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Moment transfer columns to slabs
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Vijayanarayanan
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Joined: 01 Oct 2008
Posts: 16

PostPosted: Tue Dec 30, 2008 4:05 pm    Post subject: Re: Moment transfer columns to slabs Reply with quote

Dear Vaisakh G,

Generally flat slab in high seismic regions are to be designed for gravity loads only. The entire lateral load has to be transferred through the lateral resisting system, such as shear wall, which we have to design.

Now, when a design earthquake occurs the shear wall takes all the lateral load. But in doing so it undergoes a lateral drift. As the shear wall under go lateral displacement it induces the same amount of drift on the column and the flat slab. When the columns undergo this drift, the vertical load that is transmitted through the column gets displaced by the same amount (P-delta effect).This results in a extra unbalanced moment at the slab column connection. This unbalanced moment has to be resisted by the slab column connection, with the help of shear capacity at the slab critical section and the moment capacity of the slab column connection.

Several researchers have tried to quantify the proportion in which the unbalanced moment has been resisted. After extensive research they have concluded that 40% of unbalanced moment is transferred through the shear action and the rest is transferred through the flexural action.

To incorporate this effect in the preliminary design stage, pne can follow the procedure given below,

1) Design the flat slab for gravity load.
2) Design the lateral load system based on the lateral load and the permitted lateral drift.
3) For the design inter story drift and the gravity shear ratio, refer to the ACI 318-05 section 21.11 to check whether the slab column connection is safe against punching shear or not.

When you design the slab column connection for the effect for lateral drift you would notice that we make use of the term gravity shear ratio. This term is basically a ratio of gravity shear to the shear capacity of the section.


The above procedure delineated is just a rough guideline, Pls. do not rely upon it for designing the structure.

Hope I have made at least some portion of your doubt clear.

Vijaya narayanan.AR
MTech student,
IIT Kanpur.




vaisakhg wrote:
Sir,
I came across a confusion whether the moment transfer from slabs to columns is to be considered in case of checking the punching shear. In usual practice, as i know, only the axial load is considered. But from literatre, i came across the fact that some portion of the moment is transfered by the eccentricity of shear. if that is the case that had to be added to the shear stress. Can you please give me a guidance?
Yours truly,

Vaisakh G
+971 50 6844981

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arunkashikar
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Joined: 14 Oct 2008
Posts: 91

PostPosted: Wed Dec 31, 2008 5:31 am    Post subject: Moment transfer columns to slabs Reply with quote

ACI 352.1 R89, “Recommendations for Design of Slab-Column Connections in Monolithic Reinforced Concrete Structures” can be used to design the slab-column joint.  

ACI 352 talks about beam column joint

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Arun Kashikar


Board line: +91 22 2571 9000 | Direct: +91 22 2571 9058 | Cell: 9819055576
Fax: +91 22 2571 9300 | Email: akashikar@hirco.com (akashikar@hirco.com) | Website: www.hirco.com


From: vijaya narayanan [mailto:forum@sefindia.org]
Sent: Tuesday, December 30, 2008 9:35 PM
To: econf@sefindia.org
Subject: [ECONF] Re: Moment transfer columns to slabs



Dear Vaisakh G,

Generally flat slab in high seismic regions are to be designed for gravity loads only. The entire lateral load has to be transferred through the lateral resisting system, such as shear wall, which we have to design.

Now, when a design earthquake occurs the shear wall takes all the lateral load. But in doing so it undergoes a lateral drift. As the shear wall under go lateral displacement it induces the same amount of drift on the column and the flat slab. When the columns undergo this drift, the vertical load that is transmitted through the column gets displaced by the same amount (P-delta effect).This results in a extra unbalanced moment at the slab column connection. This unbalanced moment has to be resisted by the slab column connection, with the help of shear capacity at the slab critical section and the moment capacity of the slab column connection.

Several researchers have tried to quantify the proportion in which the unbalanced moment has been resisted. After extensive research they have concluded that 40% of unbalanced moment is transferred through the shear action and the rest is transferred through the flexural action.

To incorporate this effect in the preliminary design stage, pne can follow the procedure given below,

1) Design the flat slab for gravity load.
2) Design the lateral load system based on the lateral load and the permitted lateral drift.
3) For the design inter story drift and the gravity shear ratio, refer to the ACI 318-05 section 21.11 to check whether the slab column connection is safe against punching shear or not.

When you design the slab column connection for the effect for lateral drift you would notice that we make use of the term gravity shear ratio. This term is basically a ratio of gravity shear to the shear capacity of the section.


The above procedure delineated is just a rough guideline, Pls. do not rely upon it for designing the structure.

Hope I have made at least some portion of your doubt clear.

Vijaya narayanan.AR
MTech student,
IIT Kanpur.




vaisakhg wrote:    
Sir,
I came across a confusion whether the moment transfer from slabs to columns is to be considered in case of checking the punching shear. In usual practice, as i know, only the axial load is considered. But from literatre, i came across the fact that some portion of the moment is transfered by the eccentricity of shear. if that is the case that had to be added to the shear stress. Can you please give me a guidance?
Yours truly,

Vaisakh G
+971 50 6844981

Posted via Email
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View user's profile Send private message
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