IBC Response Spectrum Analysis -- Reaction Question

I am currently working on an IBC 2012 project where I am using the response spectrum command to define the seismic loads.  I am working on a 3-D model with three rows of columns in the direction of the seismic load. 

I have specified a CQC combination method and the Ss and S1 values manually.  I am scaling the spectrum by a factor of 0.625 for the x-direction (I/R where I =1.25 and R=2) with linear interpolation and 5% damping.

When I review the reactions output, all I see are column Fy (vertical) reactions acting in the same direction (positive).  I would expect to see one of the end columns to have a negative reaction for the overturning moment uplift.  What input am I getting wrong?

Parents
  • There is no problem with the way you have set up the model. The modal combination methods used for combining the results from the various modes in a response spectrum analysis, leads to all positive numbers. This is explained in the following post

    http://communities.bentley.com/learn_communities/ask_an_expert/f/343551/p/113598/347534#347534

    More discussion on how to get signed results from a response spectrum analysis is available at

    http://communities.bentley.com/products/structural/structural_analysis___design/f/5932/p/91095/260867#260867

    Last but not the least, there is a very good book which contains loads of information related to topics like these. You can find more details about this book at



Reply
  • There is no problem with the way you have set up the model. The modal combination methods used for combining the results from the various modes in a response spectrum analysis, leads to all positive numbers. This is explained in the following post

    http://communities.bentley.com/learn_communities/ask_an_expert/f/343551/p/113598/347534#347534

    More discussion on how to get signed results from a response spectrum analysis is available at

    http://communities.bentley.com/products/structural/structural_analysis___design/f/5932/p/91095/260867#260867

    Last but not the least, there is a very good book which contains loads of information related to topics like these. You can find more details about this book at



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