Why does the Deflected Plot from a Response Spectrum Analysis look “strange”?
The results from any Response Spectrum Analysis (RSA) will all be positive. This is due to the modal combination methods used to obtain the RSA results. Since the results are all positive, ( instead of being positive and negative ) the deflected plot will be discontinuous at locations where the result would normally have been negative if it had been computed by a regular static analysis. To get a better feel for this, you can compare the deflected shapes of two simple models. Take a single bay portal frame subjected to a lateral load applied to one of the corners and perform a static analysis. Then subject the same model to dynamic loading via an RSA. If you zoom in on the results for the RSA, you will see that both columns are going UP, instead of one column going up and one going down. You should see a discontinuity at the location where the column in the statics solution would be going down.
