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Pointed discussions on tall buildings - Use of Flat Slab -co
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Dr. N. Subramanian
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PostPosted: Wed Nov 21, 2012 2:11 pm    Post subject: Reply with quote

Dear Er Kapil,

The failures that occurred are mostly in flat slab systems without shear walls.

Several failures of flat slab structures have been reported in the literature, which include New York Coliseum on May 9, 1955 (waffle slab), 2000 Commonwealth Avenue: January 5, 1971, Five story Harbour Cay Condominium collapse at Cocoa Beach, Florida, March 27, 1981(11 workers killed and 23 injured), The Tropicana Casino parking garage in Atlantic City, New Jersey: October 30,2003, Four story warehouse at Ontario, Canada: January 4, 1978, five story Sampoong Department store, Seoul, Korea: June 29th 1995 (The collapse is the largest peacetime disaster in South Korean history - 502 people died, 6 missing, and 937 sustained injuries), Piper’s Row Car Park, Wolverhampton, UK, 1997, Geneva, Switzerland 1976, Bluche, Switzerland 1981, Cagliari, Italy 2004, and parking garage flat slab at Gretzenbach, Switzerland, 2004. In addition several flat plate systems failed during earthquakes.

Many slab column connections in flat-plate structures were damaged and failed after the 1985 Mexico City earthquake, the 1989 Loma Prieta earthquake, and the 1994 Northridge earthquake. In the event of punching failure at a connection, bottom slab reinforcement anchored through the columns has been observed to be an effective means of preventing or delaying collapse; lack of such reinforcement has been observed to result in catastrophic failures (Moehle and Mahin 1991).

Several flat plate structures collapsed in a progressive manner. The basic mechanism of this failure consists of one overloaded slab-column connection failing in punching shear, subsequently overloading the slab below, resulting in progressive collapse of several floors in a building.

Though engineers construct dual systems of shear walls with flat slabs, no guidelines are available to show the amount of shear walls that are necessary for the system to be safe in case of EQ(it will be surely higher than the amount of shear walls necessary in the case of rigid frames). I suggest Prof. Murty, the moderator of this conference, (I appreciate him for doing yeomen service in the area of EQ resistant design and also for authoring the excellent booklet EQ tips, which is very useful for anyone to understand the behavior, design and detailing of structures in EQ) to conduct some expt. investigations at IITM to provide guidelines on this aspect, and also to provide a overview of the behaviour of such dual systems in case of larger earthquakes- how the walls and flat plates interact, failure mechanisms, how punching shear can be prevented, etc.

Transfer Girder Behavior:

It is also necessary to do experiments for the so called 'Floating columns' that terminate at a transfer girder. Design Engineers do not know about the behaviour at the beam-column junction where the columns are terminating, under EQ loads. Guidelines are necessary to detail such beam-column junctions, so that failure is prevented. As the transfer girders are usually provided at the first floor level to accommodate column free parking or shopping areas, such a joint assumes at most importance, as the failure at the point will trigger catastrophic failure of the whole building.

A study of the combination of flat plate-shear wall-transfer girder system is also desirable

Best wishes,
NS
kapildingare wrote:
Resp. Sefians,
     As  Er. Shekhar has pointed ,
1) Flat plate with shear walls is most economical form of construction.
2) Along with that there are two other form of structural combinations that can be used to resist gravity and lateral loads as he has noted.
But  Resp  Dr Subramanian and Resp. Agrawal  has noted there are many cases of failures when in severe earthquake zone flat plates with shear walls as lateral load resisting system are used.
What I feel  is Er Shekhar has given vital information but  what Dr Subramanian is advising is more interesting .
BECAUSE  we can design  and  construct a building which can resist DESIGNED earthquake induced forces  with all three structural combinations Er Shekhar has mentioned but in case of earthquake of more intensities than designed one(as it is expected ) mode of failure or type and extent of damage /failure   will be different for all the three structural combinations. My opinion it will be worst in case of flat plate with shear walls.
I would like to get expert opinion on relative extent of damage with respect to these three forms when such structure is exposed to severe earthquake than designed earthquake.

                                                                                      Kapil Dingare
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va
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PostPosted: Wed Nov 21, 2012 2:50 pm    Post subject: Coupled shear wall and slenderness moments Reply with quote

Dear Friends.
I would like to post few practical design issues in the forum for the consideration of experts –
<![if !supportLists]>1. <![endif]>Coupled shear wall – It is very difficult to design the short span coupling beam connecting shear walls due to very heavy shear and moment. The depth of beam is limited to 700mm due to floor height restrictions. If the moments are released, the coupling effect is lost. How to address this problem?
<![if !supportLists]>2. <![endif]>Slenderness ratio of shear wall – AS per the code IS456 the effective length of column/ shear wall ( compression member) depends on the relative junction stiffness of beams and columns. In case of shear walls, its stiffness is very large compared to beam and Beta1 and Beta 2 is approaching to 1.0 as per Fig 27 in Annexe E. Thus , the effective length of shear wall as column becomes very high in the major direction Therefore, it amounts to very large slenderness moments in major direction which makes it difficult for design. This results in very large reinforcement percentage. Alternatively if P-Delta analysis is carried out, slenderness moment is not be considered and the increase in moments is very small. Which approach is correct for design of shear walls?

Regards.
Hemant Vadalkar
Consulting Engineer Mumbai.

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Dr. N. Subramanian
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PostPosted: Wed Nov 21, 2012 5:12 pm    Post subject: Re: Coupled shear wall and slenderness moments Reply with quote

Dear All,

I request the Moderators to take active part in the discussions and share  not only their knowledge but also post documents that will be useful to the participants.

For example the following can be easily answered by Prof. Murty who has guided several students on Shear walls, and also published papers on them.

Best wishes,
NS

va wrote:
Dear Friends.
I would like to post few practical design issues in the forum for the consideration of experts –
<if>1. <endif>Coupled shear wall – It is very difficult to design the short span coupling beam connecting shear walls due to very heavy shear and moment. The depth of beam is limited to 700mm due to floor height restrictions. If the moments are released, the coupling effect is lost. How to address this problem?
<if>2. <endif>Slenderness ratio of shear wall – AS per the code IS456 the effective length of column/ shear wall ( compression member) depends on the relative junction stiffness of beams and columns. In case of shear walls, its stiffness is very large compared to beam and Beta1 and Beta 2 is approaching to 1.0 as per Fig 27 in Annexe E. Thus , the effective length of shear wall as column becomes very high in the major direction Therefore, it amounts to very large slenderness moments in major direction which makes it difficult for design. This results in very large reinforcement percentage. Alternatively if P-Delta analysis is carried out, slenderness moment is not be considered and the increase in moments is very small. Which approach is correct for design of shear walls?

Regards.
Hemant Vadalkar
Consulting Engineer Mumbai.

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prof.arc
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PostPosted: Wed Nov 21, 2012 11:46 pm    Post subject: Pointed discussions on tall buildings - Use of Flat Slab -co Reply with quote

I had seen some MS-Buildings having flat slabs only and apparently no
core shear walls
except for lift well
I could not get an answer regarding modelling such systems except a
vague statement that commercial software was used
I wonder whether modelling columns as 3D beam elements and the flat
slab as some sort of FEM elements or even equivalent beam is
appropriate
I hope some engineers of this forum who might have modelled such flat
slab system - without beams - would come forward to share their
thoughts

ARC

On 11/20/12, Econf_Moderator <forum@sefindia.org> wrote:
Quote:
Dear All:

Not all of us  design tall buildings. But  most likely, a lot of us will be
designers or project engineers for tall buildings in years to come. And
needless to say, almost all of us have opinions about how things ought or
ought not to be done.


We welcome comments from all engineers regarding issues they may want to
comment on, on design and construction of tall buildings. If we look around
us in Mumbai, NCR and most of the metros of Mumbai, majority of the new
construction is tall buildings- and here I am using the IBC definition of 50
m. And almost everyone has done tall buildings, if I use the Chinese
definition of 28m.


To start the ball rolling,


What is the general opinion of SEFIans to the use of flat slab with core
shear wall system as a structural system for tall buildings?
- Should it be allowed in Zone V?
- Up to what max height?
- What are the conditions one may impose for use of such a system for tall
buildings?


Look forward ot hearing form you all on this topic


best regards,


CVR Murty
Swaminathan Krishnan
Alpa Sheth


Econference Moderators








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N. Prabhakar
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PostPosted: Thu Nov 22, 2012 6:30 am    Post subject: Reply with quote

Dear Sefians,

We have to consider the limitations to the use of flat slab, along with shear walls, in the type of tall buildings.

In tall buildings meant for the usage of offices and commerical purpose, flat slab can be used where light-weight partitions (with timber/steel/aluminium frame with light-weight board cladding)are generally provided as compartments on any location of the floor.  The load of light-weight partitions on any location of the floor will not pose any design problem.

But, the same cannot be said about the case of tall buildings meant for residential purpose, where the internal walls are usually brick or hollow concrete blocks.  In view of the heavy weight of these walls, these walls are to be located only on the column strip of the flat slab, and cannot be located anywhere on the floor.  Due to this restriction, it is impractical to provide flat slab in tall buildings meant for residential purpose.  

I trust you will agree with the above observations.

With best wishes,

N. Prabhakar
Chartered Structural Engineer
Vasai (E)
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deviationz
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PostPosted: Thu Nov 22, 2012 7:58 am    Post subject: Reply with quote

A flat slab building with only shear walls for lateral load resistance can be used provided adequate consideration of deformation-compatibility is done at the slab-column junctions.

I believe there are buildings in California that rely only on core walls for lateral load resistance, but I also think that they have been using Performance Based Design for those buildings as required by the Building Departments.
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Dr. N. Subramanian
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PostPosted: Thu Nov 22, 2012 5:11 pm    Post subject: Reply with quote

Dear Er.

Will you please explain what you mean by "adequate consideration of deformation-compatibility is done at the slab-column junctions"? How to achieve this deformation-compatibility?

Best wishes,
NS

deviationz wrote:
A flat slab building with only shear walls for lateral load resistance can be used provided adequate consideration of deformation-compatibility is done at the slab-column junctions.

I believe there are buildings in California that rely only on core walls for lateral load resistance, but I also think that they have been using Performance Based Design for those buildings as required by the Building Departments.
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ahujavipul
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PostPosted: Thu Nov 22, 2012 6:11 pm    Post subject: Pointed discussions on tall buildings - Use of Flat Slab -co Reply with quote

I can offer my paper posted in the previous E-conference on flat slabs

http://www.sefindia.org/forum/viewtopic.php?t=4392&start=10

Hope this helps

Regards,

Vipul Ahuja

Dr. N. Subramanian wrote:
Dear Er.

Will you please explain what you mean by "adequate consideration of deformation-compatibility is done at the slab-column junctions"? How to achieve this deformation-compatibility?

Best wishes,
NS

deviationz wrote:
A flat slab building with only shear walls for lateral load resistance can be used provided adequate consideration of deformation-compatibility is done at the slab-column junctions.

I believe there are buildings in California that rely only on core walls for lateral load resistance, but I also think that they have been using Performance Based Design for those buildings as required by the Building Departments.
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PostPosted: Sat Nov 24, 2012 6:19 am    Post subject: Pointed discussions on tall buildings - Use of Flat Slab -co Reply with quote

When we analyze the slab in any finite element software, the support condition are well defined and slab acts a a flat plate with no relative displacement between any support but in real world each column and wall support deflect / settles differently according to the relative stiffness and load share hence the secondary stresses developed due to differential settlement of various support points can not be accounted in. To avoid this while creating a slab in any finite element programme we need to transfer the slab with deformation from any 3D analysis software I.e. ETABS. Regards irfan - from Vodafone
From: "Dr. N. Subramanian" <forum@sefindia.org>
Date: Thu, 22 Nov 2012 22:41:34 +0530
To: <econf34289@sefindia.org>
ReplyTo: econf34289@sefindia.org
Subject: [E-CONF] Re: Pointed discussions on tall buildings - Use of Flat Slab -co

     Dear Er.  Will you please explain what you mean by "adequate consideration of deformation-compatibility is done at the slab-column junctions"? How to achieve this deformation-compatibility?  Best wishes, NS         deviationz wrote:                A flat slab building with only shear walls for lateral load resistance can be used provided adequate consideration of deformation-compatibility is done at the slab-column junctions.  I believe there are buildings in California that rely only on core walls for lateral load resistance, but I also think that they have been using Performance Based Design for those buildings as required by the Building Departments.     
        --

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Ranjith.Chandunni
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PostPosted: Sat Nov 24, 2012 9:00 am    Post subject: Pointed discussions on tall buildings - Use of Flat Slab -co Reply with quote

Dear SEFIans,

To answer the query regarding movement joints raised by Sangeeta, I would like to add the following:

Ideally it’s the best practice to separate the structures of tower from podium with movement joints simplifying the complexities of interaction between the two structures and simplifying differential settlement issues. However this is easily said than done in the real practice owing to having inadequate stability system for podium and having to deal with too many joints which often becomes an expensive affair. A practical option would be to have at least one movement between two towers on same podium with part of podium attached to each towers. How big that part of podium should be is left to the judgement of the engineer giving due consideration to size of podium part, plan geometry, number floors and temperature effects.

Continuing the joints through the foundation is not required for obvious reason that there is hardly any relative horizontal movement at foundation level however due care should be given to shrinkage and temperature effect by design and by carefully planning concrete pouring sequence. One has to also consider differential settlement issues on raft between tower and podium.

Whether to start the movement joint at first basement level or above ground level is a question which needs some careful consideration. From practical perspective its good not to have joints in basement or even ground level slab which to a large extent can be engineered by providing post pour strips. However, what about the interaction of one tower on the other under dynamic lateral loads and what’s its effect on a continuous floor of basement/podium if no joints are present? A complex issue to analyse and trying to model this effects with FEM could be highly challenging. It’s important to understand the real effect of interaction between the two towers, the magnitude and nature of the influence and it’s quite possible that real nature of influence can get lost in a complex modelling.

A simplified approach would be to adopt stick modelling techniques appropriately representing mass and stiffness properties of towers and podium in stick models. In order to understand the behaviour under seismic loading, a time history analysis is required to be carried out. To understand behaviour under wind loading, information about along wind and across wind dynamic components of wind loads is required which can only be obtained from wind tunnel studies. Many of these dynamic effects act out of phase on the structure at times. Are these data available in the first place?

One has to decide to what extent one is willing to go analysing these effects in order to minimise or eliminate movement joints. Is such an approach necessary for the project. Don’t be surprised to find some really high in plane forces being developed in the floors if towers get connected at basement or podium levels. The behaviour can get complicated if openings are present which is common due to ramps, stairs, atriums etc.

Other than the raft, one can possibly eliminate the movement joints in basement walls provided shrinkage and temperature effects are taken care of besides the interaction effects from the tower/podium structures. The interaction effects are substantially minimal on basement walls even if movement joints are present in the floors and if the basement walls are not very close to the main cores of the tower.

Regards,

Ranjith Chandunni
Buro Happold



From: SANGEETA WIJ [mailto:forum@sefindia.org]
Sent: 22 November 2012 13:07
To: econf34289@sefindia.org
Subject: [E-CONF] Re: Pointed discussions on tall buildings - Use of Flat Slab -co



Dear Alpa
I had raised a query a few weeks ago to ask the following:
· whether there is a rule as to what % of tower columns must go to the basement raft level(hypothetically, the architect wants all tower columns to be supported only on transfer girders, let’s take a typical tower of G+20 and three basements which are full plot size). How does the overall tower stability get affected in such a case, especially in zone-IV EQ?
· What are the guidelines for Seismic separation/Expansion joints in such large basements(say it’s a 100 acre plot with 20 scattered towers).Do we or do we not give joints in the non-tower basement structure? If yes, then at what spacing? (chances are, that each part of basement may carry more than one tower, making it a very complex modelling challenge)
· When does the wind tunnel test become mandatory, between two adjacent high rise blocks?

Regards
Sangeeta Wij

From: shekhar at shekharpana... [mailto:forum@sefindia.org (forum@sefindia.org)]
Sent: 21 November 2012 10:36
To: econf34289@sefindia.org (econf34289@sefindia.org)
Subject: [E-CONF] Re: Pointed discussions on tall buildings - Use of Flat Slab -co



Lets get going....

The three basic framing systems to resist lateral loads in high-rise buildings are: (1) Frames, (2)shear walls coupled or acting individually and (3) frames interacting with shear walls.
As an economical form of construction a shear wall structure incorporating a flat plate system is almost ideal.The flat plate system is very efficient in resisting gravity loading while the shear wall provides the resistance to lateral loads.

- Shekhar Panandiker

Please reply on our new id:
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