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Flat slab -lateral load resisting frame??? -Clarification
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alpa_sheth
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Joined: 26 Jan 2003
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PostPosted: Fri Jan 16, 2009 7:30 am    Post subject: Flat slab -lateral load resisting frame??? -Clarification Reply with quote

I may mention that I had used ACI-318-05 while referencing the clause nos.
The appropriate Clause nos as per ACI 318-08 are different. In fact 318-08 would require a re-write of the mail!!

Alpa
----- Original Message ----- From: Alpa Sheth (alpa_sheth@vakilmehtasheth.com)
To: general@sefindia.org (general@sefindia.org)
Sent: Friday, January 16, 2009 12:16 PM
Subject: Flat slab -lateral load resisting frame???



Hi:

At onset let me apologise for writing on this topic after the econf has concluded. I was indisposed, hence this tardiness.

I am not quite sure I understood what was being discussed as a lateral load resisting system for flat slab structures. I also noticed a reluctance in many designers to share fully how they are approaching this issue and what they do on their projects. The topic was fairly obfuscated. I think SEFI is meant for threadbare discussions and please let's talk to each other with trust and honesty, never mind if others may differ in their design philosophy.

I have personally never used the flat slab "column strip & column" system as a lateral load resisting system. I have also not come across any journal paper/ literature either which explains "frame behaviour" of such a system under lateral loads and also what are the allowable stresses you would take, how do you mobilise moment and shear capacity at slab-column junction and so on, though there are many conference papers on this subject, mainly from South and South East Asia where this system is very popular. I would be happy if someone could direct me to journal papers on this topic.

I too think it is sage to provide shear walls to take 100% of lateral loads. If you have a good spandrel beam along perimeter, you could perhaps design it is as a dual system - shear walls + perimeter frame. And you would want to design the flat slab system as a gravity load system designed for the induced moments resulting from storey deformations under lateral loads. Some on the forum have already discussed this. I also provide ductile detailing to the flat slab columns as a form of redundancy. It is also a requirement of sorts if you design for gravity system as per ACI 318 clause 21.11


I think Intermediate Frame Concept is referenced out of context here. The provisions of Clause 21.12.6 are not meant to give carte blanche to use flat slab system for lateral load resistance. I think the clause is meant to address the moments induced from storey deformations under lateral loads. Without prejudice to that statement, refer Clause 21.12.3. You need to check shear capacity of the slabs considering earthquake force twice that of codal value.


Regards,
Alpa Sheth

[quote] ----- Original Message -----
From: vipul_ahuja (forum@sefindia.org)
To: econf@sefindia.org (econf@sefindia.org)
Sent: Wednesday, January 14, 2009 3:18 PM
Subject: [ECONF] Re: Moderators' Concluding Remarks


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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PostPosted: Fri Jan 16, 2009 9:57 am    Post subject: Flat slab -lateral load resisting frame??? -Clarification Reply with quote

We should design the columns for gravity loads and the lateral loads should be transferred to shear walls / core walls . By this method I have designed a 25 storey high rise building in Gulf. And it has worked well.
Vijay

--- On Fri, 16/1/09, alpa_sheth <forum@sefindia.org> wrote:

Quote:
From: alpa_sheth <forum@sefindia.org>
Subject: [SEFI] Fw: Flat slab -lateral load resisting frame??? -Clarification
To: general@sefindia.org
Date: Friday, 16 January, 2009, 1:31 PM

I may mention that I had used ACI-318-05 while referencing the clause nos.
The appropriate Clause nos as per ACI 318-08 are different. In fact 318-08 would require a re-write of the mail!!

Alpa
----- Original Message ----- From: Alpa Sheth (alpa_sheth@vakilmehtasheth.com (alpa_sheth@vakilmehtasheth.com))
To: general@sefindia.org (general@sefindia.org) (general@sefindia.org (general@sefindia.org))
Sent: Friday, January 16, 2009 12:16 PM
Subject: Flat slab -lateral load resisting frame???



Hi:

At onset let me apologise for writing on this topic after the econf has concluded. I was indisposed, hence this tardiness.

I am not quite sure I understood what was being discussed as a lateral load resisting system for flat slab structures. I also noticed a reluctance in many designers to share fully how they are approaching this issue and what they do on their projects. The topic was fairly obfuscated. I think SEFI is meant for threadbare discussions and please let's talk to each other with trust and honesty, never mind if others may differ in their design philosophy.

I have personally never used the flat slab "column strip & column" system as a lateral load resisting system. I have also not come across any journal paper/ literature either which explains "frame behaviour" of such a system under lateral loads and also what are the allowable stresses you would take, how do you mobilise moment and shear capacity at slab-column junction and so on, though there are many conference papers on this subject, mainly from South and South East Asia where this system is very popular. I would be happy if someone could direct me to journal papers on this topic.

I too think it is sage to provide shear walls to take 100% of lateral loads. If you have a good spandrel beam along perimeter, you could perhaps design it is as a dual system - shear walls + perimeter frame. And you would want to design the flat slab system as a gravity load system designed for the induced moments resulting from storey deformations under lateral loads. Some on the forum have already discussed this. I also provide ductile detailing to the flat slab columns as a form of redundancy. It is also a requirement of sorts if you design for gravity system as per ACI 318 clause 21.11


I think Intermediate Frame Concept is referenced out of context here. The provisions of Clause 21.12.6 are not meant to give carte blanche to use flat slab system for lateral load resistance. I think the clause is meant to address the moments induced from storey deformations under lateral loads. Without prejudice to that statement, refer Clause 21.12.3. You need to check shear capacity of the slabs considering earthquake force twice that of codal value.


Regards,
Alpa Sheth

Quote:
----- Original Message -----
From: vipul_ahuja (forum@sefindia.org (forum@sefindia.org))
To: econf@sefindia.org (econf@sefindia.org) (econf@sefindia.org (econf@sefindia.org))
Sent: Wednesday, January 14, 2009 3:18 PM
Subject: [ECONF] Re: Moderators' Concluding Remarks


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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PostPosted: Fri Jan 16, 2009 1:53 pm    Post subject: Flat slab -lateral load resisting frame??? -Clarification Reply with quote

Dear Alpa,

I would like to agree that flat slab frames should not be part of a lateral frame system, however when this practice is rampant in some parts of the world and the ACI allows it for low to moderate seismic risk zones, it would take more than a hunch to throw the concept out of the window. I have put forth argument based on joint rotation capacity based on ASCE-41 and my paper etc. but there is still a mountain to climb.

My interpretation of ACI-05 is as follows:

Sec 21.2.2.4, “members assumed not to be part of the lateral-force resisting system shall comply with 21.11”. So this is out of the discussion--we agree this is what it should be.

The section on Intermediate Frames is 21.12. This section states “If a two-way slab system without beams is treated as a part of a frame resisting E…reinforcement details in any span resisting moments caused by lateral force shall satisfy 21.12.6".

This section (21.12.6) gives all sorts of detailing requirement for flat slabs when used as frames. Further 21.12.6.7 states “two way shear caused by factored gravity loads shall not exceed 0.4xphixVc…”. So there are severe restrictions on the use of intermediate frames.

Given these parameters in this section, intermediate frames are allowed to resist earthquake forces in moderate or low risk areas. (also see 21.2.1.3).

There is one other point of perimeter beam--I don't think it is necessary if the flat slab system is not being designed as part of the lateral system (unless required as a collector etc)

I don’t have ACI-08 to study changes to these sections cited above, but if there are changes it will be appreciated if they are shared.

Warm regards

Vipul Ahuja
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Dr. N. Subramanian
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PostPosted: Wed Jan 21, 2009 7:14 pm    Post subject: Flat slab -lateral load resisting frame??? -Clarification Reply with quote

Dear Mr. Vipul Ahuja,

I thank you for your well researched article and also your discussions on this topic.

I just want to inform all about the article by Mrs. Alpa Sheth "Use of Intermediate RC MRF in moderate Seismic Zones, in ICJ, Nov. 03, pp. 1431-1435, that gives insights to Intermediate RC MRF, which may be adopted in Zone III.

I take this opportunity to send my best wishes to Mrs. Alpa for speedy recovery!

Best regards
Subramanian

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 Fri, 1/16/09, vipul_ahuja <forum@sefindia.org> wrote:
Quote:
From: vipul_ahuja <forum@sefindia.org>
Subject: [SEFI] Re: Flat slab -lateral load resisting frame??? -Clarification
To: general@sefindia.org
Date: Friday, January 16, 2009, 7:23 PM

Dear Alpa,

I would like to agree that flat slab frames should not be part of a lateral frame system, however when this practice is rampant in some parts of the world and the ACI allows it for low to moderate seismic risk zones, it would take more than a hunch to throw the concept out of the window. I have put forth argument based on joint rotation capacity based on ASCE-41 and my paper etc. but there is still a mountain to climb.

My interpretation of ACI-05 is as follows:

Sec 21.2.2.4, “members assumed not to be part of the lateral-force resisting system shall comply with 21.11”. So this is out of the discussion--we agree this is what it should be.

The section on Intermediate Frames is 21.12. This section states “If a two-way slab system without beams is treated as a part of a frame resisting E…reinforcement details in any span resisting moments caused by lateral force shall satisfy 21.12.6".

This section (21.12.6) gives all sorts of detailing requirement for flat slabs when used as frames. Further 21.12.6.7 states “two way shear caused by factored gravity loads shall not exceed 0.4xphixVc…”. So there are severe restrictions on the use of intermediate frames.

Given these parameters in this section, intermediate frames are allowed to resist earthquake forces in moderate or low risk areas. (also see 21.2.1.3).

There is one other point of perimeter beam--I don't think it is necessary if the flat slab system is not being designed as part of the lateral system (unless required as a collector etc)

I don’t have ACI-08 to study changes to these sections cited above, but if there are changes it will be appreciated if they are shared.

Warm regards

Vipul Ahuja
     



     


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ahujavipul
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PostPosted: Thu Jan 22, 2009 5:01 pm    Post subject: Flat slab -lateral load resisting frame??? -Clarification Reply with quote

Dear Dr Subramanian,

Thank you for the compliments, though I feel dwarfed when I see your contribution to SEFI  & the profession.

I am just wondering if the article you referred can be posted on this site for people to see.

Best wishes.

Vipul Ahuja
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PostPosted: Thu Jan 22, 2009 7:08 pm    Post subject: Flat slab -lateral load resisting frame??? -Clarification Reply with quote

Dear Mr. Vipul,

Thanks for the kind words.

I do not have the PDF of the paper by Mrs. Alpa- I have only the Journal copy. Perhaps she can post it.

Regards
Subramanian

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 Thu, 1/22/09, ahujavipul <forum@sefindia.org> wrote:
Quote:
From: ahujavipul <forum@sefindia.org>
Subject: [SEFI] Re: Flat slab -lateral load resisting frame??? -Clarification
To: general@sefindia.org
Date: Thursday, January 22, 2009, 10:31 PM

Dear Dr Subramanian,

Thank you for the compliments, though I feel dwarfed when I see your contribution to SEFI & the profession.

I am just wondering if the article you referred can be posted on this site for people to see.

Best wishes.

Vipul Ahuja
     



     


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PostPosted: Fri Jan 23, 2009 7:53 am    Post subject: Intermmediate frames Reply with quote

Dear Dr. Subramanian and Vipul:

Many thanks for good wishes.

Yes, there was such a paper. Here it is. Infact this has been proposed/adopted in the draft IS 1893 Code too but as Dr Sudhir Jain said, when and in what form it will come out is anybody's guess!!!

Cheers,
Alpa



[quote] ----- Original Message -----
From: drnsmani (forum@sefindia.org)
To: general@sefindia.org (general@sefindia.org)
Sent: Thursday, January 22, 2009 5:00 AM
Subject: [SEFI] Re: Flat slab -lateral load resisting frame??? -Clarification


Dear Mr. Vipul Ahuja,

I thank you for your well researched article and also your discussions on this topic.

I just want to inform all about the article by Mrs. Alpa Sheth "Use of Intermediate RC MRF in moderate Seismic Zones, in ICJ, Nov. 03, pp. 1431-1435, that gives insights to Intermediate RC MRF, which may be adopted in Zone III.

I take this opportunity to send my best wishes to Mrs. Alpa for speedy recovery!

Best regards
Subramanian

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 Fri, 1/16/09, vipul_ahuja wrote:
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PostPosted: Fri Jan 23, 2009 5:22 pm    Post subject: Flat slab -lateral load resisting frame??? -Clarification Reply with quote

Dear Dr. Subramanian,

I think this paper was written more for Intermediate Frames using beams & columns rather than flat slabs--though there is a passing mention that there needs to be a code section on them. For example none of the provisions of ACI 318-05, chapter 21.12.6 that I have addressed in my previous correspondence appears in this paper.

Of course the paper reflects excellent work but it’s not entirely relevant to this discussion.

Thanks & regards

Vipul Ahuja
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Dr. N. Subramanian
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PostPosted: Fri Jan 23, 2009 6:40 pm    Post subject: Flat slab -lateral load resisting frame??? -Clarification Reply with quote

Dear Mr. Vipul,

Absolutely. I just wanted others to know about this paper, as many may not know about Intermediate MRF provisions. Your discussions and paper provide more insight into the design of flat slabs.

As Ms.Alpa and Prof. Jain have noted, it takes several years for BIS to print the code and hence no one knows when the several draft codes (IITK_GSDMA) available in NICEE site will see the light of the day! However they will surely provide some guidance to several engineers. For this we should thank Prof. Jain and his associates.

regards
Subramanian

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 Fri, 1/23/09, ahujavipul <forum@sefindia.org> wrote:
Quote:
From: ahujavipul <forum@sefindia.org>
Subject: [SEFI] Re: Flat slab -lateral load resisting frame??? -Clarification
To: general@sefindia.org
Date: Friday, January 23, 2009, 10:52 PM

Dear Dr. Subramanian,

I think this paper was written more for Intermediate Frames using beams & columns rather than flat slabs--though there is a passing mention that there needs to be a code section on them. For example none of the provisions of ACI 318-05, chapter 21.12.6 that I have addressed in my previous correspondence appears in this paper.

Of course the paper reflects excellent work but it˘s not entirely relevant to this discussion.

Thanks & regards

Vipul Ahuja
     



     


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thensenamar mp
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PostPosted: Sat Jan 24, 2009 1:00 pm    Post subject: Reply with quote

Vipul Ahuja Sir,
Can u post your paper on flat slab in sefi so that other who have not seen your paper can benefit from it

Thanks
Thensenamr mp
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