www.sefindia.org

STRUCTURAL ENGINEERING FORUM OF INDIA [SEFI]

 Forum SubscriptionsSubscriptions DigestDigest Preferences   FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups  RegisterRegister FAQSecurity Tips FAQDonate
 ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log in to websiteLog in to websiteLog in to websiteLog in to forum 
Warning: Make sure you scan the downloaded attachment with updated antivirus tools  before opening them. They may contain viruses.
Use online scanners
here and here to upload downloaded attachment to check for safety.

proposed draft clause for PT/non-PT flat slabs in IS 1893
Goto page 1, 2  Next
 
This forum is locked: you cannot post, reply to, or edit topics.This topic is locked: you cannot edit posts or make replies. Thank Post    www.sefindia.org Forum Index -> E-Conferences-2008 [ Flat Slab Design issues ]
View previous topic :: View next topic  
Author Message
ahujavipul
General Sponsor
General Sponsor


Joined: 26 Jan 2003
Posts: 230

PostPosted: Mon Dec 22, 2008 9:35 am    Post subject: proposed draft clause for PT/non-PT flat slabs in IS 1893 Reply with quote

I have written a paper in the recently concluded ICI world Conference held in Delhi (Greater Noida) recently. It was on PT flat slabs--earthquake engineering and other aspects. I will post the paper on SEFI if I can get assurance from ICI that it wont be a copyright infringement. None the less I was asked to write up a first draft based on that paper, which is as follows (comments are of course welcome):

7.11.2.A Implementation of deformation Compatibility for Design of RCC Building with Flat Slabs.

Flat slabs and column structures shall not be considered as frames in resisting the lateral forces except as mentioned in this clause.

Flat slab structures shall be designed such that shear walls take 100% of the lateral forces. For this condition the analysis shall consider columns with moment releases top and bottom at all levels. The response reduction factor for this shear wall & foundation analysis shall be taken as specified in Table 7 for ductile shear walls (without dual systems) and shall be designed with ductile details (for shear walls, coupling beams & all foundations) conforming with the requirements of IS 13920.

Columns and incidental beams shall be analysed separately in another analysis with R=3 with no moment releases for columns and all gross section properties with ductile details conforming with the requirements of IS 13920.

For verifying whether special shear reinforcing is required at the column slab joint to take into account deformation compatibility as mentioned in 7.11.2 the entire structure shall be reanalyzed for the cracked sections with R=3, but with cracked sections (usually taken as 0.5 E as an approximation). If the inter storey drift is within limit of 0.005, the flat slab and its connection with the column need not be specially design for moment or shear transfer on account of lateral seismic forces.

If the storey drift is more than as mentioned above, the structures shall be reanalyzed for R=2 (with gross sections), and the moment in the column shall be used to design the flat slab and the slab column connection. Further for columns & slab-connections that are stressed
at over 60% of gravity load carrying capacity, R shall be taken as 1 for their respective designs. However the design moment of the slab-column connection shall not be less than the residual moment capacity of the column.

Commentary:

The approach in analyzing for various levels of R factor stems from the understanding that various elements can take different amounts of rotations in their inelastic state. This depends on axial load levels for columns, state of punching shear stress for slab column connections etc. However as a simplification for the designer, single values of R for all states of stress have been adopted. This approximation has a limitation that it could be unconservative for high states of stress (above 60 % of gravity load capacity). In such cases designer may adopt R=1 for design of columns & connection as suggested by 7.11.2.  Reference has been made to ASCE-41-03 for estimating rotation capacity of various components and to ACI-318 (2005) for drift requirements and hence recommending the values of R factor.

Regards

Vipul Ahuja


Last edited by ahujavipul on Tue Dec 23, 2008 7:58 am; edited 1 time in total
Back to top
View user's profile Send private message
Thankful People
1 user(s) is/are thankful for this post.
dipakdgaikwad
...
...


Joined: 01 Apr 2008
Posts: 77
Location: Mumbai

PostPosted: Mon Dec 22, 2008 11:38 am    Post subject: proposed draft clause for PT/non-PT flat slabs in IS 1893 Reply with quote

Dear Sir
In your earlier mail you says:
Columns and incidental beams shall be analysed separately in another analysis with R=3 with no moment releases for columns and all gross section properties with ductile details conforming with the requirements of IS 13920.
Why ductile detailing if you are using R=3 which is for "Ordinary RC moment-resisting fram".

Thanks And Regards
Dipak D Gaikwad
Back to top
View user's profile Send private message
ahujavipul
General Sponsor
General Sponsor


Joined: 26 Jan 2003
Posts: 230

PostPosted: Mon Dec 22, 2008 5:46 pm    Post subject: proposed draft clause for PT/non-PT flat slabs in IS 1893 Reply with quote

Please read the commentary at the end of the proposed code section. Also the "R" factors used here are really from the point of view of the ductility of the component rather than of the whole structure. Some components that have been determined to be less ductile will have a lesser "R" than ones having larger ductilty.

Regards

Vipul Ahuja
Back to top
View user's profile Send private message
Vijayanarayanan
SEFI Member
SEFI Member


Joined: 01 Oct 2008
Posts: 16

PostPosted: Tue Dec 23, 2008 7:49 am    Post subject: Re: proposed draft clause for PT/non-PT flat slabs in IS 189 Reply with quote

Respected Sir,

Provision of shear wall to a flat slab system would reduce the deflection to a considerable level. this primarily due to the high in plane stiffness of the shear wall. When a higher R (5) value of the shear wall is considered it is rather implied the deflection associated with it would be large compared to the columns which have lower value of R(3). Since the drift associated with the column, having the value of R=3, is less the effect of drift associated punching would decrease.

The connection between the shear wall and the slab would largely depend upon the transfer of forces from the slab to the wall, this becomes very critical with excess drift that would be associated with the increased ductility of the wall (At least at the vicinity of the connection where the forces are to be transferred). It would actually boil down to how far we are detailing the reinforcement in the slab wall connection. It is indeed we do a check for the slab column connection after the preliminary analysis, which in this case I feel that would not be the critical case. I Personally feel that the entire problem is being shifted form the slab column connection to the shear wall slab connection. In the event of improper detail of the slab wall connection the entire structure would collapse immediately, as the columns cannot resist the lateral load induced. Compared to the localized failure of the slab column connection (Assuming that Integrity reinforcement are placed as per ACI 352.1R-89).  

For this reason I personally feel that there should be a clause which would help the designers the way and means by which the slab and the shear wall can be connected.

If more than 60% of the shear capacity of the slab is used by the gravity load, is it absolutely necessary to have a response reduction factor of 1?? The design load may turn out to be way too high.
Cant we just improve the connections shear capacity by providing shear reinforcement??? If we increase with the shear capacity of the section can we do away with the last statement of the proposed clause??

I may be wrong in understanding the concept put forth too. If I am could you please correct me sir?.

thanking you,
Vijaya Narayanan,
MTech student,
IIT Kanpur.



ahujavipul wrote:
have written a paper in the recently concluded ICI world Conference held in Delhi (Greater Noida) recently. It was on PT flat slabs--earthquake engineering and other aspects. I will post the paper on SEFI if I can get assurance from ICI that it wont be a copyright infringement. None the less I was asked to write up a first draft based on that paper, which is as follows (comments are of course welcome):

7.11.2.A Implementation of deformation Compatibility for Design of RCC Building with Flat Slabs.

Flat slabs and column structures shall not be considered as frames in resisting the lateral forces except as mentioned in this clause.

Flat slab structures shall be designed such that shear walls take 100% of the lateral forces. For this condition the analysis shall consider columns with moment releases top and bottom at all levels. The response reduction factor for this shear wall & foundation analysis shall be taken as specified in the current version of IS 1893 for ductile shear walls and shall be designed with ductile details (for shear walls, coupling beams & all foundations) conforming with the requirements of IS 13920.

Columns and incidental beams shall be analysed separately in another analysis with R=3 with no moment releases for columns and all gross section properties with ductile details conforming with the requirements of IS 13920.

For verifying whether special shear reinforcing is required at the column slab joint to take into account deformation compatibility as mentioned in 7.11.2 the entire structure shall be reanalyzed for the cracked sections with R=3, but with cracked sections (usually taken as 0.5 E as an approximation). If the inter storey drift is within limit of 0.005, the flat slab and its connection with the column need not be specially design for moment or shear transfer on account of lateral seismic forces.

If the storey drift is more than as mentioned above, the structures shall be reanalyzed for R=2, and the moment in the column shall be used to design the flat slab and the slab column connection. Further for columns & slab-connections that are stressed at over 60% of gravity load carrying capacity, R shall be taken as 1 for their respective designs. However the design moment of the slab-column connection shall not be less than the residual moment capacity of the column.

Commentary:

The approach in analyzing for various levels of R factor stems from the understanding that various elements can take different amounts of rotations in the inelastic state. This depends on axial load levels for columns, state of punching shear stress for slab column connections etc. However as a simplification for the designer, single values of R for all states of stress have been adopted. This approximation has a limitation that it could be unconservative for high states of stress (above 60 % of gravity load capacity). In such cases designer may adopt R=1 for design of columns & connection as suggested by 7.11.2. Reference has been made to ASCE-41-03 for estimating rotation capacity of various components and to ACI-318 (2005) for drift requirements.

Regards

Vipul Ahuja
Back to top
View user's profile Send private message
ahujavipul
General Sponsor
General Sponsor


Joined: 26 Jan 2003
Posts: 230

PostPosted: Tue Dec 23, 2008 1:56 pm    Post subject: proposed draft clause for PT/non-PT flat slabs in IS 1893 Reply with quote

What I am trying to do is get away from the literal interpretation of 7.11.2 which states that in order to enforce deformation compatibility we need to design for R times the displacement ie R=1!! So I have tried to rationalise this by saying that the components that can stay ductile to that level of displacement need not be designed for R=1, but something lesser (ie larger R).

You do have a point about the connection of shear wall to slab. This is a shear connection and probably has very little ductility involved. I have not seen much literature/test info  to comment on the R value of this component. Conventionally these connections have been designed for the R value taken for the structure. Also you may well find that if indeed the shear walls are sparse, then only these stresses will be high.

I hope you will also appreciate for high values of stress columns & slab column connections can also also have very limited ductility and so R=1 is justified for those components under the severe conditions--and not for all conditions as IS-1893 suggests.

ACI-318 shows a plot between state of punching shear due to gravity & storey drift ratio. Points above the line require reinforcing, points below the line don't. The 0.005 drift number is from there. This & other issues are addressed in the ICI paper referenced. I thought some of the detailing issues belong to IS 13920 or IS 456. It may be noted that stud rails are still unavailable in India; conventional stirrups have been shown to be ineffective in slabs thinner than 250 mm

Thanks & regards

Vipul Ahuja
Back to top
View user's profile Send private message
Vijayanarayanan
SEFI Member
SEFI Member


Joined: 01 Oct 2008
Posts: 16

PostPosted: Thu Dec 25, 2008 10:12 am    Post subject: Re: proposed draft clause for PT/non-PT flat slabs in IS 189 Reply with quote

Respected Sir,

Thank you for the detailed reply.

I do believe that the various components of the flat slab system can be designed for higher level of ductility. Going in for R=1 for all the components, apart form the connections, would be disastrous economically. Moreover the weight of the entire structure would increase if we design the structure for a value of R=1, this again would attract large seismic force than the structure which has been designed for a R=2 or 3.

But I do feel all the connections involved, slab column connections and the slab wall connections, has to be designed for R=1, irrespective of whether the earthquake probability is sever or not. The reason behind this suggestion is that when we allow certain level of ductility, which ofcourse is very difficult in case of the type of connections involved in flat slab structures, could induce certain level of damage to the connection. This damage could be very critical in the load carrying capacity of the connection. Disruption of the load path could lead to localized failure too.

I am unable to comprehend why such a popular type of shear reinforcement is unavailable in India. Using slab drop is no solution for drift associated punching. Yes,it is possible to completely eliminate the problem of drift associated punching by providing enough lateral resisting system to control the drift to around 0.005. But I am pretty sure such a solution would rater turn out to be a costly affair.

Is there any other type of shear reinforcement that are being used in India?? (Apart form stirrups)

The graph drift ratio Vs gravity shear present in ACI 318(08 ), consist of results form data set collected form the RC specimens only. Further research PT slab were carried out by Thomas.H.K.Kang and a separate plot of Drift ratio Vs gravity shear has been presented in one of his paper. It is interesting to note that the tolerance limit of drift for which no shear reinforcement needs to be provided, for any level shear capacity already used up by gravity shear , is as high as 0.015. The paper, for reference, is available in Concrete International, April 2007 edition, with the title "Post tensioned flat slab column connection".

The result form this particular paper makes me wonder whether PT Flat slabs are better off than RC Flat slab when it comes to punching shear under lateral drift??

Finally, could you post your article form the ICI journal it would help understand the problem better.

Vijaya Narayanan
MTech Student
IIT Kanpur

ahujavipul wrote:

I hope you will also appreciate for high values of stress columns & slab column connections can also also have very limited ductility and so R=1 is justified for those components under the severe conditions--and not for all conditions as IS-1893 suggests.

ACI-318 shows a plot between state of punching shear due to gravity & storey drift ratio. Points above the line require reinforcing, points below the line don't. The 0.005 drift number is from there. This & other issues are addressed in the ICI paper referenced. I thought some of the detailing issues belong to IS 13920 or IS 456. It may be noted that stud rails are still unavailable in India; conventional stirrups have been shown to be ineffective in slabs thinner than 250 mm


Thanks & regards

Vipul Ahuja
Back to top
View user's profile Send private message
ahujavipul
General Sponsor
General Sponsor


Joined: 26 Jan 2003
Posts: 230

PostPosted: Thu Dec 25, 2008 11:06 am    Post subject: proposed draft clause for PT/non-PT flat slabs in IS 1893 Reply with quote

Dear Vijaya,

I think shear walls are probably the most effective way to limit the drift and hence the punching shear stress demand on the connections. Even designing connections for R=1 for all cases would make structures uneconomical. As I have mentioned in the proposed code the R=1 for connections & columns is to be used in rare cases when gravity induced stresses are high as well as EQ drifts are high.

I have also mentioned elsewhere to feel free to give your suggestions on any changes to this. Based on your comment, We could have two drift limits --one for RC flat slabs (0.005) & one for PT (0.015). I wouldn't mind looking at the paper you  are referencing.

In general I have also tried to keep the code section simpler to use while being conservative. For example the R values have far more variation than mentioned & we should almost have a table of R values for every component based on variations in stress levels, percentage of steel etc.  Even net to gross section properties depend on axial stress--however we are doing only one (0.5E)--just to keep it simple. Thats the prime reason for posting this--so everyone can react.

I am not sure whether it would be a copyright problem to post my ICI paper on the SEFI web site. However I am sure you will be able to find the ICI World Conference 2008 CD on campus (library etc.). If you still can't please contact me separately via email.

Regards,

Vipul Ahuja
Back to top
View user's profile Send private message
dipakdgaikwad
...
...


Joined: 01 Apr 2008
Posts: 77
Location: Mumbai

PostPosted: Fri Dec 26, 2008 12:49 pm    Post subject: Proposed draft clause for PT/non-PT flat slabs in IS 1893 Reply with quote

Dear All
          Is it possible to have high-rise building with some floors with Flat slab and some with normal beam slab?
          If yes how to analyze such building for seismic?
        
Thanks And Regards
Dipak D Gaikwad
Back to top
View user's profile Send private message
Vijayanarayanan
SEFI Member
SEFI Member


Joined: 01 Oct 2008
Posts: 16

PostPosted: Sat Dec 27, 2008 6:50 am    Post subject: Re: Proposed draft clause for PT/non-PT flat slabs in IS 189 Reply with quote

Respected Dipak,

Theoretically, it is possible to have a combination of both in a single building. But, practically i am not sure.

The lateral load resistance is largely depends upon the stiffness of the structure. It is a well know fact that the flat slab structure has low stiffness associated to it. When the combination of framed structure used along with flat slab structure, the problem of soft  story would come in to effect. In order to avoid it one can go in for bracing to provide lateral stiffness equivalent to the beam column story.

The problems associated with drift to the flat slab structure has to be kept in mind while designing the structure. Which means that the drift has to be kept as low as possible. The drift induced punching shear depends largely on the gravity load in the column. If the framed story is built over the Flat slab story, then there are ample chance that the gravity axial load would increase on the column.

It would be very difficult to adopt a suitable response reduction factor to reduce the lateral load.

Personally, I would not go in for such a combination of system if i have an alternate option.

Vijay Narayanan
MTech student,
IIT Kanpur.



dipakdgaikwad wrote:
Dear All
          Is it possible to have high-rise building with some floors with Flat slab and some with normal beam slab?
          If yes how to analyze such building for seismic?        
Thanks And Regards
Dipak D Gaikwad
Back to top
View user's profile Send private message
ahujavipul
General Sponsor
General Sponsor


Joined: 26 Jan 2003
Posts: 230

PostPosted: Tue Dec 30, 2008 4:43 am    Post subject: proposed draft clause for PT/non-PT flat slabs in IS 1893 Reply with quote

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
Back to top
View user's profile Send private message
Display posts from previous:   
This forum is locked: you cannot post, reply to, or edit topics.This topic is locked: you cannot edit posts or make replies. Thank Post    www.sefindia.org Forum Index -> E-Conferences-2008 [ Flat Slab Design issues ] All times are GMT
Goto page 1, 2  Next
Page 1 of 2

 

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You can attach files in this forum
You can download files in this forum


© 2003, 2008 SEFINDIA, Indian Domain Registration
Publishing or acceptance of an advertisement is neither a guarantee nor endorsement of the advertiser's product or service. advertisement policy