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Moderators' Concluding Remarks

 
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dcrai
E-Conference Moderator


Joined: 26 Jan 2003
Posts: 9

PostPosted: Mon Jan 12, 2009 6:00 am    Post subject: Moderators' Concluding Remarks Reply with quote

Dear Colleagues,
It has indeed been an interesting fortnight of discussion over flat slab systems. Over this period we have had the chance to deliberate on wide range of issues covering the entire spectrum of flat slab modelling, analysis, design and practice. Though the e-Conference has come to an end, we encourage our colleagues to carry on with their discussions, at SEFI general forum.  
We were happy to see that postings have covered issues related to research, practice, and code provisions and many of them have been quite comprehensive in discussing them. Though these posts themselves are excellent source of information on flats slabs system, we have tried to summarise the discussion as below:  
1. General issues
a. Suitability of flat slab structures
It was rather evident form the discussion that the flat slabs systems are suitable for commercial structures requiring large span and open office space. Though flat slab structures have many drawbacks in terms of inadequate understanding of its behaviour under seismic action, its advantages such as large head room, reduced height of the building, easy shuttering and faster construction has made it popular among the clients and the construction industry.  
b. Geometry of flat slab structures
As far as the geometry of the flat slab structures are concerned, it is better to have a symmetric disposition of columns and the lateral resisting systems. Seismic behaviour of symmetric system has found to be better than the asymmetric system. It has been suggested that the use of single panel flat slabs systems are to be avoided.  

2. Behaviour under seismic action

The behaviour of the flat slab structure under seismic forces is a subject of active research and development because of their poor performance in the past earthquakes.
a. Drift-induced (seismic) Punching Shear
Generally, flat slabs under high seismic zones are required to be designed only for gravity loads and the entire lateral load is to be resisted by the lateral load resisting frame in place. However, the drift induced in the lateral direction has been found to induce additional unbalanced moment which enhances the shear stress demand at the slab column critical section. For this reason it is recommended that the inter story drift be restricted for flat slabs structures and/or limit the permissible shear stress allowed under gravity loads. A plot between gravity shear and inter story drift has been made available, by the research fraternity and ACI 318-08 (clause 21.13.6) code, with which you can determine whether the slab column connection designed needs additional shear reinforcement to resist punching shear failure or not.  
b. Lateral resistance of flat slab system
As stated in the previous section, flat slab systems are designed to resist only gravity load in high seismic zones. However, one may argue that it is possible to design the flat slab system to resist lateral load as well. If we design the flat slab system to resist the lateral load then the following questions come to mind:  
1) Can we design the flat slab system to resist the 100% lateral load? If yes, what should be the value of response reduction factor for such flat slab system, which accounts for its ability to undergo inelastic deformation without losing stability and integrity?  Recommendations have been made to the research community to ascertain the value of response reduction factor through experimental and analytical studies.  
2) If the lateral resistance offered by the flat slab system found to be inadequate then can we complement the lateral resistance by providing shear wall or moment resisting frame? If so, can we term such a system as a dual system? If yes, what should be the value of response reduction factor for such dual systems? Furthermore, what methodology needs to be adopted for proportioning the lateral load between the flat slab system and the other lateral force resisting system?
3) Can such a type of combined/dual lateral resisting system be used in high seismic zones?

3. Design issues
a. Shear reinforcement in slab
The use of shear reinforcement has been found to be an attractive option to improve the behaviour of the slab column connection under lateral loading. The shear fixture such as stud type reinforcement has been show to have improved shear strength as well as lateral deformation capacity. Apart form the stud reinforcement, there are different types of shear reinforcement developed and tested for their efficiency to improve the capacity and the ductility of the slab column connection. However, it has been shown that the stirrups are effective only if the slab depth is more than 250 mm.
b. Effectiveness of drop and edge beam
The effect of presence of drop and/or edge beam on the overall behaviour of the flat slab system under lateral loads has been found to have insufficient ductility. The drops are not effective in the regions where high unbalanced moments are expected to occur due to lateral drift. This is primarily due to the fact that the unbalanced moment results in change in the shear crack pattern which would render the drop ineffective. Similarly, edge beams along with the peripheral columns could provide a certain level of frame action in resisting the induced lateral loads. But, this frame action may not be adequate enough for the strength and deformability requirements under the induced seismic actions.
c. Detailing of tendons
The effect of detailing a tendon to pass through the column was discussed with a simple example. (Refer to post tensioned slab post) It is evident form this example that the forces induced, are at the discretion of the designer to consider it or not. It is advisable to lay the tendons individually rather than through the duct, at least in the vicinity of the slab column connection.
d. Sunken portion and slab cut offs
The effect of sunken portion or cut-outs in the slab would largely depend upon the location and size of it. ACI 318-08 (clause 11.11.6) has recommended suitable solution for the same. As far as possible one must try to avoid opening near the slab column critical section.  

4. Analysis of flat slab systems
In order to design the structure for probable internal forces, the analyses for determining the design forces have to be properly carried out. Major issues regarding modelling, analysis were deliberated during the e- Conference. Many finite element based soft wares are available which could be used for analysis and design of flat slab structures, such as ADAPT, SAFE. These software are capable of handling not only RC flat slab but also the effect of PT as well. There was some discussion on how to model shear walls while analysing the flat slab system.  

5. Other issues
One can not emphasize enough that no matter how good the analysis and design is, the behaviour of the structure would largely depend on quality of construction and detailing provided. We could have had more posts highlighting the problems associated in the field. However, we are sure that the materials posted on the use of post tensioning with flat slab system may have helped in understanding the practical issues related to the same.  
The cost effectiveness of flat slab system was deliberated upon. In general, the flat slabs have been found to be competitive for a span of 10 to 12 m. Geographical location and optimisation of design plays a vital role on the cost effectiveness of the flat lab systems.  

Finally, we would like to thank you for taking part in this e-Conference on flat slabs.  It has indeed been a wonderful learning experience for all of us. We would like to thank all the participants for their valuable post and comments during the entire fortnight of discussions. We hope that the discussion provided will prove a stepping stone for further inquiry and learning.

Thank you and wish you a very happy new year!

Moderators
Durgesh Rai and Manmohan Kalgal
with assistance from Vijaya Narayanan, A R
Quote:
--

Durgesh C Rai
Dept. of Civil Engineering
Indian Institute of Technology Kanpur
Kanpur 208016, India
Ph: +91 512 259 7717
http://home.iitk.ac.in/~dcrai/


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PostPosted: Mon Jan 12, 2009 7:36 am    Post subject: Moderators' Concluding Remarks Reply with quote

Friends,

I have been silent spectator all along during the eConference due to preoccupation with pressures of meetings on matters away from structural engineering, in the aftermath of 26/11 in Mumbai. But that should not prevent me from adding concluding words.

I wish to express gratitude to Durgesh Rai and Manmohan Kalgal and their assistants Vijaya Narayanan, A R ... for moderating and summing up the eConference.

Sudhir Badami



On 12/01/2009, dcrai <forum@sefindia.org (forum@sefindia.org)> wrote: [quote]  Dear Colleagues,
It has indeed been an interesting fortnight of discussion over flat slab systems. Over this period we have had the chance to deliberate on wide range of issues covering the entire spectrum of flat slab modelling, analysis, design and practice. Though the e-Conference has come to an end, we encourage our colleagues to carry on with their discussions, at SEFI general forum.
We were happy to see that postings have covered issues related to research, practice, and code provisions and many of them have been quite comprehensive in discussing them. Though these posts themselves are excellent source of information on flats slabs system, we have tried to summarise the discussion as below:
1. General issues
a. Suitability of flat slab structures
It was rather evident form the discussion that the flat slabs systems are suitable for commercial structures requiring large span and open office space. Though flat slab structures have many drawbacks in terms of inadequate understanding of its behaviour under seismic action, its advantages such as large head room, reduced height of the building, easy shuttering and faster construction has made it popular among the clients and the construction industry.
b. Geometry of flat slab structures
As far as the geometry of the flat slab structures are concerned, it is better to have a symmetric disposition of columns and the lateral resisting systems. Seismic behaviour of symmetric system has found to be better than the asymmetric system. It has been suggested that the use of single panel flat slabs systems are to be avoided.

2. Behaviour under seismic action
The behaviour of the flat slab structure under seismic forces is a subject of active research and development because of their poor performance in the past earthquakes.
a. Drift-induced (seismic) Punching Shear
Generally, flat slabs under high seismic zones are required to be designed only for gravity loads and the entire lateral load is to be resisted by the lateral load resisting frame in place. However, the drift induced in the lateral direction has been found to induce additional unbalanced moment which enhances the shear stress demand at the slab column critical section. For this reason it is recommended that the inter story drift be restricted for flat slabs structures and/or limit the permissible shear stress allowed under gravity loads. A plot between gravity shear and inter story drift has been made available, by the research fraternity and ACI 318-08 (clause 21.13.6) code, with which you can determine whether the slab column connection designed needs additional shear reinforcement to resist punching shear failure or not.
b. Lateral resistance of flat slab system
As stated in the previous section, flat slab systems are designed to resist only gravity load in high seismic zones. However, one may argue that it is possible to design the flat slab system to resist lateral load as well. If we design the flat slab system to resist the lateral load then the following questions come to mind:
1) Can we design the flat slab system to resist the 100% lateral load? If yes, what should be the value of response reduction factor for such flat slab system, which accounts for its ability to undergo inelastic deformation without losing stability and integrity? Recommendations have been made to the research community to ascertain the value of response reduction factor through experimental and analytical studies.
2) If the lateral resistance offered by the flat slab system found to be inadequate then can we complement the lateral resistance by providing shear wall or moment resisting frame? If so, can we term such a system as a dual system? If yes, what should be the value of response reduction factor for such dual systems? Furthermore, what methodology needs to be adopted for proportioning the lateral load between the flat slab system and the other lateral force resisting system?
3) Can such a type of combined/dual lateral resisting system be used in high seismic zones?

3. Design issues
a. Shear reinforcement in slab
The use of shear reinforcement has been found to be an attractive option to improve the behaviour of the slab column connection under lateral loading. The shear fixture such as stud type reinforcement has been show to have improved shear strength as well as lateral deformation capacity. Apart form the stud reinforcement, there are different types of shear reinforcement developed and tested for their efficiency to improve the capacity and the ductility of the slab column connection. However, it has been shown that the stirrups are effective only if the slab depth is more than 250 mm.
b. Effectiveness of drop and edge beam
The effect of presence of drop and/or edge beam on the overall behaviour of the flat slab system under lateral loads has been found to have insufficient ductility. The drops are not effective in the regions where high unbalanced moments are expected to occur due to lateral drift. This is primarily due to the fact that the unbalanced moment results in change in the shear crack pattern which would render the drop ineffective. Similarly, edge beams along with the peripheral columns could provide a certain level of frame action in resisting the induced lateral loads. But, this frame action may not be adequate enough for the strength and deformability requirements under the induced seismic actions.
c. Detailing of tendons
The effect of detailing a tendon to pass through the column was discussed with a simple example. (Refer to post tensioned slab post) It is evident form this example that the forces induced, are at the discretion of the designer to consider it or not. It is advisable to lay the tendons individually rather than through the duct, at least in the vicinity of the slab column connection.
d. Sunken portion and slab cut offs
The effect of sunken portion or cut-outs in the slab would largely depend upon the location and size of it. ACI 318-08 (clause 11.11.6) has recommended suitable solution for the same. As far as possible one must try to avoid opening near the slab column critical section.

4. Analysis of flat slab systems
In order to design the structure for probable internal forces, the analyses for determining the design forces have to be properly carried out. Major issues regarding modelling, analysis were deliberated during the e- Conference. Many finite element based soft wares are available which could be used for analysis and design of flat slab structures, such as ADAPT, SAFE. These software are capable of handling not only RC flat slab but also the effect of PT as well. There was some discussion on how to model shear walls while analysing the flat slab system.

5. Other issues
One can not emphasize enough that no matter how good the analysis and design is, the behaviour of the structure would largely depend on quality of construction and detailing provided. We could have had more posts highlighting the problems associated in the field. However, we are sure that the materials posted on the use of post tensioning with flat slab system may have helped in understanding the practical issues related to the same.
The cost effectiveness of flat slab system was deliberated upon. In general, the flat slabs have been found to be competitive for a span of 10 to 12 m. Geographical location and optimisation of design plays a vital role on the cost effectiveness of the flat lab systems.

Finally, we would like to thank you for taking part in this e-Conference on flat slabs. It has indeed been a wonderful learning experience for all of us. We would like to thank all the participants for their valuable post and comments during the entire fortnight of discussions. We hope that the discussion provided will prove a stepping stone for further inquiry and learning.

Thank you and wish you a very happy new year!

Moderators
Durgesh Rai and Manmohan Kalgal
with assistance from Vijaya Narayanan, A R
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PostPosted: Mon Jan 12, 2009 8:37 am    Post subject: Moderators' Concluding Remarks Reply with quote

So very well summarized! An excellent wrap-up of a very successful Econference.


Thanks, so much Dr Rai and Dr. Kalgal for moderating this very invigorating econf. And to all of the participants for making it such a resounding success.

Sanjeev as always you have provided such exemplary supprt. We deeply appreciate it.

Warm regards,
Alpa Sheth


[quote] ----- Original Message -----
From: dcrai (forum@sefindia.org)
To: econf@sefindia.org (econf@sefindia.org)
Sent: Monday, January 12, 2009 11:45 AM
Subject: [ECONF] Moderators' Concluding Remarks


Dear Colleagues,
It has indeed been an interesting fortnight of discussion over flat slab systems. Over this period we have had the chance to deliberate on wide range of issues covering the entire spectrum of flat slab modelling, analysis, design and practice. Though the e-Conference has come to an end, we encourage our colleagues to carry on with their discussions, at SEFI general forum.
We were happy to see that postings have covered issues related to research, practice, and code provisions and many of them have been quite comprehensive in discussing them. Though these posts themselves are excellent source of information on flats slabs system, we have tried to summarise the discussion as below:
1. General issues
a. Suitability of flat slab structures
It was rather evident form the discussion that the flat slabs systems are suitable for commercial structures requiring large span and open office space. Though flat slab structures have many drawbacks in terms of inadequate understanding of its behaviour under seismic action, its advantages such as large head room, reduced height of the building, easy shuttering and faster construction has made it popular among the clients and the construction industry.
b. Geometry of flat slab structures
As far as the geometry of the flat slab structures are concerned, it is better to have a symmetric disposition of columns and the lateral resisting systems. Seismic behaviour of symmetric system has found to be better than the asymmetric system. It has been suggested that the use of single panel flat slabs systems are to be avoided.

2. Behaviour under seismic action
The behaviour of the flat slab structure under seismic forces is a subject of active research and development because of their poor performance in the past earthquakes.
a. Drift-induced (seismic) Punching Shear
Generally, flat slabs under high seismic zones are required to be designed only for gravity loads and the entire lateral load is to be resisted by the lateral load resisting frame in place. However, the drift induced in the lateral direction has been found to induce additional unbalanced moment which enhances the shear stress demand at the slab column critical section. For this reason it is recommended that the inter story drift be restricted for flat slabs structures and/or limit the permissible shear stress allowed under gravity loads. A plot between gravity shear and inter story drift has been made available, by the research fraternity and ACI 318-08 (clause 21.13.6) code, with which you can determine whether the slab column connection designed needs additional shear reinforcement to resist punching shear failure or not.
b. Lateral resistance of flat slab system
As stated in the previous section, flat slab systems are designed to resist only gravity load in high seismic zones. However, one may argue that it is possible to design the flat slab system to resist lateral load as well. If we design the flat slab system to resist the lateral load then the following questions come to mind:
1) Can we design the flat slab system to resist the 100% lateral load? If yes, what should be the value of response reduction factor for such flat slab system, which accounts for its ability to undergo inelastic deformation without losing stability and integrity? Recommendations have been made to the research community to ascertain the value of response reduction factor through experimental and analytical studies.
2) If the lateral resistance offered by the flat slab system found to be inadequate then can we complement the lateral resistance by providing shear wall or moment resisting frame? If so, can we term such a system as a dual system? If yes, what should be the value of response reduction factor for such dual systems? Furthermore, what methodology needs to be adopted for proportioning the lateral load between the flat slab system and the other lateral force resisting system?
3) Can such a type of combined/dual lateral resisting system be used in high seismic zones?

3. Design issues
a. Shear reinforcement in slab
The use of shear reinforcement has been found to be an attractive option to improve the behaviour of the slab column connection under lateral loading. The shear fixture such as stud type reinforcement has been show to have improved shear strength as well as lateral deformation capacity. Apart form the stud reinforcement, there are different types of shear reinforcement developed and tested for their efficiency to improve the capacity and the ductility of the slab column connection. However, it has been shown that the stirrups are effective only if the slab depth is more than 250 mm.
b. Effectiveness of drop and edge beam
The effect of presence of drop and/or edge beam on the overall behaviour of the flat slab system under lateral loads has been found to have insufficient ductility. The drops are not effective in the regions where high unbalanced moments are expected to occur due to lateral drift. This is primarily due to the fact that the unbalanced moment results in change in the shear crack pattern which would render the drop ineffective. Similarly, edge beams along with the peripheral columns could provide a certain level of frame action in resisting the induced lateral loads. But, this frame action may not be adequate enough for the strength and deformability requirements under the induced seismic actions.
c. Detailing of tendons
The effect of detailing a tendon to pass through the column was discussed with a simple example. (Refer to post tensioned slab post) It is evident form this example that the forces induced, are at the discretion of the designer to consider it or not. It is advisable to lay the tendons individually rather than through the duct, at least in the vicinity of the slab column connection.
d. Sunken portion and slab cut offs
The effect of sunken portion or cut-outs in the slab would largely depend upon the location and size of it. ACI 318-08 (clause 11.11.6) has recommended suitable solution for the same. As far as possible one must try to avoid opening near the slab column critical section.

4. Analysis of flat slab systems
In order to design the structure for probable internal forces, the analyses for determining the design forces have to be properly carried out. Major issues regarding modelling, analysis were deliberated during the e- Conference. Many finite element based soft wares are available which could be used for analysis and design of flat slab structures, such as ADAPT, SAFE. These software are capable of handling not only RC flat slab but also the effect of PT as well. There was some discussion on how to model shear walls while analysing the flat slab system.

5. Other issues
One can not emphasize enough that no matter how good the analysis and design is, the behaviour of the structure would largely depend on quality of construction and detailing provided. We could have had more posts highlighting the problems associated in the field. However, we are sure that the materials posted on the use of post tensioning with flat slab system may have helped in understanding the practical issues related to the same.
The cost effectiveness of flat slab system was deliberated upon. In general, the flat slabs have been found to be competitive for a span of 10 to 12 m. Geographical location and optimisation of design plays a vital role on the cost effectiveness of the flat lab systems.

Finally, we would like to thank you for taking part in this e-Conference on flat slabs. It has indeed been a wonderful learning experience for all of us. We would like to thank all the participants for their valuable post and comments during the entire fortnight of discussions. We hope that the discussion provided will prove a stepping stone for further inquiry and learning.

Thank you and wish you a very happy new year!

Moderators
Durgesh Rai and Manmohan Kalgal
with assistance from Vijaya Narayanan, A R
--auto removed--

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PostPosted: Mon Jan 12, 2009 3:21 pm    Post subject: Moderators' Concluding Remarks Reply with quote

Hi all,

I second Mr.Sudhir Badami and wish to appreciate the moderators Prof. Durgesh Rai and Mr. Manmohan Kalgal and Mr.Vijaya Narayanan, A R.

But they have not said any word about preparing a booklet on this topic, for the benefit of many young engineers, who may be undertaking the analysis, design and construction of flat slab systems.

Best wishes
Subramanian Narayanan

Dr.N.Subramanian,Ph.D.,F.ASCE, M.ACI,

Consulting Structural Engineer
Maryland, USA

See my books at: www.multi-science.co.uk/subramanian-book.htm
www.oup.co.in/search_detail.php?id=144559





--- On Mon, 1/12/09, badami <forum@sefindia.org> wrote:
Quote:
From: badami <forum@sefindia.org>
Subject: [ECONF] Re: Moderators' Concluding Remarks
To: econf@sefindia.org
Date: Monday, January 12, 2009, 1:23 PM

Friends,

I have been silent spectator all along during the eConference due to preoccupation with pressures of meetings on matters away from structural engineering, in the aftermath of 26/11 in Mumbai. But that should not prevent me from adding concluding words.

I wish to express gratitude to Durgesh Rai and Manmohan Kalgal and their assistants Vijaya Narayanan, A R ... for moderating and summing up the eConference.

Sudhir Badami



On 12/01/2009, dcrai forum@sefindia.org)> wrote:
Quote:
Dear Colleagues,
It has indeed been an interesting fortnight of discussion over flat slab systems. Over this period we have had the chance to deliberate on wide range of issues covering the entire spectrum of flat slab modelling, analysis, design and practice. Though the e-Conference has come to an end, we encourage our colleagues to carry on with their discussions, at SEFI general forum.
We were happy to see that postings have covered issues related to research, practice, and code provisions and many of them have been quite comprehensive in discussing them. Though these posts themselves are excellent source of information on flats slabs system, we have tried to summarise the discussion as below:
1. General issues
a. Suitability of flat slab structures
It was rather evident form the discussion that the flat slabs systems are suitable for commercial structures requiring large span and open office space. Though flat slab structures have many drawbacks in terms of inadequate understanding of its behaviour under seismic action, its advantages such as large head room, reduced height of the building, easy shuttering and faster construction has made it popular among the clients and the construction industry.
b. Geometry of flat slab structures
As far as the geometry of the flat slab structures are concerned, it is better to have a symmetric disposition of columns and the lateral resisting systems. Seismic behaviour of symmetric system has found to be better than the asymmetric system. It has been suggested that the use of single panel flat slabs systems are to be avoided.

2. Behaviour under seismic action
The behaviour of the flat slab structure under seismic forces is a subject of active research and development because of their poor performance in the past earthquakes.
a. Drift-induced (seismic) Punching Shear
Generally, flat slabs under high seismic zones are required to be designed only for gravity loads and the entire lateral load is to be resisted by the lateral load resisting frame in place. However, the drift induced in the lateral direction has been found to induce additional unbalanced moment which enhances the shear stress demand at the slab column critical section. For this reason it is recommended that the inter story drift be restricted for flat slabs structures and/or limit the permissible shear stress allowed under gravity loads. A plot between gravity shear and inter story drift has been made available, by the research fraternity and ACI 318-08 (clause 21.13.6) code, with which you can determine whether the slab column connection designed needs additional shear reinforcement to resist punching shear failure or not.
b. Lateral resistance of flat slab system
As stated in the previous section, flat slab systems are designed to resist only gravity load in high seismic zones. However, one may argue that it is possible to design the flat slab system to resist lateral load as well. If we design the flat slab system to resist the lateral load then the following questions come to mind:
1) Can we design the flat slab system to resist the 100% lateral load? If yes, what should be the value of response reduction factor for such flat slab system, which accounts for its ability to undergo inelastic deformation without losing stability and integrity? Recommendations have been made to the research community to ascertain the value of response reduction factor through experimental and analytical studies.
2) If the lateral resistance offered by the flat slab system found to be inadequate then can we complement the lateral resistance by providing shear wall or moment resisting frame? If so, can we term such a system as a dual system? If yes, what should be the value of response reduction factor for such dual systems? Furthermore, what methodology needs to be adopted for proportioning the lateral load between the flat slab system and the other lateral force resisting system?
3) Can such a type of combined/dual lateral resisting system be used in high seismic zones?

3. Design issues
a. Shear reinforcement in slab
The use of shear reinforcement has been found to be an attractive option to improve the behaviour of the slab column connection under lateral loading. The shear fixture such as stud type reinforcement has been show to have improved shear strength as well as lateral deformation capacity. Apart form the stud reinforcement, there are different types of shear reinforcement developed and tested for their efficiency to improve the capacity and the ductility of the slab column connection. However, it has been shown that the stirrups are effective only if the slab depth is more than 250 mm.
b. Effectiveness of drop and edge beam
The effect of presence of drop and/or edge beam on the overall behaviour of the flat slab system under lateral loads has been found to have insufficient ductility. The drops are not effective in the regions where high unbalanced moments are expected to occur due to lateral drift. This is primarily due to the fact that the unbalanced moment results in change in the shear crack pattern which would render the drop ineffective. Similarly, edge beams along with the peripheral columns could provide a certain level of frame action in resisting the induced lateral loads. But, this frame action may not be adequate enough for the strength and deformability requirements under the induced seismic actions.
c. Detailing of tendons
The effect of detailing a tendon to pass through the column was discussed with a simple example. (Refer to post tensioned slab post) It is evident form this example that the forces induced, are at the discretion of the designer to consider it or not. It is advisable to lay the tendons individually rather than through the duct, at least in the vicinity of the slab column connection.
d. Sunken portion and slab cut offs
The effect of sunken portion or cut-outs in the slab would largely depend upon the location and size of it. ACI 318-08 (clause 11.11.6) has recommended suitable solution for the same. As far as possible one must try to avoid opening near the slab column critical section.

4. Analysis of flat slab systems
In order to design the structure for probable internal forces, the analyses for determining the design forces have to be properly carried out. Major issues regarding modelling, analysis were deliberated during the e- Conference. Many finite element based soft wares are available which could be used for analysis and design of flat slab structures, such as ADAPT, SAFE. These software are capable of handling not only RC flat slab but also the effect of PT as well. There was some discussion on how to model shear walls while analysing the flat slab system.

5. Other issues
One can not emphasize enough that no matter how good the analysis and design is, the behaviour of the structure would largely depend on quality of construction and detailing provided. We could have had more posts highlighting the problems associated in the field. However, we are sure that the materials posted on the use of post tensioning with flat slab system may have helped in understanding the practical issues related to the same.
The cost effectiveness of flat slab system was deliberated upon. In general, the flat slabs have been found to be competitive for a span of 10 to 12 m. Geographical location and optimisation of design plays a vital role on the cost effectiveness of the flat lab systems.

Finally, we would like to thank you for taking part in this e-Conference on flat slabs. It has indeed been a wonderful learning experience for all of us. We would like to thank all the participants for their valuable post and comments during the entire fortnight of discussions. We hope that the discussion provided will prove a stepping stone for further inquiry and learning.

Thank you and wish you a very happy new year!

Moderators
Durgesh Rai and Manmohan Kalgal
with assistance from Vijaya Narayanan, A R
--auto removed--
     



     




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PostPosted: Mon Jan 12, 2009 3:24 pm    Post subject: Moderators' Concluding Remarks Reply with quote

Respected Sir,
Really this e-conference is very successful event I have ever attended. It has not only allowed us a flexible timing for attending the conference but covered a broad spectrum of flat slab construction as well. Kudos to e-conf team--Sandeep

Note: Forwarded message attached

-- Original Message --

From: "alpa_sheth" <forum@sefindia.org>
To: econf@sefindia.org
Subject: [ECONF] Re: Moderators' Concluding Remarks



------ Start of attached email. Subject: [ECONF] Re: Moderators' Concluding Remarks ------
So very well summarized! An excellent wrap-up of a very successful Econference.


Thanks, so much Dr Rai and Dr. Kalgal for moderating this very invigorating econf. And to all of the participants for making it such a resounding success.

Sanjeev as always you have provided such exemplary supprt. We deeply appreciate it.

Warm regards,
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----- Original Message -----
From: dcrai (forum@sefindia.org)
To: econf@sefindia.org (econf@sefindia.org)
Sent: Monday, January 12, 2009 11:45 AM
Subject: [ECONF] Moderators' Concluding Remarks


Dear Colleagues,
It has indeed been an interesting fortnight of discussion over flat slab systems. Over this period we have had the chance to deliberate on wide range of issues covering the entire spectrum of flat slab modelling, analysis, design and practice. Though the e-Conference has come to an end, we encourage our colleagues to carry on with their discussions, at SEFI general forum.
We were happy to see that postings have covered issues related to research, practice, and code provisions and many of them have been quite comprehensive in discussing them. Though these posts themselves are excellent source of information on flats slabs system, we have tried to summarise the discussion as below:
1. General issues
a. Suitability of flat slab structures
It was rather evident form the discussion that the flat slabs systems are suitable for commercial structures requiring large span and open office space. Though flat slab structures have many drawbacks in terms of inadequate understanding of its behaviour under seismic action, its advantages such as large head room, reduced height of the building, easy shuttering and faster construction has made it popular among the clients and the construction industry.
b. Geometry of flat slab structures
As far as the geometry of the flat slab structures are concerned, it is better to have a symmetric disposition of columns and the lateral resisting systems. Seismic behaviour of symmetric system has found to be better than the asymmetric system. It has been suggested that the use of single panel flat slabs systems are to be avoided.

2. Behaviour under seismic action
The behaviour of the flat slab structure under seismic forces is a subject of active research and development because of their poor performance in the past earthquakes.
a. Drift-induced (seismic) Punching Shear
Generally, flat slabs under high seismic zones are required to be designed only for gravity loads and the entire lateral load is to be resisted by the lateral load resisting frame in place. However, the drift induced in the lateral direction has been found to induce additional unbalanced moment which enhances the shear stress demand at the slab column critical section. For this reason it is recommended that the inter story drift be restricted for flat slabs structures and/or limit the permissible shear stress allowed under gravity loads. A plot between gravity shear and inter story drift has been made available, by the research fraternity and ACI 318-08 (clause 21.13.6) code, with which you can determine whether the slab column connection designed needs additional shear reinforcement to resist punching shear failure or not.
b. Lateral resistance of flat slab system
As stated in the previous section, flat slab systems are designed to resist only gravity load in high seismic zones. However, one may argue that it is possible to design the flat slab system to resist lateral load as well. If we design the flat slab system to resist the lateral load then the following questions come to mind:
1) Can we design the flat slab system to resist the 100% lateral load? If yes, what should be the value of response reduction factor for such flat slab system, which accounts for its ability to undergo inelastic deformation without losing stability and integrity? Recommendations have been made to the research community to ascertain the value of response reduction factor through experimental and analytical studies.
2) If the lateral resistance offered by the flat slab system found to be inadequate then can we complement the lateral resistance by providing shear wall or moment resisting frame? If so, can we term such a system as a dual system? If yes, what should be the value of response reduction factor for such dual systems? Furthermore, what methodology needs to be adopted for proportioning the lateral load between the flat slab system and the other lateral force resisting system?
3) Can such a type of combined/dual lateral resisting system be used in high seismic zones?

3. Design issues
a. Shear reinforcement in slab
The use of shear reinforcement has been found to be an attractive option to improve the behaviour of the slab column connection under lateral loading. The shear fixture such as stud type reinforcement has been show to have improved shear strength as well as lateral deformation capacity. Apart form the stud reinforcement, there are different types of shear reinforcement developed and tested for their efficiency to improve the capacity and the ductility of the slab column connection. However, it has been shown that the stirrups are effective only if the slab depth is more than 250 mm.
b. Effectiveness of drop and edge beam
The effect of presence of drop and/or edge beam on the overall behaviour of the flat slab system under lateral loads has been found to have insufficient ductility. The drops are not effective in the regions where high unbalanced moments are expected to occur due to lateral drift. This is primarily due to the fact that the unbalanced moment results in change in the shear crack pattern which would render the drop ineffective. Similarly, edge beams along with the peripheral columns could provide a certain level of frame action in resisting the induced lateral loads. But, this frame action may not be adequate enough for the strength and deformability requirements under the induced seismic actions.
c. Detailing of tendons
The effect of detailing a tendon to pass through the column was discussed with a simple example. (Refer to post tensioned slab post) It is evident form this example that the forces induced, are at the discretion of the designer to consider it or not. It is advisable to lay the tendons individually rather than through the duct, at least in the vicinity of the slab column connection.
d. Sunken portion and slab cut offs
The effect of sunken portion or cut-outs in the slab would largely depend upon the location and size of it. ACI 318-08 (clause 11.11.6) has recommended suitable solution for the same. As far as possible one must try to avoid opening near the slab column critical section.

4. Analysis of flat slab systems
In order to design the structure for probable internal forces, the analyses for determining the design forces have to be properly carried out. Major issues regarding modelling, analysis were deliberated during the e- Conference. Many finite element based soft wares are available which could be used for analysis and design of flat slab structures, such as ADAPT, SAFE. These software are capable of handling not only RC flat slab but also the effect of PT as well. There was some discussion on how to model shear walls while analysing the flat slab system.

5. Other issues
One can not emphasize enough that no matter how good the analysis and design is, the behaviour of the structure would largely depend on quality of construction and detailing provided. We could have had more posts highlighting the problems associated in the field. However, we are sure that the materials posted on the use of post tensioning with flat slab system may have helped in understanding the practical issues related to the same.
The cost effectiveness of flat slab system was deliberated upon. In general, the flat slabs have been found to be competitive for a span of 10 to 12 m. Geographical location and optimisation of design plays a vital role on the cost effectiveness of the flat lab systems.

Finally, we would like to thank you for taking part in this e-Conference on flat slabs. It has indeed been a wonderful learning experience for all of us. We would like to thank all the participants for their valuable post and comments during the entire fortnight of discussions. We hope that the discussion provided will prove a stepping stone for further inquiry and learning.

Thank you and wish you a very happy new year!

Moderators
Durgesh Rai and Manmohan Kalgal
with assistance from Vijaya Narayanan, A R
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vipul_ahuja
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Joined: 01 Jan 2009
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PostPosted: Wed Jan 14, 2009 9:48 am    Post subject: Moderators' Concluding Remarks Reply with quote

Dear Moderators:

I  would like to felicitate the excellent job done in moderating as well as summarizing the conference proceedings. In the summary I noticed some lingering questions that I felt I had addressed to some extent in the paper I posted. I will try to address them here again (extracts from the original posting is in italics):

1) Can we design the flat slab system to resist the 100% lateral load? If yes, what should be the value of response reduction factor for such flat slab system, which accounts for its ability to undergo inelastic deformation without losing stability and integrity? Recommendations have been made to the research community to ascertain the value of response reduction factor through experimental and analytical studies.

Yes, as per ACI chapter 21 there is a concept of “Intermediate Frames”, comprising flat slabs. However it also states that these are not intended for zones of high seismicity. Further there are restrictions on the gravity load punching shear stress ratio being restricted to 0.4 among other conditions & detailing requirements.

Note that the paper referenced (& ASCE 41-06), shows the amount of rotation at various performance levels for various components (at varying states of stress & detailing practices adopted). These have already been converted to “m” factors (i.e. “R” values). Incidentally the rotation allowed for intermediate frames would yield a very small value or “R”. Of course if the research community in India wishes to verify these numbers it would be a welcome step.

2) If the lateral resistance offered by the flat slab system found to be inadequate then can we complement the lateral resistance by providing shear wall or moment resisting frame? If so, can we term such a system as a dual system? If yes, what should be the value of response reduction factor for such dual systems? Furthermore, what methodology needs to be adopted for proportioning the lateral load between the flat slab system and the other lateral force resisting system?

As we know when we make a deliberate attempt to limit the drift (to restrict the drift as per the ACI curve), the shear walls stiffness will be high enough that the flat slab stiffness would be justified in ignoring. For those cases where the flat slab frame stiffness is not ignorable, detailing practices (including joint shear reinforcing) would need to be provided to ensure the slab-column joints can take the peak rotation demand.

3) Can such a type of combined/dual lateral resisting system be used in high seismic zones?

This boils down to whether the frame components can take the rotations for the levels of performance required. Though as a matter of practice in my experience even on the East Coast in the US (area of low seismic risk) designers almost always provide shear walls. All in all I am not inclined to providing Intermediate Frames. Fortunately or unfortunately that also comes accross as the intent of IS 1893-2002.

Having said that I hope it is apparent that all the research has to do is to establish the ductility values for various types of details states of stress (as shown in ASCE 41). However the basic algorithm for the design community is ready—only we need a modification of the code. Adjustments can be made as research results are made available—till then it is better to err on the safe side.

Warm regards & a Happy New Year to everyone,

Vipul Ahuja
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