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econference on tallbuildings

 
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k.gangadharan
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Joined: 01 Apr 2008
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PostPosted: Sun Nov 25, 2012 2:29 pm    Post subject: econference on tallbuildings Reply with quote

FLat slab buildings

Many of the architects and developers prefer flat slab buildings for office comlexes
as it is a column and slab building (without beams) to reduce floor height and air conditoning load. In this category there is  flat slab with slab drops at column positions and flat plates without slab drops.
It is not clear in Indian codes for how many floors or upto what heights a flat slab building can be designed in different earthquake zones where as clear directions are available in chinese codes regarding the heights & storeys in different earthquake zones as pointed out by madam Alpha Sheth (in her second mail on chinese practice.)
Many structural engineers in this forum have already expressed their concern for  this lack of provisions in Indian codes: IS 456 ,1893,&13920.
To make the structure safe against lateral loads like EQ forces & wind, we can provide shear walls with lift walls & staircase walls in suitable locations . But many architects locate lifts & staircases eccentrically and not in ideal locations.These eccentric locations result in enormous torsion and drift , as the centre of mass and centre of rigidity are wide apart.Many of the architects do not agree to the structural engineer's suggestion for symmetrical relocation of shear walls and therefore the designer has to resort to some other technique to make the fragile structure safe against earthquake  forces and other lateral forces like wind.The problem becomes more serious if it is a tall structure with one or two floors in the ground  floor is open for car parking facilities

Normally the modular grids for columns for flat slab structures are large may be 9mx 12 m or 12m x12m or even higher modules. In such cases we have to go for post tensioning the slabs for such large spans.
In one of the papers presented by consultant mr. Vipul Ahuja & Pro: B.M Ahuja ,  on Post tension flat slabs in high seismic zones, describing the deficiencies  and drawbacks in Indian practice, has pointed out that there must be separation between shear resisting elements & PT slabs so that prestressing forces are not lost to the shear resisting elements. I have  doubt regarding this  in actual practice as the shear walls are located at the boundary of the floor plate &  is also a support to the slab;  and post tensioning is done after placing the prestressing ducts are placed and concreting the slab. In such a situation it is not possible to create a separation between the shear wall support and slab and one end of the  cable duct ( other than the stressing end) is normally anchored on the shear walls itself. Mr. Vipul Ahuja may suggest some other alternative remedial measures for this.

His other recommendation  like providing prestressing strands thru' columns avoiding ducts  may be possible.

Another suggestion with respect to  Providing shear stud rails around columns in flat plates  to take punching shear is also a remote posibility in India . as nobody is manufacturing shear stud rails in India ; or otherwise we have to increase the thickness of the slab to take care of the punching shear., which is not an economical proposal. If any company is manufacturing shear stud rails in India, please provide the details of the firm in the forum

thanks & warm regards

K.Gangadharan
Structural consultant
Trivandrum

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