 2012 10:00 am    Post subject: How should one define min stiffness requirement of tall buil Reply with quote

Dear sefians,
We are designing structures with flat slab and shear walls. I have one doubt. How to check the connectivity of shear wall with slab? Normally Lift walls are one of the shear wall. If there are multiple lifts, which is normal case In high rise buildings, internal walls have only 300 mm bearing for horizontal shear. How to design this junction? Is some additional steel is required? Any codal provision?

Thanks & regards
Yogesh Kalamkar



From: alpa_sheth [mailto:forum@sefindia.org]
Sent: 20 November 2012 00:00
To: econf34289@sefindia.org
Subject: [E-CONF] Re: How should one define min stiffness requirement of tall buildings?



Dear Prof. Swaminathan,

You raise a very imp point re. stiffness -
Typically, we seem to link stiffness with max allowable deformation under the design seismic loads of say h/500 and up to elastoplastic inter story limit of h/120 for shear walls (say h/100 or frame-shear walls and so on) under such loads.
What has been seen in the recent New Zealand, Tohoku, Chile earthquakes is that the spectral accelerations experienced were many times more than design accelerations and conventional theories did not work. Hence defining stiffness in relation to allowable deformations in a design earthquake is problematic to me because then it no longer remains an absolute requirement but a relative one- relative to selected design earthquake.
I have seen in India some projects having a fundamental period of as high as 9 seconds for a building of just 220 to 230 m and the argument put forward is that the building is within allowable deformation/drift values so why should there be any objection.
So my question is- should we have some other way of defining stiffness- either as you say ratio of area of the shear walls/vertical elements to total floor area or max fundamental period of a building with relation to its height or ....
I'd like to hear from others regarding this issue as I think we seem to be building way too flexible buildings in India.


regards,
Alpa












On Mon, Nov 19, 2012 at 1:00 PM, krishnan_caltech forum@sefindia.org (forum@sefindia.org))> wrote:
:
Dear SEFIans,

I would like to welcome all members of the Structural Engineers Forum of India to this much-anticipated e-conference on Tall Building Design and Construction. The big headline from the 2010 M8.8 Maule earthquake was that of the 3000+ tall buildings (>10 stories) in Chile, 80 buildings were damaged and only one collapsed. Given the size of the event, there is a broad consensus in the global engineering community that this outcome is more than satisfactory as far as tall building performance is concerned. Much of the credit has been attributed to the building code revisions undertaken after the 1985 M7.8 Valparaiso earthquake that caused extensive damage. With few exceptions, the seismic provisions of the American Concrete Institute’s ACI-318 building code for structural concrete were adopted for the design of new reinforced concrete structures in Chile (as in India, the material of choice for tall buildings in Chile has been reinforced concrete). Damage was mostly limited to concrete crushing and spalling, and reinforcing bar buckling and fracture at the ends of thin shear walls. The shear wall boundary element detailing provisions in ACI-318 were omitted from the revised Chilean code and these thin walls lacked the required extent of confinement reinforcement. Engineers have concluded that thicker shear walls incorporating boundary elements would have prevented most of the observed damage. On the face of it, this seems to be a ratification of the ACI-318 seismic provisions and suggests that such a code could be adopted for tall building design the world over. While this may be the best course of action, this in itself may not be sufficient to produce the Chilean outcome. The ratio of wall area to floor area in Chilean tall buildings is far greater (2%-4%) than that in US tall buildings (1%-1.5%) despite the fact that they are designed nominally to the same provisions. The Chilean tall buildings are thus much stiffer than their US counterparts. The stiff Chilean tall buildings have been tested to a certain extent in this earthquake, whereas the flexible US tall buildings are yet to be tested. According to data collected by Prof. Moroni of the University of Chile, Chilean tall buildings have been getting progressively more flexible over the last 5 decades. During the same period, the damage rate in these buildings has been increasing with increasing flexibility (ref.: Prof. Jack Moehle’s EERI/PEER presentation). These observations raise important philosophical questions in the Indian context. Should Indian tall buildings go the stiff way of Chile or the flexible way of the US? How would one achieve this through the building code? I would like to see this question of “flexible vs stiff” vociferously debated in this conference. I would also like to understand and learn about the state of ductile detailing in Indian tall buildings. Do engineers and contractors recognize the importance of ductility detailing for earthquake resistance? How do we codify, implement, and regulate this? How do we educate all the stakeholders on the critical need for seismic detailing and get their “buy-in”? I hope to find answers to all these questions in this conference.

The Chilean example has also raised other important questions for India. The peak ground velocity in Concepcion was quite strong at 67cm/s; however, the peak ground displacement was only 21 cm. The duration of significant ground motion was quite long at 88 s, but not as much as the 1960 Chile and the 1964 Alaska earthquakes. Had the displacements and durations been greater, the outcome may have been quite different. Do we know what ground shaking would result in Delhi from a great earthquake on the Himalayan Frontal Thrust? Do we have models that can predict ground motion from such an event? I hope to find out answers to these questions in this conference. I hope similar seismological questions can be addressed for other large metropolitan cities in the northern belt and the western front including, but not limited to, Mumbai, Ahmedabad, Kolkata, Surat, Pune, Jaipur, Vadodara, and Allahabad.

I am also deeply concerned about the buildings on stilts that seem to be ubiquitous (at least in the south where I have traveled extensively in the last decade). We engineers have to come up with creative solutions to address the architectural drivers of such glaring seismic vulnerabilities without compromising structural (seismic) integrity. I hope to see creative solutions for such problems outlined in this conference. Tall first and second stories, termination and/or offsetting of gravity/lateral force resisting elements such as columns and walls, also fall in this category. I hope to see case histories of buildings with such idiosyncrasies and the novel approaches that SEFI engineers have undertaken to tackle such architecturally driven situations.

Last, but certainly not the least, environmental scientists warn us that global warming is going to result in more intense and violent storms and hurricanes in the coming decades. Our coastal cities are going to be most affected. The latest storms in the eastern US (New Jersey) and in eastern India (Chennai) are harbingers of storms to come. Are our tall buildings prepared to face these “Frankenstorms”? How do we deal with the moving target of wind hazard and our ever-increasing density of tall buildings in our mega-cities? During the lifetime of our building how will the wind loading patterns change with the rapidly changing landscape. How do we anticipate these changes at the design stage and build in contingencies into our designs, without being overly conservative and jacking up the costs? Seems like a pretty stiff challenge to me. I hope to hear your considered and deep thoughts on these “gray-area” questions.

In closing, I am looking forward to two weeks of exciting, rejuvenating, informative and enlightening debates and discussions on tall building design and construction in India. I sincerely hope that the SEFI community takes full advantage of this wonderful opportunity to share and elevate the state of this art, made possible by the remarkable vision and efforts of my co-moderators, Er. Alpha Sheth and Prof. C. V. R. Murty.

Swaminathan Krishnan, Co-Moderator
California Institute of Technology
http://krishnan.caltech.edu

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PostPosted: Thu Nov 22, 2012 4:26 am    Post subject: How should one define min stiffness requirement of tall buil Reply with quote

though steel buildings are not much encouraged in india........after which height it is better to go for steel building ???


sarfraj...



From: "alpa_sheth" <forum@sefindia.org>
To: econf34289@sefindia.org,  
Date: 11/19/12 11:58 PM
Subject: [E-CONF] Re: How should one define min stiffness requirement of tall buildings?



Dear Prof. Swaminathan,

You raise a very imp point re. stiffness -
Typically, we seem to link stiffness with max allowable deformation under the design seismic loads of say h/500 and up to elastoplastic inter story limit of h/120 for shear walls (say h/100 or frame-shear walls and so on) under such loads.
What has been seen in the recent New Zealand, Tohoku, Chile earthquakes is that the spectral accelerations experienced were many times more than design accelerations and conventional theories did not work. Hence defining stiffness in relation to allowable deformations in a design earthquake is problematic to me because then it no longer remains an absolute requirement but a relative one- relative to selected design earthquake.
I have seen in India some projects having a fundamental period of as high as 9 seconds for a building of just 220 to 230 m and the argument put forward is that the building is within allowable deformation/drift values so why should there be any objection.
So my question is- should we have some other way of defining stiffness- either as you say ratio of area of the shear walls/vertical elements to total floor area or max fundamental period of a building with relation to its height or ....
I'd like to hear from others regarding this issue as I think we seem to be building way too flexible buildings in India.


regards,
Alpa












On Mon, Nov 19, 2012 at 1:00 PM, krishnan_caltech forum@sefindia.org)> wrote:   :  Dear SEFIans,

I would like to welcome all members of the Structural Engineers Forum of India to this much-anticipated e-conference on Tall Building Design and Construction. The big headline from the 2010 M8.8 Maule earthquake was that of the 3000+ tall buildings (>10 stories) in Chile, 80 buildings were damaged and only one collapsed. Given the size of the event, there is a broad consensus in the global engineering community that this outcome is more than satisfactory as far as tall building performance is concerned. Much of the credit has been attributed to the building code revisions undertaken after the 1985 M7.8 Valparaiso earthquake that caused extensive damage. With few exceptions, the seismic provisions of the American Concrete Institute’s ACI-318 building code for structural concrete were adopted for the design of new reinforced concrete structures in Chile (as in India, the material of choice for tall buildings in Chile has been reinforced concrete). Damage was mostly limited to concrete crushing and spalling, and reinforcing bar buckling and fracture at the ends of thin shear walls. The shear wall boundary element detailing provisions in ACI-318 were omitted from the revised Chilean code and these thin walls lacked the required extent of confinement reinforcement. Engineers have concluded that thicker shear walls incorporating boundary elements would have prevented most of the observed damage. On the face of it, this seems to be a ratification of the ACI-318 seismic provisions and suggests that such a code could be adopted for tall building design the world over. While this may be the best course of action, this in itself may not be sufficient to produce the Chilean outcome. The ratio of wall area to floor area in Chilean tall buildings is far greater (2%-4%) than that in US tall buildings (1%-1.5%) despite the fact that they are designed nominally to the same provisions. The Chilean tall buildings are thus much stiffer than their US counterparts. The stiff Chilean tall buildings have been tested to a certain extent in this earthquake, whereas the flexible US tall buildings are yet to be tested. According to data collected by Prof. Moroni of the University of Chile, Chilean tall buildings have been getting progressively more flexible over the last 5 decades. During the same period, the damage rate in these buildings has been increasing with increasing flexibility (ref.: Prof. Jack Moehle’s EERI/PEER presentation). These observations raise important philosophical questions in the Indian context. Should Indian tall buildings go the stiff way of Chile or the flexible way of the US? How would one achieve this through the building code? I would like to see this question of “flexible vs stiff” vociferously debated in this conference. I would also like to understand and learn about the state of ductile detailing in Indian tall buildings. Do engineers and contractors recognize the importance of ductility detailing for earthquake resistance? How do we codify, implement, and regulate this? How do we educate all the stakeholders on the critical need for seismic detailing and get their “buy-in”? I hope to find answers to all these questions in this conference.

The Chilean example has also raised other important questions for India. The peak ground velocity in Concepcion was quite strong at 67cm/s; however, the peak ground displacement was only 21 cm. The duration of significant ground motion was quite long at 88 s, but not as much as the 1960 Chile and the 1964 Alaska earthquakes. Had the displacements and durations been greater, the outcome may have been quite different. Do we know what ground shaking would result in Delhi from a great earthquake on the Himalayan Frontal Thrust? Do we have models that can predict ground motion from such an event? I hope to find out answers to these questions in this conference. I hope similar seismological questions can be addressed for other large metropolitan cities in the northern belt and the western front including, but not limited to, Mumbai, Ahmedabad, Kolkata, Surat, Pune, Jaipur, Vadodara, and Allahabad.

I am also deeply concerned about the buildings on stilts that seem to be ubiquitous (at least in the south where I have traveled extensively in the last decade). We engineers have to come up with creative solutions to address the architectural drivers of such glaring seismic vulnerabilities without compromising structural (seismic) integrity. I hope to see creative solutions for such problems outlined in this conference. Tall first and second stories, termination and/or offsetting of gravity/lateral force resisting elements such as columns and walls, also fall in this category. I hope to see case histories of buildings with such idiosyncrasies and the novel approaches that SEFI engineers have undertaken to tackle such architecturally driven situations.

Last, but certainly not the least, environmental scientists warn us that global warming is going to result in more intense and violent storms and hurricanes in the coming decades. Our coastal cities are going to be most affected. The latest storms in the eastern US (New Jersey) and in eastern India (Chennai) are harbingers of storms to come. Are our tall buildings prepared to face these “Frankenstorms”? How do we deal with the moving target of wind hazard and our ever-increasing density of tall buildings in our mega-cities? During the lifetime of our building how will the wind loading patterns change with the rapidly changing landscape. How do we anticipate these changes at the design stage and build in contingencies into our designs, without being overly conservative and jacking up the costs? Seems like a pretty stiff challenge to me. I hope to hear your considered and deep thoughts on these “gray-area” questions.

In closing, I am looking forward to two weeks of exciting, rejuvenating, informative and enlightening debates and discussions on tall building design and construction in India. I sincerely hope that the SEFI community takes full advantage of this wonderful opportunity to share and elevate the state of this art, made possible by the remarkable vision and efforts of my co-moderators, Er. Alpha Sheth and Prof. C. V. R. Murty.

Swaminathan Krishnan, Co-Moderator
California Institute of Technology
http://krishnan.caltech.edu












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gautam chattopadhyay
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PostPosted: Thu Nov 22, 2012 5:38 am    Post subject: How should one define min stiffness requirement of tall buil Reply with quote

Alpa, stiffness is geometric cum deformation property of a structural element. If you redefine it in terms of area, instead of moment of inertia, I feel assimilation of stiffness matrix will be queersome. Let us not redefine the fundamantals, rather simulate more conservative hystogram/ time history to accommodate the awesome earthquakes we have experienced in the recent past. 

On Thu, Nov 22, 2012 at 11:00 AM, sarfaraj.husain <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           though steel buildings are not much encouraged in india........after which height it is better to go for steel building ???


sarfraj...



From: "alpa_sheth"
To: econf34289@sefindia.org (econf34289@sefindia.org),
Date: 11/19/12 11:58 PM
Subject: [E-CONF] Re: How should one define min stiffness requirement of tall buildings?



Dear Prof. Swaminathan,

You raise a very imp point re. stiffness -
Typically, we seem to link stiffness with max allowable deformation under the design seismic loads of say h/500 and up to elastoplastic inter story limit of h/120 for shear walls (say h/100 or frame-shear walls and so on) under such loads.
What has been seen in the recent New Zealand, Tohoku, Chile earthquakes is that the spectral accelerations experienced were many times more than design accelerations and conventional theories did not work. Hence defining stiffness in relation to allowable deformations in a design earthquake is problematic to me because then it no longer remains an absolute requirement but a relative one- relative to selected design earthquake.
I have seen in India some projects having a fundamental period of as high as 9 seconds for a building of just 220 to 230 m and the argument put forward is that the building is within allowable deformation/drift values so why should there be any objection.
So my question is- should we have some other way of defining stiffness- either as you say ratio of area of the shear walls/vertical elements to total floor area or max fundamental period of a building with relation to its height or ....
I'd like to hear from others regarding this issue as I think we seem to be building way too flexible buildings in India.


regards,
Alpa












On Mon, Nov 19, 2012 at 1:00 PM, krishnan_caltech forum@sefindia.org (forum@sefindia.org))> wrote: : Dear SEFIans,

I would like to welcome all members of the Structural Engineers Forum of India to this much-anticipated e-conference on Tall Building Design and Construction. The big headline from the 2010 M8.8 Maule earthquake was that of the 3000+ tall buildings (>10 stories) in Chile, 80 buildings were damaged and only one collapsed. Given the size of the event, there is a broad consensus in the global engineering community that this outcome is more than satisfactory as far as tall building performance is concerned. Much of the credit has been attributed to the building code revisions undertaken after the 1985 M7.8 Valparaiso earthquake that caused extensive damage. With few exceptions, the seismic provisions of the American Concrete Institute’s ACI-318 building code for structural concrete were adopted for the design of new reinforced concrete structures in Chile (as in India, the material of choice for tall buildings in Chile has been reinforced concrete). Damage was mostly limited to concrete crushing and spalling, and reinforcing bar buckling and fracture at the ends of thin shear walls. The shear wall boundary element detailing provisions in ACI-318 were omitted from the revised Chilean code and these thin walls lacked the required extent of confinement reinforcement. Engineers have concluded that thicker shear walls incorporating boundary elements would have prevented most of the observed damage. On the face of it, this seems to be a ratification of the ACI-318 seismic provisions and suggests that such a code could be adopted for tall building design the world over. While this may be the best course of action, this in itself may not be sufficient to produce the Chilean outcome. The ratio of wall area to floor area in Chilean tall buildings is far greater (2%-4%) than that in US tall buildings (1%-1.5%) despite the fact that they are designed nominally to the same provisions. The Chilean tall buildings are thus much stiffer than their US counterparts. The stiff Chilean tall buildings have been tested to a certain extent in this earthquake, whereas the flexible US tall buildings are yet to be tested. According to data collected by Prof. Moroni of the University of Chile, Chilean tall buildings have been getting progressively more flexible over the last 5 decades. During the same period, the damage rate in these buildings has been increasing with increasing flexibility (ref.: Prof. Jack Moehle’s EERI/PEER presentation). These observations raise important philosophical questions in the Indian context. Should Indian tall buildings go the stiff way of Chile or the flexible way of the US? How would one achieve this through the building code? I would like to see this question of “flexible vs stiff†vociferously debated in this conference. I would also like to understand and learn about the state of ductile detailing in Indian tall buildings. Do engineers and contractors recognize the importance of ductility detailing for earthquake resistance? How do we codify, implement, and regulate this? How do we educate all the stakeholders on the critical need for seismic detailing and get their “buy-in†? I hope to find answers to all these questions in this conference.

The Chilean example has also raised other important questions for India. The peak ground velocity in Concepcion was quite strong at 67cm/s; however, the peak ground displacement was only 21 cm. The duration of significant ground motion was quite long at 88 s, but not as much as the 1960 Chile and the 1964 Alaska earthquakes. Had the displacements and durations been greater, the outcome may have been quite different. Do we know what ground shaking would result in Delhi from a great earthquake on the Himalayan Frontal Thrust? Do we have models that can predict ground motion from such an event? I hope to find out answers to these questions in this conference. I hope similar seismological questions can be addressed for other large metropolitan cities in the northern belt and the western front including, but not limited to, Mumbai, Ahmedabad, Kolkata, Surat, Pune, Jaipur, Vadodara, and Allahabad.

I am also deeply concerned about the buildings on stilts that seem to be ubiquitous (at least in the south where I have traveled extensively in the last decade). We engineers have to come up with creative solutions to address the architectural drivers of such glaring seismic vulnerabilities without compromising structural (seismic) integrity. I hope to see creative solutions for such problems outlined in this conference. Tall first and second stories, termination and/or offsetting of gravity/lateral force resisting elements such as columns and walls, also fall in this category. I hope to see case histories of buildings with such idiosyncrasies and the novel approaches that SEFI engineers have undertaken to tackle such architecturally driven situations.

Last, but certainly not the least, environmental scientists warn us that global warming is going to result in more intense and violent storms and hurricanes in the coming decades. Our coastal cities are going to be most affected. The latest storms in the eastern US (New Jersey) and in eastern India (Chennai) are harbingers of storms to come. Are our tall buildings prepared to face these “Frankenstorms†? How do we deal with the moving target of wind hazard and our ever-increasing density of tall buildings in our mega-cities? During the lifetime of our building how will the wind loading patterns change with the rapidly changing landscape. How do we anticipate these changes at the design stage and build in contingencies into our designs, without being overly conservative and jacking up the costs? Seems like a pretty stiff challenge to me. I hope to hear your considered and deep thoughts on these “gray-area†questions.

In closing, I am looking forward to two weeks of exciting, rejuvenating, informative and enlightening debates and discussions on tall building design and construction in India. I sincerely hope that the SEFI community takes full advantage of this wonderful opportunity to share and elevate the state of this art, made possible by the remarkable vision and efforts of my co-moderators, Er. Alpha Sheth and Prof. C. V. R. Murty.

Swaminathan Krishnan, Co-Moderator
California Institute of Technology
http://krishnan.caltech.edu












-- ­­
     



     



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RKBhola
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Joined: 05 Dec 2009
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PostPosted: Thu Nov 22, 2012 9:32 am    Post subject: How should one define min stiffness requirement of tall buil Reply with quote

Welcome SEFIians,
 
The issue of stiffness for tall buildings is important as it is linked to the overall behaviour of the building on a 'macro' level.  This also gets linked to the 'soft storey' that we see in almost all tall buildings, due to the presence of basement/stilt/podium parking.  Impact of the 'Aspect Ratio' (Height/Plan Dimension) is also a factor which should determine the orientation of the shear walls.
 
Should we consider the buildings in Chile as stiff because they have a wall area of 2-3%, or the ones in the US as flexible as the wall area is 1-2%?  I'm sure there are numerous other factors to be considered, and it will be helpful if we can enumerate these, and also get some detailed information about their relative impact on the stiffness.
 
In some recent discussions with senior academicians, we have come across the viewpoint that the relative stiffness between the stilt and typical floor levels should be 100% - even though the Code would permit 80% stiffness before classifying the storey as a 'soft storey'.  What is the viewpoint of the other experts on this panel?  Even when we do define a storey as 'soft' in my opinion it should not be a 'threshold' whereafter the Seismic Loads are to be increased to 2.50 times - a more gradual approach to the enhancement of the Seismic Forces would seem to be more in order.  Do we have some data/information about the same?
 
Given the complexity of the buildings that all of us come across as designers, one can be sure that there can be no straightforward formulae to address the above.  However, if we can discuss some design factors that we have arrived at by our experience - it will help the budding designers to come up with better performing structures.
 
Regards
 
Rajendra Kumar Bhola
CIVTECH


On Wed, Nov 21, 2012 at 9:30 PM, sarfaraj.husain <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           though steel buildings are not much encouraged in india........after which height it is better to go for steel building ???


sarfraj...



From: "alpa_sheth"
To: econf34289@sefindia.org (econf34289@sefindia.org),
Date: 11/19/12 11:58 PM
Subject: [E-CONF] Re: How should one define min stiffness requirement of tall buildings?



Dear Prof. Swaminathan,

You raise a very imp point re. stiffness -
Typically, we seem to link stiffness with max allowable deformation under the design seismic loads of say h/500 and up to elastoplastic inter story limit of h/120 for shear walls (say h/100 or frame-shear walls and so on) under such loads.
What has been seen in the recent New Zealand, Tohoku, Chile earthquakes is that the spectral accelerations experienced were many times more than design accelerations and conventional theories did not work. Hence defining stiffness in relation to allowable deformations in a design earthquake is problematic to me because then it no longer remains an absolute requirement but a relative one- relative to selected design earthquake.
I have seen in India some projects having a fundamental period of as high as 9 seconds for a building of just 220 to 230 m and the argument put forward is that the building is within allowable deformation/drift values so why should there be any objection.
So my question is- should we have some other way of defining stiffness- either as you say ratio of area of the shear walls/vertical elements to total floor area or max fundamental period of a building with relation to its height or ....
I'd like to hear from others regarding this issue as I think we seem to be building way too flexible buildings in India.


regards,
Alpa












On Mon, Nov 19, 2012 at 1:00 PM, krishnan_caltech forum@sefindia.org (forum@sefindia.org))> wrote: : Dear SEFIans,

I would like to welcome all members of the Structural Engineers Forum of India to this much-anticipated e-conference on Tall Building Design and Construction. The big headline from the 2010 M8.8 Maule earthquake was that of the 3000+ tall buildings (>10 stories) in Chile, 80 buildings were damaged and only one collapsed. Given the size of the event, there is a broad consensus in the global engineering community that this outcome is more than satisfactory as far as tall building performance is concerned. Much of the credit has been attributed to the building code revisions undertaken after the 1985 M7.8 Valparaiso earthquake that caused extensive damage. With few exceptions, the seismic provisions of the American Concrete Institute’s ACI-318 building code for structural concrete were adopted for the design of new reinforced concrete structures in Chile (as in India, the material of choice for tall buildings in Chile has been reinforced concrete). Damage was mostly limited to concrete crushing and spalling, and reinforcing bar buckling and fracture at the ends of thin shear walls. The shear wall boundary element detailing provisions in ACI-318 were omitted from the revised Chilean code and these thin walls lacked the required extent of confinement reinforcement. Engineers have concluded that thicker shear walls incorporating boundary elements would have prevented most of the observed damage. On the face of it, this seems to be a ratification of the ACI-318 seismic provisions and suggests that such a code could be adopted for tall building design the world over. While this may be the best course of action, this in itself may not be sufficient to produce the Chilean outcome. The ratio of wall area to floor area in Chilean tall buildings is far greater (2%-4%) than that in US tall buildings (1%-1.5%) despite the fact that they are designed nominally to the same provisions. The Chilean tall buildings are thus much stiffer than their US counterparts. The stiff Chilean tall buildings have been tested to a certain extent in this earthquake, whereas the flexible US tall buildings are yet to be tested. According to data collected by Prof. Moroni of the University of Chile, Chilean tall buildings have been getting progressively more flexible over the last 5 decades. During the same period, the damage rate in these buildings has been increasing with increasing flexibility (ref.: Prof. Jack Moehle’s EERI/PEER presentation). These observations raise important philosophical questions in the Indian context. Should Indian tall buildings go the stiff way of Chile or the flexible way of the US? How would one achieve this through the building code? I would like to see this question of “flexible vs stiff†vociferously debated in this conference. I would also like to understand and learn about the state of ductile detailing in Indian tall buildings. Do engineers and contractors recognize the importance of ductility detailing for earthquake resistance? How do we codify, implement, and regulate this? How do we educate all the stakeholders on the critical need for seismic detailing and get their “buy-in†? I hope to find answers to all these questions in this conference.

The Chilean example has also raised other important questions for India. The peak ground velocity in Concepcion was quite strong at 67cm/s; however, the peak ground displacement was only 21 cm. The duration of significant ground motion was quite long at 88 s, but not as much as the 1960 Chile and the 1964 Alaska earthquakes. Had the displacements and durations been greater, the outcome may have been quite different. Do we know what ground shaking would result in Delhi from a great earthquake on the Himalayan Frontal Thrust? Do we have models that can predict ground motion from such an event? I hope to find out answers to these questions in this conference. I hope similar seismological questions can be addressed for other large metropolitan cities in the northern belt and the western front including, but not limited to, Mumbai, Ahmedabad, Kolkata, Surat, Pune, Jaipur, Vadodara, and Allahabad.

I am also deeply concerned about the buildings on stilts that seem to be ubiquitous (at least in the south where I have traveled extensively in the last decade). We engineers have to come up with creative solutions to address the architectural drivers of such glaring seismic vulnerabilities without compromising structural (seismic) integrity. I hope to see creative solutions for such problems outlined in this conference. Tall first and second stories, termination and/or offsetting of gravity/lateral force resisting elements such as columns and walls, also fall in this category. I hope to see case histories of buildings with such idiosyncrasies and the novel approaches that SEFI engineers have undertaken to tackle such architecturally driven situations.

Last, but certainly not the least, environmental scientists warn us that global warming is going to result in more intense and violent storms and hurricanes in the coming decades. Our coastal cities are going to be most affected. The latest storms in the eastern US (New Jersey) and in eastern India (Chennai) are harbingers of storms to come. Are our tall buildings prepared to face these “Frankenstorms†? How do we deal with the moving target of wind hazard and our ever-increasing density of tall buildings in our mega-cities? During the lifetime of our building how will the wind loading patterns change with the rapidly changing landscape. How do we anticipate these changes at the design stage and build in contingencies into our designs, without being overly conservative and jacking up the costs? Seems like a pretty stiff challenge to me. I hope to hear your considered and deep thoughts on these “gray-area†questions.

In closing, I am looking forward to two weeks of exciting, rejuvenating, informative and enlightening debates and discussions on tall building design and construction in India. I sincerely hope that the SEFI community takes full advantage of this wonderful opportunity to share and elevate the state of this art, made possible by the remarkable vision and efforts of my co-moderators, Er. Alpha Sheth and Prof. C. V. R. Murty.

Swaminathan Krishnan, Co-Moderator
California Institute of Technology
http://krishnan.caltech.edu












-- ­­
     



     



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mailingprabu
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PostPosted: Thu Nov 22, 2012 4:25 pm    Post subject: How should one define min stiffness requirement of tall buil Reply with quote

Quote:

--Original message--
From: RKBhola
Sent:  22/11/2012, 6:48  pm
To: econf34289@sefindia.org
Subject: [E-CONF] Re: How should one define min stiffness requirement of tall buil
  Welcome SEFIians,

The issue of stiffness for tall buildings is important as it is linked to the overall behaviour of the building on a 'macro' level. This also gets linked to the 'soft storey' that we see in almost all tall buildings, due to the presence of basement/stilt/podium parking. Impact of the 'Aspect Ratio' (Height/Plan Dimension) is also a factor which should determine the orientation of the shear walls.

Should we consider the buildings in Chile as stiff because they have a wall area of 2-3%, or the ones in the US as flexible as the wall area is 1-2%? I'm sure there are numerous other factors to be considered, and it will be helpful if we can enumerate these, and also get some detailed information about their relative impact on the stiffness.

In some recent discussions with senior academicians, we have come across the viewpoint that the relative stiffness between the stilt and typical floor levels should be 100% - even though the Code would permit 80% stiffness before classifying the storey as a 'soft storey'. What is the viewpoint of the other experts on this panel? Even when we do define a storey as 'soft' in my opinion it should not be a 'threshold' whereafter the Seismic Loads are to be increased to 2.50 times - a more gradual approach to the enhancement of the Seismic Forces would seem to be more in order. Do we have some data/information about the same?

Given the complexity of the buildings that all of us come across as designers, one can be sure that there can be no straightforward formulae to address the above. However, if we can discuss some design factors that we have arrived at by our experience - it will help the budding designers to come up with better performing structures.

Regards

Rajendra Kumar Bhola
CIVTECH


On Wed, Nov 21, 2012 at 9:30 PM, sarfaraj.husain forum@sefindia.org)> wrote:
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