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Post tensioned slabs
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kajal.chopra
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PostPosted: Fri Dec 26, 2008 6:57 am    Post subject: Reply with quote

Hi Dipak/Kedar,

Theoretically speaking, the idea of varying the profile of the tendon on either side of the column (or over the full span) is to counteract the bending moments due to external loading.

Now, near the columns wherein one would normally have hogging bending moments we have the tendon lying above the centroidal axis of the slab.

Again, theoretically speaking, if one keeps the tendon profile constant on either side of the column, there is a fair chance of the slab sagging down in this region due to prestressing effect if the bending moment due to external loading is not sufficient enough.

In the drawing, I have attached which is from a worked example in the British code- Technical Report 43, we have the tendon profile varying on either side of the tendon too.

Theoretically speaking, this is how it should be.

Kedar/Dipak please let us all know if there is any practical reason  to keep the  tendon on either side of the column staright as in the drawing sent by Kedar?

Ajay



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nitinengr
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PostPosted: Fri Dec 26, 2008 7:52 am    Post subject: Regarding Tendon effect passing through columns Reply with quote

Dear All,

As per my experince in the design,

I can recommmand that atleast two tendons should pass through the columns also this is specify in the indian code also.

I think we shuold not forgot the Vertical component of the Tendons which helps in the punching shear also.

Also in the design if you see the bending moment is not affected much.

Thanks
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Kedar
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PostPosted: Fri Dec 26, 2008 2:22 pm    Post subject: Reply with quote

@Kajal
@Deepak

Thnx dear !

regards
Kedar
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Kedar
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PostPosted: Fri Dec 26, 2008 3:24 pm    Post subject: Reply with quote

Hi Kajal/Deepak


@ Deepak
The moment in column will be created if there
is eccentricity of tendon(s).


@Kajal


For the asymmetric profile of tendons over the end-to-end span
one need to check the,Plz note that there can be local asymmetry as compared to end-to-end span


1. Jacking arrangement.
2. Sequence of jacking the tendons.
3. Magnitude of tension applied during sequence.


Depending upon the LL application sequence the jacking of tendons needs to be controlled for PT structural elements.


so if one knows the utility of the slab one can find the why tendons are asymmetrical possibly.


Guys Plz correct me if I am wrong.

regards
Kedar
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PRADEEP GARG
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PostPosted: Sun Dec 28, 2008 2:06 pm    Post subject: Modelling of Flat Slab- Column Str. in STAAD/E-TAB Reply with 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

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?

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!)
c. You can have floor height windows since there are no beams
d. Since Shuttering is simpler, it takes less time to provide and remove shuttering (mentioned in your question 3).
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?

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.
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.

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?).

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.
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.
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.

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.

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

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?

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.

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.

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

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

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.
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.

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

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?

There are many softwares with IS code modules fitted in.

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 ?

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.

Hope this answers the curiosity to some extent!

regards

Kalgal







Attachments:
PT in Building Sector.pdf








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vikram.jeet
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PostPosted: Mon Dec 29, 2008 9:10 am    Post subject: [ECONF] Re: Post tensioned slabs Reply with quote

Explanations through simplified illustrations as given by Er Kidar
are really praisewothy as understanding provided to the concept is  
very clear

The answers to queries on flat slab concept by Mr Kalgal shows  
his excellent working experience on the designing of such structures
and definately deserve high regards. He being one of the moderators
of this econf, fully deserve the status

regards

vikramjeet

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kajal.chopra
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PostPosted: Tue Dec 30, 2008 3:52 am    Post subject: To Mr Kedar Reply with quote

Hello Kedar,

All calculations that you have provided stand valid when the slab and column act monotlithically.Fine.

Sorry, am here with a very basic question.Are there not cases when the slab and column are not monolithically cast?

Please help!
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Kedar
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PostPosted: Tue Dec 30, 2008 7:19 am    Post subject: Reply with quote

Hi Kajal



The kind of situation you are proposing
is almost difficult to exist.


bcoz


1.If there is no slab n column connection
then where the tendons are passing then?



I never saw tendon are hanging in air.}<-------------Extreme assumption



2.If at all slab and column are connected via }<-----Moderate assumption
pseudo joint definitely the forces needs to be}
considered on column as a routine structural
analysis &  design procedure.Which we have
iterated in our earlier postings.


As an amendment to earlier posted explanatory example
The moment in coulum will be due to


1.Moment due to Horizontal component or shear
= 13.13 x 4.2 =55.5 kN.m


(Assuming cantilever action,plz note that this would be the extreme
assumption resulting in to max. moment,since in actual there would
combined deformation of slab and column hence full cantilever action may not exist what we are assuming here,but for the sake of quick calculations and conservative estimation of forces,we can definitely take this liberty to assume from practical work-out point of view)


2.Moment due to Vertical component
=15.0 kN.m (estimated earlier)


Hence total moment on column
=55.5+15.0
=70.5 kN.m


Plz note that this magnitude is low for all practical purpose
as compared to size of column we have considered
i.e. 900 x 900


I feel this also applicable to example you posted earlier too.



Plz correct me if I am mistaken.



regards
Kedar
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kajal.chopra
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PostPosted: Tue Dec 30, 2008 8:34 am    Post subject: Reply with quote

Hi Kedar,

I meant that the slab is cast in a separate operation and the columns are cast in another separate operation (which could happen?)

Then, the forces between the slab and column need not be in proportion to stifnesses.Remeber, in moment distribution we distribute the forces between members meeting at a joint ASSUMING that the two members are monolithically cast.

I am saying that the slab and column may not be monolithically cast but may be cast in a sepaarte operation.

Please correct me wherever i am wrong.
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Kedar
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PostPosted: Tue Dec 30, 2008 9:03 am    Post subject: Reply with quote

Hi Kajal


Plz read point No. 2 in previous posting.


regards
Kedar
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