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Post tensioned slabs
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kajal.chopra
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Joined: 26 Jan 2003
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PostPosted: Tue Dec 30, 2008 10:01 am    Post subject: Reply with quote

See Kedar,

in your point no 1 you have arrived at the shear going to the column 13.13 KN by distributing the total shear in proportion to the stiffnesses of column and slab.Right?

My question is that if the slab and column are cast in two different operations, can we distribute the total shear in proportion to the stiffnesses of the slab and column?

Distribution of shear in proportion to stiffnesses is valid only if a complete monlolthic action exists between slab and column.Right?

Now, if the two that is slab and column are cast in 2 separate operations, can there be a complete monloithic action ensured between the two?
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Kedar
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Joined: 23 Jul 2008
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PostPosted: Tue Dec 30, 2008 10:26 am    Post subject: Reply with quote

Hi Kajal



If the joint is NOT monolithic one can  not
distribute shear in proportion with stiffness.



But therotically if the slab is forced fit one can expect
slight constraint for deflection  of column,hence there
could be transfer of shear in the form of push/pull
to slab.



This is similar to concept of introducing stiffness reduction factor for
joint as a whole,hence shear distribution procedure won't change i.e. if there is absolutely no connection between slab and column the stiffness reduction factor will become ZERO and factor will become ONE if there is monolithic connection giving us previous results.




But Practically its less important.




regards
Kedar
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amarjeetsingh
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PostPosted: Thu Jan 01, 2009 8:21 am    Post subject: Post tensioned slabs Reply with quote

dear all,

lots of informative tech material about PT/ Conventional Flat slab has been discussed in the last fewdays in this forum. it will be appreciative if the material is complied and posted as reference for all of us.

thanks in anticipation


regrds

amarjeet singh

On 12/30/08, Kedar <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
  Hi Kajal



If the joint is NOT monolithic one can not
distribute shear in proportion with stiffness.



But therotically if the slab is forced fit one can expect
slight constraint for deflection of column,hence there
could be transfer of shear in the form of push/pull
to slab.



This is similar to concept of introducing stiffness reduction factor for
joint as a whole,hence shear distribution procedure won't change i.e. if there is absolutely no connection between slab and column the stiffness reduction factor will become ZERO and factor will become ONE if there is monolithic connection giving us previous results.




But Practically its less important.




regards
Kedar







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rbchopda
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Joined: 30 Aug 2008
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PostPosted: Thu Jan 01, 2009 2:12 pm    Post subject: Post tensioned slabs Reply with quote

Dear sir,

very good suggestion, i strongly support this.

Regards
er.rbchopda@rediffmail.com (er.rbchopda@rediffmail.com)


On Thu, Jan 1, 2009 at 3:29 PM, amarjeet singh <forum@sefindia.org (forum@sefindia.org)> wrote:
[quote]  dear all,

lots of informative tech material about PT/ Conventional Flat slab has been discussed in the last fewdays in this forum. it will be appreciative if the material is complied and posted as reference for all of us.

thanks in anticipation


regrds

amarjeet singh

On 12/30/08, Kedar forum@sefindia.org (forum@sefindia.org))> wrote:
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Dr. N. Subramanian
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PostPosted: Thu Jan 01, 2009 4:09 pm    Post subject: Post tensioned slabs Reply with quote

Dear all,
I
t is a good suggestion by Mr. Amajeet Singh. As I indicated in my earlier email, a special publication similar in line with ACI Special publication, may be compiled, with some design examples of both RCC and PT slabs and also idealization of flat slabs for computer analysis, etc. The document may be posted in SEFIndia site and published as a book.

Regards
Subramanaian

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, 1/1/09, amarjeet singh <forum@sefindia.org> wrote:
[quote]From: amarjeet singh <forum@sefindia.org>
Subject: [ECONF] Re: Post tensioned slabs
To: econf@sefindia.org
Date: Thursday, January 1, 2009, 3:29 PM

dear all,

lots of informative tech material about PT/ Conventional Flat slab has been discussed in the last fewdays in this forum. it will be appreciative if the material is complied and posted as reference for all of us.

thanks in anticipation


regrds

amarjeet singh

On 12/30/08, Kedar forum@sefindia.org)> wrote:        --auto removed--

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Rajiv
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PostPosted: Mon Jan 05, 2009 1:08 pm    Post subject: Post tensioned slabs Reply with quote

Dear Mr. Bhowmick, Mr. Garg and Dr. Kalgal:

Many thanks for discussing so many things in one single post. Staged analysis is important in many rapid projects where a time cycle is defined for use and reuse of forms. The only way of doing it is to consider time dependent material behaviour in solving the problem. Linear analysis software can't handle this complex problem.

However there are many software available which can do this. For example, SAP2000 and Adina for staged analysis. PT slabs can be designed using SAFE P/T, ADAPT, RAPT etc.

Thanks and regards

Rajiv



On Sun, 28 Dec 2008 PRADEEP GARG wrote :
Quote:
Dear Mr. Alok Bhowmick & Mr Kalgal

Thanks for discussing some very pertinent question and
answers on Flat Slabs and PT slab.

I have one more querry in this regard :

The modelling of slab-beam-column-shear wall structures
for analysis and design is quite well known when using
softwares like STAAD, E-Tabs etc.

How should flat slab- column -shear wall be modelled in
these softwares for analysis and design ?

Pradeep Garg


--- On Thu, 12/25/08, bsec <forum@sefindia.org> wrote:


Quote:
From: bsec <forum@sefindia.org>
Subject: [ECONF] Re: Post tensioned slabs
To: econf@sefindia.org
Date: Thursday, December 25, 2008, 5:26 PM

Dear Mr Kalgal,

Thanks very much for such a crisp and prompt
response. It answers my querries to a large extent
Quote:

Alok Bhowmick

-----Original Message-----
From: mkalgal [mailto:forum@sefindia.org]
Sent: Thursday, December 25, 2008 4:43 PM
To: econf@sefindia.org (econf@sefindia.org)
Subject: [ECONF] Re: Post tensioned slabs

Dear Alok

A very big list indeed!

Will try to answer as briefly as possible

1. What are the key benefits of Flat Slab
construction vis--vis a normal beam and slab
construction?
Quote:

a. Absence of beams leads to greater head room.
b. For a given head room (which in case of beam slab
construction is to the soffit of the beam), the storey
height can be reduced, which leads to lesser weight on
columns and foudation, reduction in cost of walling,
plaster and painting etc. (If we can reduce 300mm in
each floor, you get one floor more for the same height
as would be required for a 10 storey beam-slab
buliding!)
Quote:
c. You can have floor height windows since there are
no beams
Quote:
d. Since Shuttering is simpler, it takes less time to
provide and remove shuttering (mentioned in your
question 3).
Quote:
e. Bar-bending work is simpler

2. When and under what circumstances one should
decide to go for Flat Slab Construction for a given
project? What is the economic span range for flat slab
construction? Is there any guideline?
Quote:

First part is answered above. Economical spans depend
on the loading and number of spans. In RCC, flat
plate/slabs are provided between 6m to 10m spans. It
can be shown that PT slabs are more economical than RC
Flat slabs for spans between 9m and 12m spans.
Quote:
Do not ask for a flat slab to be more economical than
beam-slab! Beams slab construction is generally more
economical and robust as compared to flat slab systems.
Quote:

3. Post Tensioning of Flat Slab helps not only to
bring in material economy in the structure, but also
helps to speed up construction as the de-shuttering can
be done much faster. Is it a practice in India to go
for RCC Flat Slab (rather than PT Slabs?).
Quote:

There are hundreds of PT flat slab building built and
hundreds more are being built. There are more than 6 PT
companies working all over India and their hands are
full.
Quote:
Some consultants prefer RC flat slabs over PT flat
slabs, more so in seismic zones IV and V. Nothing wrong
in it, but my personal opinion is if RC flat slab is OK;
there should be no bar for PT flat slabs in these
zones.
Quote:
In-fact, since PT flat slabs are slightly less thick
than RC flat slabs, the seismic mass is lesser and the
seismic demand on the system reduces.
Quote:

4. For a typical multistoried building, What would be
the time saving with Flat Slab? Is there any cost
saving also in case one goes for Flat Slab
construction? Specific case study, if done for Indian
projects may be shared by experts in the field.
Quote:

Construction companies/consultants could come out
with cost-cmparison studies that they would have.
Quote:

5. What are the pre-requisites for Flat Slab
Construction, in terms of geometry of the multistoried
building design that the architects must know. Is there
any documented guideline in this regard?
Quote:

There are no rigid pre-requisites as per codes. It is
preferable to have regular column disposition so that
if one wants to use a equivalent frame method (EFM) to
cross check his finite element method (FEM), it would
be simpler. But column positioning is many a times
usurped by architects! Just like it is better to have a
symmetrical building in terms of geometry and mass from
seismic resistance criteria - but how many architects
care? A typical reply - "you can't curtail my freedom
for creative designs!" I have suggested in my mails in
this and other sections some useful articles to read.
Quote:

6. Upto what span length, RCC Flat Slab is economical,
beyond which one should go for PT Slabs in India. What
is the maximum span length upto which one can go for
Flat Slab with PT.
Quote:

Answered under question 2 above.
As for maximum span, it is a function of loading,
number of bays, the thickness that one is ready to
provide and so on. The company where I worked had
provided 12m to 14m spans in many buildings. When
moments become difficult to handle, we would provide PT
band beams between the columns
Quote:

7. For a PT Flat Slab, what are the various methods
of post tensioning which are available in India? Any
guideline?
Quote:

In India is invariably bonded post-tensioning that is
adopted. A comparison between bonded and unbonded PT is
given in an article of mine which I had written a few
years ago and have appended it with this mail.
Quote:
As for guidelines, many are available abroad (on the
internet as well) and proposed IS 1343 (a draft is
enclosed with one of my replies in this E-conf) also
attempts to give some guidelines.
Quote:

8. What is the choice of design methods for Flat Slab
Construction?
Quote:

Please see my article appended.

9. I am aware that there are softwares (like ADAPT),
which are available for design of Flat Slab. Are these
softwares applicable for Indian Projects, where one has
to follow the Indian codes?
Quote:

There are many softwares with IS code modules fitted
in.
Quote:

10. How important is the construction stage check for
Flat Slab? Rapid construction, I believe can lead to
loading of slab at a time when it has not gained the
full strength! Is it a practice to carry out
construction stage check and is it a governing load
case ?
Quote:

Yes. In PT slabs, the shutters are removed
immediately upon stressing, which is about 3-4 days.
Although it is re-propped, many a times the props are
inadequate. We must make sure that the props are
adequate and stay put for the required number of days
or else do a construction stage analysis. This can be
critical since there is chance of the live load to dead
load ratio being lesser in long-span flat slabs. Also
since the speed is so high (one slab every 6-10 days)
the second slab below would receive loads from two
floors higher and if not supported, could fail.
Quote:

Hope this answers the curiosity to some extent!

regards

Kalgal







Attachments:
PT in Building Sector.pdf (http://www.sefindia.org/for-
um/files/pt_in_building_sector_839.pdf)








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