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proposed draft clause for PT/non-PT flat slabs in IS 1893
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Jalil A. Sheikh
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PostPosted: Tue Dec 30, 2008 12:30 pm    Post subject: proposed draft clause for PT/non-PT flat slabs in IS 1893 Reply with quote

DEAR VIPUL AHUJA

IF WE CONSIDER AN INDIPENDENT LATERAL FORCE SYSTEM LIKE SHEAR WALL AND CONSIDER "R" VALUE BASED ON DUCTILITY OF THIS COMPONENT (SHEAR WALL), WE HAVE TO ADHERE MIN. STIRRUPS REQUIREMENTS GIVEN IN DUCTILITY CODE FOR COLUMNS & BEAMS? WHAT TO DO FOR THE FRAME WHICH WILL DEFORM AS MUCH AS SHEAR WALL ALLOW (UPTO THE ALLOWABLE DUCTILITY).

JALIL A. SHEIKH

--- On Tue, 30/12/08, ahujavipul <forum@sefindia.org> wrote:

Quote:
From: ahujavipul <forum@sefindia.org>
Subject: [ECONF] Re: proposed draft clause for PT/non-PT flat slabs in IS 1893
To: econf@sefindia.org
Date: Tuesday, 30 December, 2008, 4:43 AM

Dear Deepak,

As Vijaya has said this is a theoretical condition, but even if it was considered, and an independent lateral force resisting system such as shear walls is adopted, you would simply consider "R" values based on ductility of components rather than of the whole building as one--ie to comply with the deformation compatibility clause. Remember, here the frame (whether beam-column frame or flat slab-column ) is not being designed to resist lateral forces, but only to deform as much as the shear walls allow (up to the allowable ductility).

Best wishes

Vipul Ahuja






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Vijayanarayanan
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PostPosted: Tue Dec 30, 2008 1:52 pm    Post subject: Re: proposed draft clause for PT/non-PT flat slabs in IS 189 Reply with quote

Respected JALIL A. SHEIKH ji,

Yes, it is required to design a lateral resiting frame to cater to the seismic demand and the flat slab are designed to resist the lateral drift and the gravity load alone.

When the lateral resisting frame undergoes lateral drift it is bound to induce the same amount of drift on the flat slab and the supporting column. It is important to note that under this drift the column should not fail. For this reason one has to go in for ductile detailing of the column no matter whether we assume the total lateral load to be taken by the shear wall or not.

One has to proportion the stiffness of the shear wall to such an extent that the deformation induced in the column, slab, slab column connection to be as minimum as possible.

Vijaya narayanan,
MTech student,
IIT Kanpur.

Jalil A. Sheikh wrote:
DEAR VIPUL AHUJA

IF WE CONSIDER AN INDIPENDENT LATERAL FORCE SYSTEM LIKE SHEAR WALL AND CONSIDER "R" VALUE BASED ON DUCTILITY OF THIS COMPONENT (SHEAR WALL), WE HAVE TO ADHERE MIN. STIRRUPS REQUIREMENTS GIVEN IN DUCTILITY CODE FOR COLUMNS & BEAMS? WHAT TO DO FOR THE FRAME WHICH WILL DEFORM AS MUCH AS SHEAR WALL ALLOW (UPTO THE ALLOWABLE DUCTILITY).

JALIL A. SHEIKH

--- On Tue, 30/12/08, ahujavipul <forum> wrote:

Quote:
From: ahujavipul <forum>
Subject: [ECONF] Re: proposed draft clause for PT/non-PT flat slabs in IS 1893
To: econf@sefindia.org
Date: Tuesday, 30 December, 2008, 4:43 AM

Dear Deepak,

As Vijaya has said this is a theoretical condition, but even if it was considered, and an independent lateral force resisting system such as shear walls is adopted, you would simply consider "R" values based on ductility of components rather than of the whole building as one--ie to comply with the deformation compatibility clause. Remember, here the frame (whether beam-column frame or flat slab-column ) is not being designed to resist lateral forces, but only to deform as much as the shear walls allow (up to the allowable ductility).

Best wishes

Vipul Ahuja






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ahujavipul
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PostPosted: Tue Dec 30, 2008 7:40 pm    Post subject: proposed draft clause for PT/non-PT flat slabs in IS 1893 Reply with quote

If you look up ASCE-41-06 (or the older version FEMA 356) you will find ductility factors ("m" ie R factors) for various components such as beams, columns, flat-slab/column connections under varying details of ductile detailing. For example a column with or without ductle detailing will have different "m" (or R) factors. It also depends on the axial stress on the column. So if you have limited the drift (based on cracked section) to less than the graph shown in ACI chapter 21 (or 0.005 as mentioned in the proposed draft), you need not worry about the flat slab-column connection.

Theoretically you are free to detail the column as you wish--with or without ductility, so long as you use the proper ductility factor associated with it. For example a component may have a R=3 for ductile detailing but R=1 for non-ductile detailing. However I feel it would not be prudent to design columns in a PT flat slab system in a non-ductile way, especially in Zones IV & V (and perhaps for important structures in Zone III).

I have seen some comments stating the shear walls should be designed for 75% and PT flat slab 25%. I wish to emphasize that this would not be in line with the thought process of the code section being proposed--nor in line with the existing code.

Best wishes

Vipul Ahuja
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dcrai
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PostPosted: Wed Dec 31, 2008 2:51 pm    Post subject: proposed draft clause for PT/non-PT flat slabs in IS 1893 Reply with quote

Dear Colleagues,

For buildings in high seismic regions, the current US practice is that
the flat slab systems will cater to the needs of gravity loads alone and
the entire seismic load will be resisted by the ‘dedicated’ lateral
force resisting system. Such a flat slab system is generally referred as
“Non-participating/ Non-seismic” frames. Even though the flat slab are
designed to resist the gravity load only, checks have to be performed to
prevent ‘overstressing’ of slab column connection due to lateral drift.

Over the last decade, much of the research effort was directed to
establish a relation between the lateral drift ratio and the increased
shear stress induced by it on the critical section of slab column
connection. As the shear induced by the unbalanced moment cannot be
accurately determined, the approach has been to restrict the gravity
load induced shear and thereby letting the available unused shear
capacity to be utilized during seismic actions.

Such a proposal before ACI for RC and PT can be summarised as follows:

1) For gravity shear ratio of 0.6 and greater, the maximum allowable
drift (without special shear reinforcement)

for RC = 0.005

for PT = 0.015

2) For gravity shear ratio in the range 0 to 0.6, the maximum allowable
drift (without special shear reinforcement),

Decreases linearly for RC from 0.035 to 0.005

Decreases linearly for PT form 0.045 to 0.015

The above proposal supported by the experimental evidence indicates that
the PT slab is better than RC in resisting additional shear under
seismic conditions. One may ask how relevant the above proposal is for
kinds of flat slabs that are being constructed in India. In other words,
do we know what percentage of such flat slabs would require special type
of shear reinforcement to satisfy the above criteria in high seismic
zones, say in Zones IV and V?

Wishing you all a very happy and successful New Year,
and hoping to hear more from you on the above and other flat slabs
issues in 2009!

Moderators



ahujavipul wrote:
Quote:
If you look up ASCE-41-06 (or the older version FEMA 356) you will
find ductility factors ("m" ie R factors) for various components such
as beams, columns, flat-slab/column connections under varying details
of ductile detailing. For example a column with or without ductle
detailing will have different "m" (or R) factors. It also depends on
the axial stress on the column. So if you have limited the drift
(based on cracked section) to less than the graph shown in ACI chapter
21 (or 0.005 as mentioned in the proposed draft), you need not worry
about the flat slab-column connection.

Theoretically you are free to detail the column as you wish--with or
without ductility, so long as you use the proper ductility factor
associated with it. For example a component may have a R=3 for ductile
detailing but R=1 for non-ductile detailing. However I feel it would
not be prudent to design columns in a PT flat slab system in a
non-ductile way, especially in Zones IV & V (and perhaps for important
structures in Zone III).

I have seen some comments stating the shear walls should be designed
for 75% and PT flat slab 25%. I wish to emphasize that this would not
be in line with the thought process of the code section being
proposed--nor in line with the existing code.

Best wishes

Vipul Ahuja






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vipul_ahuja
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PostPosted: Thu Jan 01, 2009 9:20 am    Post subject: proposed draft clause for PT/non-PT flat slabs in IS 1893 Reply with quote

I am posting my paper (submitted at the ICI World Conference, Greater Noida, 2008)  "POST TENSION FLAT SLABS—EARTHQUAKE ENGINEERING & MISC ASPECTS IN HIGH SEISMIC ZONES IN THE INDIA ENVIRONMENT" here for reference.

Happy New Year to everyone.

Vipul Ahuja



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vipul_ahuja
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PostPosted: Thu Jan 01, 2009 9:39 am    Post subject: proposed draft clause for PT/non-PT flat slabs in IS 1893 Reply with quote

In response to the comment from the Moderators:

Practically for most routine buildings with shear walls we find the the drift is almost always less than .002 (ie half of the current requirement--0.004 of IS 1893). So with cracked sections a building with well planned shear walls, the drift ratio it is unlikely to exceed 0.004 and probably never 0.005.

Also see my paper posted separately highlighting quality control problems with PT construction in India. Research may be for ideal conditions where all of the intended PT force does go into the slab. I feel for Indian conditions a single drift limit for both PT & non-PT slabs (0.005) is prudent and makes the code simpler to use--which is why I put it in the proposed draft.

Warm regards & a Happy New Year

Vipul Ahuja
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dipakdgaikwad
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PostPosted: Fri Jan 02, 2009 9:11 am    Post subject: Re: proposed draft clause for PT/non-PT flat slabs in IS 189 Reply with quote

Dear JALIL A. SHEIKH & All
Place see the "Clause-11 of Draft IS 13920"[available on NICEE website]
It is giving three different ductile detailing requirements for gravity column according to Induced moments and shear under latter displacement (calculated for R=2) combined with factored gravity moment
comparing with design moment and shear strength of frame.

Thanks and Regards
Dipak D Gaikwad
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ahujavipul
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PostPosted: Sun Jan 04, 2009 3:09 am    Post subject: proposed draft clause for PT/non-PT flat slabs in IS 1893 Reply with quote

Dear Dipak

The issue here is design loads (IS 1893)--what you are talking about is design (IS 13920).Though some reconciliation is required between the proposed IS13920 & IS 1893 proposal, I have now posted the PT paper as previously discussed for the rationale.

Thanks and Regards

Vipul Ahuja
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dipakdgaikwad
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PostPosted: Sat Jan 10, 2009 10:18 am    Post subject: Flat slab highrise building analysis for WIND Load Reply with quote

(Flat slab+Corewall+Gravity Column) highrise building analysis for WIND Load.

Any specially care like seismic required?

Thanks And Regards
Dipak D Gaikwad
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vipul_ahuja
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PostPosted: Tue Jan 13, 2009 1:47 pm    Post subject: Flat slab highrise building analysis for WIND Load Reply with quote

Dear Dipak,

As already outlined in my paper, an independent lateral force system should always be provided for PT flat slabs--even if wind controls design.

Though you may look up chapter 21 of ACI-2005 or later versions that there is a concept of "intermediate frames", ie when the flat slab column system is designed to take up the lateral forces.  Many severe conditions have been put when using this approach including limiting the gravity punching shear ratio to low levels (0.4).

Also when comparing wind and earthquake forces, do not simply compare wind & EQ with R=4. Remember, you would need to compare with R=3 & even other lesser values for other components. It can be that some components are wind controlled while others are earthquake controlled.

Thanks & regards,

Vipul Ahuja
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