×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

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!
  • Students Click Here

*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

Jobs

P-delta + Modal Spetral Analysis

P-delta + Modal Spetral Analysis

P-delta + Modal Spetral Analysis

(OP)
Is there an option in SAP2000 to take into account the p-delta effect when you are working with load combinations which have a Response Spectrum load case?

I know that it makes no sense to combine p-delta with a response spectrum result because it loose the sign. But how can I have this amplification into account?

Currently I'm scaling up the drifts and forces by the ASCE 7-05 Section 12.8.7 coefficient. Is there another way? Regards,

Sebastian

 

RE: P-delta + Modal Spetral Analysis

This isn't a full answer to your question, but modal analysis cases can be based on the stiffness of a nonlinear P-delta case. Those "stressed" modal cases can then be used as the basis for response spectrum or modal time history cases. For example, you could define a static nonlinear p-delta load case which includes selfweight, equipment loads, and maybe even a percentage of live load. You could then define a modal case to use the stiffness of that nonlinear load case. In some designs, it can make a significant difference in modal results.

As you already pointed out, there isn't a reasonable way to include response spectrum (RS) results in a P-delta case. You could define nonlinear P-delta load cases which include static earthquake loads, but not RS loads. Alternatively, a direct integration time history analysis for seismic loads could consider P-delta

I don't have it in front of me at this moment, but doesn't 12.8.7 refer to amplification of static results?

RE: P-delta + Modal Spetral Analysis

(OP)
Dear masomenos,
               in my opinion it doesn't say that this amplification is only valid for static results.

According with ASCE 7.05 Section 12.8.7: "Where the stability coefficient (teta) is greater than 0.10 but less than or equal to the incremental factor related to P-delta effects on displacements and member forces shall be determined by rational analysis. Alternatively, it is permitted to multiply displacements and member forces by l.O/(l - teta)."

Regarding you answer, according with ASCE I need to take p-delta effect into account if teta is > 0.10. The "stressed" modal cases could amplify the forces but I think that it isn't the same. Thanks, regards.

Sebastian

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!


Resources