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What is natural period for first mode of 20 storey flat slab building

What is natural period for first mode of 20 storey flat slab building

What is natural period for first mode of 20 storey flat slab building

(OP)
Dear all,

Can anyone suggest a common & proper first fundamental period for 20-storey building using column, flat plate and a shear wall system? I am dealing with a design of one building. It has been found that the first mode (in torsion) has a period about 5.7 sec.

Thank  

RE: What is natural period for first mode of 20 storey flat slab building

approximations for a 20 story building should be pretty close.  0.1 x N would be 2 sec.  Probably valid in the orthogonal directions.  It sounds like you have no perimeter walls, and a fairly large plate.  Its not that hard to imagine given your period that you have some serious torsional irregularity.  You might consider making it a duel system with an outboard moment frame.  Its done quite often.

RE: What is natural period for first mode of 20 storey flat slab building

I would avoid torsional first mode shapes, especially  in 20 storey building!!!
I think that period of 5.7s isn't unusual, since you have very soft torsional layout. Listen to dougantholz advices.

RE: What is natural period for first mode of 20 storey flat slab building

(OP)
Dear friends

Thanks for all useful advices, esp. dougantholz. I have tries adding edge beams of size 800x400 to avoid serious reduction in clear height. The period is now gone to 4.17 s. Still the torsional mode is dominant. I would have question whether we need to completely eliminate this torsion susceptibility, i.e. first mode shall not be a torsional mode, if so.

Sincere  

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