What is natural period for first mode of 20 storey flat slab building 1

[dollarbulldog]

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

 

[dougantholz]

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.

 

[drile007]

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.

 

[dollarbulldog]

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