seismic design used period and extreme torsional irregularity in a residential building

[abdallah hamdan]

while designing a 7-story residential building, Etabs calculated the fundamental period as 1.85, and the approximated Period Ta was 0.53, the building is located in seismic zone 2B with Z=0.2g, and the period used for calculating base shear was 1.4 Ta which is 0.74, isn't the used period far from the fundamental period, should I decrease it to be near the approximated Period ? by adding shear walls?
another question, the building has extreme torsional irregularity, does increase the accidental torsion at each level by an amplification factor, Ax, only, safe for design?
the building is in the attachments if it helps
the code used is UBC 97
thanks in advance

 

[HTURKAK]

The picture attached implies the first mode is torsional mode ..and the bldg has extreme torsional irregularity..

Will you post the first three mode shapes to get better responds ?









Tim was so learned that he could name a
horse in nine languages: so ignorant that he bought a cow to ride on.
(BENJAMIN FRANKLIN )

 

[abdallah hamdan]

here are the first 3 mode shapes, another question, can I use response spectrum analysis when the first mode is torsional?

 

[HTURKAK]

UBC 97 allows the use of RSA for this case. ( Refer to 1631.5 Response Spectrum Analysis.)
1630.7 Horizontal Torsional Moments. ( and formula (30-16))

But i will suggest you to add some shear walls keeping symmetry to minimize the torsional effects.







Tim was so learned that he could name a
horse in nine languages: so ignorant that he bought a cow to ride on.
(BENJAMIN FRANKLIN )

 

[driftLimiter]

Have you checked drift on this building? With a period that long I would expect significant drift perhaps too much.

 

[kissymoose]

Have you checked drift on this building? With a period that long I would expect significant drift perhaps too much.

no surprise to hear this coming from you

 

[driftLimiter]

lol maybe i just should have posted a blank reply and let the OP guess what I mean.