×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Are you an
Engineering professional?
Join Eng-Tips Forums!
• Talk With Other Members
• Be Notified Of Responses
• Keyword Search
Favorite Forums
• Automated Signatures
• 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.

How to calculate drift of response spectra analysis?

How to calculate drift of response spectra analysis?

(OP)
When perform reponse spectra load case in X direction, there is always some forces in Y direction. Sometimes the forces in Y direction is pretty large or even close to force in X direction. Therefore the drift in Y direction is close to the drift in X direction.

So how to check the story drift? Just check drift in X direction by neglecting drift in Y direction or use squre root of the sum of suqres of X and Y directions?

Thanks

RE: How to calculate drift of response spectra analysis?

what I understand is that when we define Response spectrum cases we already define them the method for summing up the modes either by CQC or SRSS method therefore the drifts we get are already combination from some of the modes shapes.
What I do usually is that I define Response Spectrum function with the name SX+SY, SX+0.3SY and SY+0.3SX and check the drifts on this by checking Story drift option of ETABS. If i get problem at specific point then I just go to that point and try to check out in more detail.

Thanks
Zeeshan

RE: How to calculate drift of response spectra analysis?

Hi...
The earthquake waves are comming from one direction. You don't need to sum the influences from two ortognal directions.
Just analyse the influnce in two main directions (X i Y in most cases) separately.

RE: How to calculate drift of response spectra analysis?

However, it depends on your local code. Eurocode 8 demands taking 30% of influence from ortogonal direction.
So, it would be X+0,3Y or Y+0,3X.

RE: How to calculate drift of response spectra analysis?

hi, lisica.
you mentioned that we needn't to sum the influences from two orthogonal directions. However, the earthquake wave is 3D and can not accurately predicted. So many seismic records show that the ratio of PGA for the two orthogonal directions is approximately 1:0.85. By square root of the sum of suqres of X and Y directions with the above factors, we could get the orthogonal effects, Ex+0.3Ey or Ey+0.3Ex.
About the story drift check, I do agree with Zeeshan's viewpoints. Please refer to Chapter 4 Commentary of NEHRP 2003 for more details.
Shulu Zhang

RE: How to calculate drift of response spectra analysis?

OK, I agree with you.
My code requires using Ex+0.3Ey or Ey+0.3Ex, so I am using this relation.

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.

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!