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PT flat slab - drop panel, edge beams and design

 
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arunkashikar
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PostPosted: Mon Dec 22, 2008 4:58 am    Post subject: PT flat slab - drop panel, edge beams and design Reply with quote

Most of the flat slabs used now-a-days is Post tensioned (PT). My question is specific to PT flat slab.

I had already put these questions on general forum and some deliberations are already done on these issues, but I think these needs to be deliberated in much more details in the e-conference and hence repeating these questions.
<![if !supportLists]>1) <![endif]> Is there a practical way of avoiding drop panels in PT flat slabs by using some kind of shear reinforcement or stud-rail. This is very common outside India but very uncommon here. Avoiding drop panel will make the PT slab cheaper as well as faster to construct. Can we discuss on the technical/practical problems or limitations in India because of which we still continue to use drop panels?
<![if !supportLists]>2) <![endif]>Current design practice in building industry in India is – The PT slab is designed by PT vendor, while the rest of the building is designed by structural consultant. Which in my opinion makes the design in-efficient as:
<![if !supportLists]>a. <![endif]>PT slab is designed only for gravity load, without thinking about effect of lateral load
<![if !supportLists]>b. <![endif]>Passive reinforcement provided to take care of lateral load may be much higher in this case as PT strands are not at all considered to resist lateral load. ( Whether it can be used or not from stress reversal perspective can also be debated)
<![if !supportLists]>c. <![endif]>If in bridge industry PT is always designed by consultant, why it can’t be done for buildings?
<![if !supportLists]>3) <![endif]>Can we also avoid edge beams by designing PT slab itself for all forces which are supposed to be taken by edge beam, which will make PT slab really flat, helping in faster construction. This is also a common practice outside India.

Regards,
Arun Kashikar

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ahujavipul
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PostPosted: Mon Dec 22, 2008 5:35 am    Post subject: Re : PT flat slab - drop panel, edge beams and design Reply with quote

This mail has been re posted under an appropriate subject heading

Last edited by ahujavipul on Mon Dec 22, 2008 9:51 am; edited 1 time in total
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Vijayanarayanan
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PostPosted: Mon Dec 22, 2008 7:40 am    Post subject: Re: PT flat slab - drop panel, edge beams and design Reply with quote

Dear sefians,

The concept of using the PT Flat slab for lateral resistance seems to be out of reach. This is primarily due to the fact that the lateral stiffness of any flat slab is very low. For this reason the flat slabs are primarily designed to resist the gravity load alone. Furthermore the use of PT along with the flat slab decreases the lateral stiffness. One has to use certain kind of lateral load resisting system, such as shear wall, along with the flat slab system in order to resist the lateral load. In my opinion, the design of PT slab to resist the lateral load would become very complex.

The use of drop panel, form laboratory test, has been found to increase the shear strength alone. The most interesting fact out of these test were with respect to ductility, the provision of drops does not alter the ductility of the slab column connection. In case of seismic design one intends to have maximum ductility to the system. For this reason, I personally feel that, the use of drop panel is not going to solve the problem of punching shear.

The use of shear stud has been found to increase the ductility, but further research is required when the flat slab is subjected to high level of unbalanced moment, induced due to lateral loading.

There are many other types of shear reinforcement, at least in research level, which can be put to use too. The comparative statement of such reinforcement and their limitations are the need of the hour.

The edge beam to a certain extent helps in resisting the lateral load and the controlling the deflection.

I personally feel that there should be a compatibility between the ease for construction, lower cost with the need to provide adequate measures to make the structure safe.


Vijaya Narayanan.A.R
M.Tech student,
IIT Kanpur
  

arunkashikar wrote:
Most of the flat slabs used now-a-days is Post tensioned (PT). My question is specific to PT flat slab.

I had already put these questions on general forum and some deliberations are already done on these issues, but I think these needs to be deliberated in much more details in the e-conference and hence repeating these questions.
<if>1) <endif> Is there a practical way of avoiding drop panels in PT flat slabs by using some kind of shear reinforcement or stud-rail. This is very common outside India but very uncommon here. Avoiding drop panel will make the PT slab cheaper as well as faster to construct. Can we discuss on the technical/practical problems or limitations in India because of which we still continue to use drop panels?
<if>2) <endif>Current design practice in building industry in India is – The PT slab is designed by PT vendor, while the rest of the building is designed by structural consultant. Which in my opinion makes the design in-efficient as:
<if>a. <endif>PT slab is designed only for gravity load, without thinking about effect of lateral load
<if>b. <endif>Passive reinforcement provided to take care of lateral load may be much higher in this case as PT strands are not at all considered to resist lateral load. ( Whether it can be used or not from stress reversal perspective can also be debated)
<if>c. <endif>If in bridge industry PT is always designed by consultant, why it can’t be done for buildings?
<if>3) <endif>Can we also avoid edge beams by designing PT slab itself for all forces which are supposed to be taken by edge beam, which will make PT slab really flat, helping in faster construction. This is also a common practice outside India.

Regards,
Arun Kashikar

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sahi
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Joined: 02 Oct 2008
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PostPosted: Mon Dec 22, 2008 8:44 am    Post subject: PT flat slab - drop panel, edge beams and design Reply with quote

There are two parts of discussion. One is for Dr & prof in IITs. Other
is field construction. for discussion and written articals you should
listen Dr of IIT who have not seen many slabs.
If you are interested in construction then you should keep things
simple and those under stood by people working in trade. Design and
construction should be conservative enough and economical considering
prevailing quality of site workers and mistakes created by them. If
there is failure, no body will read your equations derived by IIT DRs.

On 12/22/08, vijaya narayanan <forum@sefindia.org> wrote:
Quote:
Dear sefians,

The concept of using the PT Flat slab for lateral resistance seems to be out
of reach. This is primarily due to the fact that the lateral resistance of
any flat slab is very low. For this reason the flat slabs are primarily
designed to resist the gravity load alone. Furthermore the use of PT along
with the flat slab decreases the lateral resistance. One has to use certain
kind of lateral load resisting system, such as shear wall, along with the
flat slab system in order to resist the lateral load. In my opinion, the
design of PT slab to resist the lateral load would become very complex.

The use of drop panel, form laboratory test, has been found to increase the
shear strength alone. The most interesting fact out of these test were with
respect to ductility, the provision of drops does not alter the ductility of
the slab column connection. In case of seismic design one intends to have
maximum ductility to the system. For this reason, I personally feel that,
the use of drop panel is not going to solve the problem of punching shear.

The use of shear stud has been found to increase the ductility, but further
research is required when the flat slab is subjected to high level of
unbalanced moment, induced due to lateral loading.

There are many other types of shear reinforcement, at least in research
level, which can be put to use too. The comparative statement of such
reinforcement and their limitations are the need of the hour.

The edge beam to a certain extent helps in resisting the lateral load and
the controlling the deflection.

I personally feel that there should be a compatibility between the ease for
construction, lower cost with the need to provide adequate measures to make
the structure safe.


Vijaya Narayanan.A.R
M.Tech student,
IIT Kanpur















arunkashikar wrote:
Quote:
Most of the flat slabs used now-a-days is Post tensioned (PT). My question
is specific to PT flat slab.

I had already put these questions on general forum and some deliberations
are already done on these issues, but I think these needs to be
deliberated in much more details in the e-conference and hence repeating
these questions.
<if>1) <endif> Is there a practical way of avoiding drop panels in PT flat
slabs by using some kind of shear reinforcement or stud-rail. This is very
common outside India but very uncommon here. Avoiding drop panel will make
the PT slab cheaper as well as faster to construct. Can we discuss on the
technical/practical problems or limitations in India because of which we
still continue to use drop panels?
<if>2) <endif>Current design practice in building industry in India is –
The PT slab is designed by PT vendor, while the rest of the building is
designed by structural consultant. Which in my opinion makes the design
in-efficient as:
<if>a. <endif>PT slab is designed only for gravity load, without thinking
about effect of lateral load
<if>b. <endif>Passive reinforcement provided to take care of lateral load
may be much higher in this case as PT strands are not at all considered to
resist lateral load. ( Whether it can be used or not from stress reversal
perspective can also be debated)
<if>c. <endif>If in bridge industry PT is always designed by consultant,
why it can't be done for buildings?
<if>3) <endif>Can we also avoid edge beams by designing PT slab itself for
all forces which are supposed to be taken by edge beam, which will make PT
slab really flat, helping in faster construction. This is also a common
practice outside India.

Regards,
Arun Kashikar











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ahujavipul
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PostPosted: Thu Dec 25, 2008 5:37 am    Post subject: PT flat slab - drop panel, edge beams and design Reply with quote

Dear Mr. Vijaya

You have highlighted several good points regarding ductility of the slab column joint. I am myself convinced its just good to avoid the connection from at least being part of the lateral force resisting system. If the drift is very small--less than 0.005--assuming a cracked section analysis (see my code recommendation elsewhere on the posts), ACI states no shear reinforcing is required. And this stands to reason as well. Further we should not dabble at high states of axial stress due to gravity loads in columns & punching stress for joints.

Once we have made a statement that shear walls will take 100% of lateral forces I don't know why we should make a fuss over a perimeter beam forming a frame. Most likely well placed perimeter shear walls (in few bays) will be far stiffer than a perimeter frame--making the frame redundant.

Using lower "R" factors for design of connections also ensures ductility (also addressed in reference above)

Regards

Vipul Ahuja
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