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How Tall Building (No. of Storeys) We Should Prefer in India

 
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subhash_mehrotra
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PostPosted: Fri Nov 30, 2012 1:57 pm    Post subject: How Tall Building (No. of Storeys) We Should Prefer in India Reply with quote

Dear All.

As I was busy in connection with the marriage of my daughter upto November 27, 2012. †I could not interact earlier.† Now that the E-conference has been extended upto 5th December, 2012, it gives me an opportunity to interact in the balance period.

Based on my 40 years of experience in the design and construction of more than 600 tall buildings all over India in the storey range of 10 to 40, I wish to share some of key points in a number of mails:

How tall building (No. of storeys) we should prefer in Indian conditions: One of the most important governing factor is human comfort.† We should keep in mind that whereas in residential tall towers apart from healthy people, old, sick and children live, in office towers the workers are supposed to be healthy and fit and in case of severe wind etc, they can be guided to vacate the building.† Considering permissible lateral deflection of H/500.† The taller the building, more is the lateral sway.† For 60 mtr. high tower (about 20 storeys), the permissible lateral deflection† is 120 mm; for 90 mtr height †(about 30 storeys) this would be 180 mm †and for 210 mtr height (70 storeys) it would be about 420 mm.†  

The actual deflections shall be close to these lateral permissible values, if we have provided adequate lateral stiffness.† To control lateral deflection, it is advisable to keep column and shear wall sizes same throughout the height and we should only change the reinforcement.

As pointed out by Alpa Sheth, the maximum vibration acceleration permitted by Chinese code for residential buildings is 0.15 m/S2 and for office 0.25 m/S2 (no such restriction in IS Code).† This gives us enough guidance for caring for human comfort in residential towers.

Based on my actual experience of deflection during earthquake and wind of only lesser intensities as, I would prefer residential tower to be† 20 Ė 30 storey only with upper limit of 40 storey.

Ultimately the decision of client is final but we can always advise him for more towers of less no. storeys.

Commercial Building :† We can go upto 80 storeys or may be more but some of following crucial factors shall be kept in mind :

a)††††† Some of RMC suppliers are very good, some are mediocre / poor.† We have seen frequent failures of concrete supplied by RMC.† Many times there is delay in transport of mixed concrete or sometimes even after it is delivered there is delay in pouring.† This results in setting of concrete.† †To make it workable water is added at site which thus reduces its strength.

b)††††† In design we follow strong columns, week beam philosophy but at site we have seen that columns fail more frequently.† Most of the times there is good quantity of beam, slab concrete.† Thus concrete from RMC / Batching plant is efficiently poured. †But for column the quantity is very less and thus it gets neglected.† Beam column junction concrete is another problem, the contractor tends to pour same mix in beam column junction as is for beam.

c)††††† Many times concrete cubes fail for column but are not reported to the consultant.† It is only after distress appears, it is brought to the notice and by that time it could be too late.† We have seen failure of concrete in single columns in a 25 storey office building and when tested for strength by core test it was 50% of required strength.† Similarly, we have seen failure of concrete in a large number of columns in 15 storeyed Residential Tower and many more similar failures.

To conclude this mail, I would say that ďWe have to create awareness not only amongst Design Engineers but amongst Site Engineers also as collapse of any tall building can lead to halt /progress of future of tall buildings in IndiaĒ.  


Best regards,

S C Mehrotra
Mehro Consultants  


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PostPosted: Sat Dec 01, 2012 6:43 am    Post subject: Reply with quote

Resp S C Mehrotra ,

                          With such wide experience in actual design,I would really like to get your views on selecting framing system,location adv disadv of selecting shear walls as lateral load resisting system,relative imp of bracing system in concrete multy storey frames,and effects of using RCC walls for all walls internal aswell as external for multy storey rcc frames.

                      Thank you.

                                                       Kapil Dingare
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sarfaraj.husain
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PostPosted: Sat Dec 01, 2012 7:40 am    Post subject: How Tall Building (No. of Storeys) We Should Prefer in India Reply with quote

fantastic ( & to the point) posting by Mr. Mehrotra .....shows his rich experience in design & detailing of multistorey rcc structures...would like to get some more from him ( his exp .regarding steel tall buildings)....

sarfraj



From: "subhash_mehrotra" <forum@sefindia.org>
To: econf34289@sefindia.org,  
Date: 11/30/12 08:01 PM
Subject: [E-CONF] How Tall Building (No. of Storeys) We Should Prefer in Indian Conditions



Dear All.

As I was busy in connection with the marriage of my daughter upto November 27, 2012. I could not interact earlier. Now that the E-conference has been extended upto 5th December, 2012, it gives me an opportunity to interact in the balance period.

Based on my 40 years of experience in the design and construction of more than 600 tall buildings all over India in the storey range of 10 to 40, I wish to share some of key points in a number of mails:

How tall building (No. of storeys) we should prefer in Indian conditions:
One of the most important governing factor is human comfort. We should keep in mind that whereas in residential tall towers apart from healthy people, old, sick and children live, in office towers the workers are supposed to be healthy and fit and in case of severe wind etc, they can be guided to vacate the building. Considering permissible lateral deflection of H/500. The taller the building, more is the lateral sway. For 60 mtr. high tower (about 20 storeys), the permissible lateral deflection is 120 mm; for 90 mtr height (about 30 storeys) this would be 180 mm and for 210 mtr height (70 storeys) it would be about 420 mm.

The actual deflections shall be close to these lateral permissible values, if we have provided adequate lateral stiffness. To control lateral deflection, it is advisable to keep column and shear wall sizes same throughout the height and we should only change the reinforcement.

As pointed out by Alpa Sheth, the maximum vibration acceleration permitted by Chinese code for residential buildings is 0.15 m/S2 and for office 0.25 m/S2 (no such restriction in IS Code). This gives us enough guidance for caring for human comfort in residential towers.

Based on my actual experience of deflection during earthquake and wind of only lesser intensities as, I would prefer residential tower to be 20 ‚Äď 30 storey only with upper limit of 40 storey.

Ultimately the decision of client is final but we can always advise him for more towers of less no. storeys.

Commercial Building
: We can go upto 80 storeys or may be more but some of following crucial factors shall be kept in mind :

a) Some of RMC suppliers are very good, some are mediocre / poor. We have seen frequent failures of concrete supplied by RMC. Many times there is delay in transport of mixed concrete or sometimes even after it is delivered there is delay in pouring. This results in setting of concrete. To make it workable water is added at site which thus reduces its strength.

b) In design we follow strong columns, week beam philosophy but at site we have seen that columns fail more frequently. Most of the times there is good quantity of beam, slab concrete. Thus concrete from RMC / Batching plant is efficiently poured. But for column the quantity is very less and thus it gets neglected. Beam column junction concrete is another problem, the contractor tends to pour same mix in beam column junction as is for beam.

c) Many times concrete cubes fail for column but are not reported to the consultant. It is only after distress appears, it is brought to the notice and by that time it could be too late. We have seen failure of concrete in single columns in a 25 storey office building and when tested for strength by core test it was 50% of required strength. Similarly, we have seen failure of concrete in a large number of columns in 15 storeyed Residential Tower and many more similar failures.

To conclude this mail, I would say that ‚ÄúWe have to create awareness not only amongst Design Engineers but amongst Site Engineers also as collapse of any tall building can lead to halt /progress of future of tall buildings in India‚ÄĚ.


Best regards,

S C Mehrotra
Mehro Consultants






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Nedian
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PostPosted: Sat Dec 01, 2012 8:11 am    Post subject: How Tall Building (No. of Storeys) We Should Prefer in India Reply with quote

Dear†Kapil Dingare,
†You can find my understanding to your email in my response;
†For tall buildings, having height to width ratio is large, building has greater slenderness ratio and behave as slender against the later force agents like wind and seismic forces, to do so, for code limit the later drift and lateral sway of the building so that it may not create any problems for the occupants at the top floor of the building, if lateral sway will be more, top floor sway will be more and floor will laterally move more then code restrictions and all the time, stories move to and fro and occupants have serviceability problems (itís just background), now I would like to come towards your questions what I understand,
1.†Generally, in concrete buildings, bracings is not in practice unlike steel†structures
∑† † † † †2.††For resisting large mass of the concrete building, we have to provide†concrete†shear walls (walls from the raft foundation to the desired level as per design), they are very stiffer in its plan and capable of resisting large lateral moments and shear forces and stiffer the building (normally effective at the periphery of the building, it has also large discussion beyond the scope of your question)
∑† † † †3.†Bracings on the other hand not applicable as it is difficult to provide shuttering for concrete bracings and not has such capacity as compare to concrete shear walls, such joints and steel congestions and proper pouring of concrete and anchorage of steel bars are not difficult in construction
∑† † † †4.†Intension of bracing to take the tensile forces (usually) and concrete is not good in tension, therefore bracing is not recommended

Thanks and best regards,


On Sat, Dec 1, 2012 at 9:43 AM, kapildingare <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           Resp S C Mehrotra ,

                          With such wide experience in actual design,I would really like to get your views on selecting framing system,location adv disadv of selecting shear walls as lateral load resisting system,relative imp of bracing system in concrete multy storey frames,and effects of using RCC walls for all walls internal aswell as external for multy storey rcc frames.

                     Thank you.

                                                       Kapil Dingare
     



     


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subhash_mehrotra
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Joined: 16 Nov 2012
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PostPosted: Wed Dec 05, 2012 2:29 pm    Post subject: How Tall Building (No. of Storeys) We Should Prefer in India Reply with quote

Dear Kapilji,
 
As desired by you, I am also forwarding my views:
 
1.      The overriding consideration in the choice of structural system in tall buildings is to provide resistance to lateral loading due to earthquake and wind, which are dynamic in nature.
 
2.      The choice of the structural system depends on form and functional requirement of the building and should satisfy safety and serviceability.  However, with the predominant increase in height, structural requirement becomes the primary design criteria.
 
3.      Floor System:
 
RCC construction usually consists of one way or two way beam and slab systems and flat slab plates with or without drops.  In larger spans waffle slab system or two way grid slabs can be used.  For speeding up construction composite construction of steel and concrete flooring are also used.
     4.    Framing Systems:
i)          Rigid frame structure consisting of rigidly connected beam and columns, which can carry both vertical as well as lateral loads, as it provides bending stiffness in the plane of bent.  Further providing storey height knee-bracing or single or double diagonal bracing or chevron bracing, in either concrete or steel, higher lateral resistance can be moblised. Steel bracing can be connected to RCC frames through insert plates.
Unbraced frames deflect primarily in SHEAR MODE under lateral load.
Further integrating with shear walls, shear cores or perimeter tubular frames lateral stiffness can be augmented.¬† The combination of providing shear wall on outer perimeter with inner shear wall core performs better.¬†  
Providing in-fills of brick or block can also enhance lateral resistance.  Under dynamic loading due to wind and earthquake, the in fills can get displaced and the behaviour of frames drastically alters and undermines their capacity to resist lateral forces.
ii)¬†¬†¬†¬†¬†¬†¬†¬† Flat plate and Flat Slab with drops monolithic with columns and walls are to be used with caution specially in earthquake zones ‚Äď IV & V.¬† Being more flexible, under dynamic loading, the system is likely to loose considerable flexural stiffness due to cracking and undermine the structural integrity of the total system both vertically and laterally.¬† The P_‚ąÜ effect on vertical members can be aggravated and can lead to failure of structure. Punching shear stress in flat plate / flat slab with drop under earthquake is very critical.¬† It is always preferred to provide shear reinforcement in flat plate / flat slab with drop to cater for earthquake forces.¬†¬† The thin joint of flat plate and column provides less ductility.¬† The flat plate / flat slab with drop should be used along with inner shear walls and perimeter beam, column, shear wall frames.¬†  
iii)        Shear Walls
a)         Shear walls have large in-plane stiffness and strength and is suited for bracing tall buildings.  They deflect as vertical cantilevers under lateral loading.  They can be plane or cores and have grater horizontal stiffness than rigid frames.  But, they are not as ductile as beam-columns frames.  Coupled wall systems improves ductile response.  Similarly, large boundary elements (width more than shear wall) shall give better ductility.
b)         Providing large number of shear wall restricts flexibility in planning internal space.  Placing them in the periphery with inner cores to house lifts and shafts should provide vertical continuity in the system rather than in staggered manner.  Because of their large in-plane stiffness, location of them should maintain plan symmetry, as far as possible, to avoid torsional distortion under lateral loading.
c)         These walls, when combined with frames provide an excellent system for lateral resistance in tall buildings.  Walls which bend to deflect in cantilever mode and frames in shear mode are constrained to adopt a common deflected shape due to in-plane rigidity of floor.  This interaction at top level produce a stiffer and stronger structure and can be economical upto 40 stories.
            Such integrated systems possess better ductility than an all wall system.
iv)¬†¬†¬†¬†¬†¬†¬† Other forms used are framed ‚Äď tube, tube-in-tube, bundled-tube, braced-tube, outrigger-braced, suspended structures and hybrid structures.
5.¬†¬†¬†¬†¬†¬†¬†¬†¬† i)¬†¬†¬†¬†¬†¬†¬†¬† Dynamic effect of wind and earthquake becomes major design factors as buildings become taller and slender, which may lead to instability of high-rise building due to second order P_‚ąÜ effects.
ii)         Effect of creep, shrinkage and temperature can also be influencing design factors.
iii)        In very tall and slender buildings providing dampers will become necessary to improve performance and enhance safety.
Regards,
S C Mehrotra

On Sat, Dec 1, 2012 at 2:05 PM, Nedian <forum@sefindia.org (forum@sefindia.org)> wrote:
[quote]  Dear¬†Kapil Dingare,
 You can find my understanding to your email in my response;
 For tall buildings, having height to width ratio is large, building has greater slenderness ratio and behave as slender against the later force agents like wind and seismic forces, to do so, for code limit the later drift and lateral sway of the building so that it may not create any problems for the occupants at the top floor of the building, if lateral sway will be more, top floor sway will be more and floor will laterally move more then code restrictions and all the time, stories move to and fro and occupants have serviceability problems (it’s just background), now I would like to come towards your questions what I understand,
1. Generally, in concrete buildings, bracings is not in practice unlike steel structures
·         2.  For resisting large mass of the concrete building, we have to provide concrete shear walls (walls from the raft foundation to the desired level as per design), they are very stiffer in its plan and capable of resisting large lateral moments and shear forces and stiffer the building (normally effective at the periphery of the building, it has also large discussion beyond the scope of your question)
·       3. Bracings on the other hand not applicable as it is difficult to provide shuttering for concrete bracings and not has such capacity as compare to concrete shear walls, such joints and steel congestions and proper pouring of concrete and anchorage of steel bars are not difficult in construction
·       4. Intension of bracing to take the tensile forces (usually) and concrete is not good in tension, therefore bracing is not recommended
 
Thanks and best regards,


On Sat, Dec 1, 2012 at 9:43 AM, kapildingare forum@sefindia.org (forum@sefindia.org))> wrote:
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Dr. N. Subramanian
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PostPosted: Wed Dec 05, 2012 3:58 pm    Post subject: Reply with quote

Dear Er Kapil,

You may find this article useful:
M. Halis Gunel and H. Emre Ilgin, A proposal for the classification of structural systems of tall buildings, Building and Environment, V. 42, No.7, July 2007, pp.2667Ė2675.

Though I also agree that the provision of bracing is difficult in concrete buildings there are a few concrete buildings with bracings:

The slender 780 3rd Avenue, in NYC,  the trussed tube structural system in concrete

Onterie Center, in Chicago using the trussed tube structural system in concrete

Best wishes,
NS

kapildingare wrote:
Resp S C Mehrotra ,

                          With such wide experience in actual design,I would really like to get your views on selecting framing system,location adv disadv of selecting shear walls as lateral load resisting system,relative imp of bracing system in concrete multy storey frames,and effects of using RCC walls for all walls internal aswell as external for multy storey rcc frames.

                      Thank you.

                                                       Kapil Dingare
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kapildingare
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PostPosted: Thu Dec 06, 2012 1:09 am    Post subject: Reply with quote

Resp Subhash Mehrotra,

               Thank you very much for your valued information.I would like your views on type of structural frame ,when multystoried frames are constructed with all walls internal as well as external as RCC structural walls concreting of that walls is done along with slab concreting with formwork like alumnium formwork like ALUFORM or like . Specially I want to know role of rcc walls ,columns and beams as compared to role of those element in regular rcc frames where infill walls are simple non structural walls.

                                            Thank you very much and expecting knowledge from you,thanks again.



                                                        Kapil Dingare
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PostPosted: Thu Dec 06, 2012 1:22 am    Post subject: Reply with quote

Resp Dr Subramanian,
            
                                   Thank you for your reply but Sir to be frank with you as we are working at small places ,really we dont have access to journals and magazines referenced by you people ,so we  cant have access to such articles which are not available on Net.So for persons like me INTERNET and platforms like SEFI where people like you are there to guide us are God's gift .Thank you ones again.

                                                       Kapil Dingare
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