×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

# Contact US

Log In

#### Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
• Talk With Other Members
• Be Notified Of Responses
To Your Posts
• Keyword Search
• One-Click Access To Your
Favorite Forums
• Automated Signatures
On Your Posts
• Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

#### Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

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

 Forum Search FAQs Links MVPs

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

(OP)
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

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

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 )

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

(OP)

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

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

#### Quote (Report abdallah hamdan here are the first 3 mode shapes, another question, can I use response spectrum analysis when the first mode is torsional?)

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 )

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

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

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

#### Quote (driftLimiter)

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

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

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

#### Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

#### Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

### Reply To This Thread

#### Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login

Close Box

# Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

• Talk To Other Members
• Notification Of Responses To Questions
• Favorite Forums One Click Access
• Keyword Search Of All Posts, And More...

Register now while it's still free!

Already a member? Close this window and log in.