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Why slabs are only designed for gravity loads and not for any lateral force (Earthquake and wind)?

 
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venkatasalamkl
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Joined: 27 Oct 2017
Posts: 13

PostPosted: Mon Dec 02, 2019 7:45 am    Post subject: Why slabs are only designed for gravity loads and not for any lateral force (Earthquake and wind)? Reply with quote

Hi All,

As a fresher with curiosity to learn how things are designed industry I need some input/clarity in certain areas.

I will start with my first question today.

#1. Why don't we design slabs for lateral forces?
               As we all know F=Ma. M is mass of slab and a is acceleration of structure. From this equation its clear that lateral earthquake forces are generated from slab mass. i.e., each finite slab mass is excited during earthquake, so the force from the finite mass from the farther end of the structure should be taken into the lateral force resisting system. Then in this case why do most of the companies in the market are designing slab as gravity only element?

With my interactions with my classmates I understand that most of the companies design slab as gravity only element.

What's reason behind for this major assumption (slabs to be designed only gravity only element)?

It would be really helpful if senior people in SEFI throws some insight on this assumption.

Thanks,
Venkat
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syed_A
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Joined: 22 Oct 2011
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Location: Hyderabad

PostPosted: Mon Dec 02, 2019 12:52 pm    Post subject: Reply with quote

Dear Engr
For better understanding with figures please refer Design of Reinforced concrete structures by N SUBRAMANIAN sir book, section 9.9, page no 349

Regards
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venkatasalamkl
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PostPosted: Mon Dec 02, 2019 10:15 pm    Post subject: Reply with quote

Thanks for sharing Syed.

As said in book there are very good stuffs for diaphragm design in ACI 318 as well.

The question here is why do companies ignore this and design as gravity only slab.

I guess our code doesn't explain much about diaphragm design.

Why is this ignored all along?

Experts/Codal committee members if anyone here in SEFI please throw your thoughts. That will be really helpful to young engineers.
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Dr. N. Subramanian
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PostPosted: Mon Dec 02, 2019 11:14 pm    Post subject: Reply with quote

Dear Er Venkatasalam,

Diaphragm is mentioned in Clause 7.6.4 of IS 1893 (Part 1):2016. See  Moehle, J.P., J.D. Hooper, D.J. Kelly, and T.R. Meyer, Seismic Design of Cast-in-PlaceConcrete Diaphragms,Chords, and Collectors-A Guide for Practicing Engineers (https://www.nehrp.gov/pdf/nistgcr10-917-4.pdf) for the complete treatment of Diaphragms.

Best wishes
NS

venkatasalamkl wrote:
Thanks for sharing Syed.

As said in book there are very good stuffs for diaphragm design in ACI 318 as well.

The question here is why do companies ignore this and design as gravity only slab.

I guess our code doesn't explain much about diaphragm design.

Why is this ignored all along?

Experts/Codal committee members if anyone here in SEFI please throw your thoughts. That will be really helpful to young engineers.
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venkatasalamkl
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Joined: 27 Oct 2017
Posts: 13

PostPosted: Sat Dec 28, 2019 11:25 pm    Post subject: Reply with quote

Thanks for sharing NEHRP guidelines sir.

I went through that now, but my concern is about Indian Standards and Indian Practices. As stated earlier, IS 1893 hardly has half a page discussion on diaphragm design. But if you compare the same with American standards (ACI 318-19 Chapter - 12 & NEHRP Technical briefs), they have separate chapter for diaphragm design.

IS 1893 talks only about selection of diaphragm (Rigid/Semi rigid). This clause will be helpful by guiding us on which type of diaphragm should be selected for analysis. But Indian standards totally ignored the DESIGN & DETAILING part. I may be unaware about written statements on diaphragm design and detailing in Indian standards. If any, Please let me know.

As IS doesn't talk about design and detailing, quite few design consultancies here ignores the fact that slab should be designed for lateral loads as well. I have personally seen few cases where slab and shear wall is connected with only 2-T12 T&B Fe415 bars, which means that it has a capacity to transfer only 187.64 kN (The axial capacity reported here ignores materials FOS and also note that there were no shear friction bars to transfer shear force) and at the same time those T&B bars has to transfer gravity moment and shear. The shear wall in that level had 280 kN lateral shear demand. So in this case the wall to slab connection will fail during an earthquake event because of insufficient rebars. This will change the entire load path of the structure for what it is designed for.

Also, in another instance I have heard engineers saying that RIGID diaphragm need not be designed for lateral loads. Is that true? From my understanding even if we are idealizing the slab as a rigid diaphragm, the slab should be designed for lateral loads. As the force is generated in the slab.

I am bit curious to know who is responsible for adding these things in Indian Standards? Is the topic included in upcoming revisions of IS 1893 / IS 456? How to contact codal committee people? Can young engineers take part in helping senior people for updating IS codes?

@Dr. Subramanian - I am sure that there are good no. of topics to be included in Indian standards and I understood this fact after reading your book on Design of Reinforced Concrete Structures. Thanks for the wonderful book!!!

Thanks,
Venkat
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