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Post tensioned slabs
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kajal.chopra
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PostPosted: Wed Dec 24, 2008 11:23 am    Post subject: Reply with quote

Yes, indeed.

But do you personally think this 106KN can be not considered?

It can cause sufficient bending in column?
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gorhemant
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PostPosted: Wed Dec 24, 2008 11:24 am    Post subject: Post tensioned slabs Reply with quote

we are transferring compressive force to concrete slab. The stresses in slab due to prestress are of reversed nature compared to one due to dead/live load.
The standard document on PT slab design TR43 (technical report 43) recommends minimum average effective prestress of 0.7MPa. It means if P is effective forces havving tributary width of B and slab thickness D ( P/(BD) shall not be less than 0.7MPa. The PT slabs are designed as Partial Prestressed Element.
PT in slab can be achieved through
Bonded Tendon
Unbonded Tendon
In India and Middle east region only Bonded system is widely used for PT slabs. The ultimate tensile strength (Pu) of 0.5inch strand is 18.3tonn. Each strand is stressed to level of 0.75Pu and after all losses the effective force in strand is in the range of 0.6Pu ( considering 80% loss of Prestress force).
If average effective stress in slab exceeds 2.5MPa limits the restraint effect due to columns/shear Wall / core needs to be considered ( Point 1.4 of TR43)

Hemant Gor
M.Tech (Structure)
Post Tension Design Specialist


On Wed, Dec 24, 2008 at 8:19 AM, kajal.chopra <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
  My only concern is:

Let us say a tendon passes through the olumn.

Now imagine the free body of the column.

In its free body, there would be a huge concentrated force acting at the location where the tendon passes, right?

Thus, this single concentrated force can cause enormous bending in the column irrespective if the stiffness of column is high/low)

Please anyone - throw some light on this issue.







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Kedar
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PostPosted: Wed Dec 24, 2008 11:30 am    Post subject: Reply with quote

Hi Kajal


In my opinion this does not cause
much structural changes.


So NO need to consider BM in
column due to shear



since

1.Shear in column=106kN
2.BM in column = 106 x 3 = 318kN.m
assuming cantilever action


The kind of column you proposed
1000 x 1000 are quite healthy in size.
They can easily take this BM (318 kN.m).


Still it is at your discretion
to let or not-to-let


regards
Kedar
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mkalgal
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PostPosted: Wed Dec 24, 2008 7:01 pm    Post subject: Reply with quote

Dear All

I am no expert on softwares which can analyse the effect of gravity loads along with effects of lateral loads, especially when prestressed.
I am recommending two articles written by Bijan Aalami, a renowned authority for you to read
http://www.adaptsoft.com/resources/ADAPT_P201_Nonpre-Bonded-PT-Bldg-Dsgnr.pdf
http://www.adaptsoft.com/resources/Structural_Modeling_Slabs_CI_Dec2005.pdf
I will try to suggest some more downloadable articles(so that copyrights are not violated)

regards

Kalgal
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kajal.chopra
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PostPosted: Thu Dec 25, 2008 12:50 am    Post subject: Reply with quote

Thanks Kedar.The discussion was indeed useful
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bsec
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PostPosted: Thu Dec 25, 2008 6:20 am    Post subject: Post tensioned slabs Reply with quote

Dear All,  

I am closely following the postings on Flat Slabs in the e-conference. I have never designed a Flat Slab in my life after passing out of the college, and therefore can not contribute much technically in the recent discussions. However, as a structural engineer, I have few technical querries which I would request the experts to clarify :  

1. What are the key benefits of Flat Slab construction vis--vis a normal beam and slab construction ?
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?
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?).  
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.  
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?
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.  
7. For a PT Flat Slab, what are the various methods of post tensioning which are available in India? Any guideline?
8. What is the choice of design methods for Flat Slab Construction?  
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?  
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 ?
11. How does our present codal provision for Flat Slabs compares with the International Practice? Which International Code is most advanced for Flat Slab design and detail?

With best wishes to all

Alok Bhowmick


-----Original Message-----
From: kajal.chopra [mailto:forum@sefindia.org]
Sent: Monday, December 22, 2008 4:03 PM
To: econf@sefindia.org
Subject: [ECONF] Re: Post tensioned slabs

Thanks for the reply.

You said , in a ductile detailed column, there is not enough space to accomodate the width of (say) 70 mm.

Let us there is a provision to accomodate that width of 70mm(supposing),
then if these tendons do pass through the column would not that high force exerted on that location of the column by the tendon cause column to bend heavily?

Would not this be detrimental for the column?Would not this effect be avoided by not passing the tendon through the column (assuming there is sufficient space for the duct to pass)?

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dipakdgaikwad
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PostPosted: Thu Dec 25, 2008 7:04 am    Post subject: TEST Post tensioned slabs Reply with quote

Dear Kedar

               I personally appreciate  Very Happy you for addressing the issue with

simple and self explanatory calculations which help me to understand the

real issue.

               Keep it up!

Very Happy  Very Happy  Very Happy  Very Happy  Very Happy  Very Happy  
Thanks And Regards
Dipak D Gaikwad
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mkalgal
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PostPosted: Thu Dec 25, 2008 11:13 am    Post subject: Reply with quote

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



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PostPosted: Thu Dec 25, 2008 11:49 am    Post subject: Post tensioned slabs Reply with quote

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
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







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dipakdgaikwad
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PostPosted: Fri Dec 26, 2008 6:21 am    Post subject: Post tensioned slabs Reply with quote

    Dear KAJAL/KEDAR
              I have not designed any PT slab but I have observed from PT consultants drg. that the profile of tendon on either side of columns normally (span/6) is kept constant. (See the attached drawing)
               Thus the vertical component will not exist on either side of column. And the vertical component on remaining part of span will be balanced by DL (Which is meant for that only).
                So I think there will not be any moment in column due to vertical dip of tendon passing through it.
               Correct me if I am wrong. Confused

Thanks and Regards
Dipak D Gaikwad    


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