QUESTION:  How to apply the 100% longitudinal Force and 30% Transverse per AASHTO in RC Pier. For multimodal analysis, do you need only one load case per direction?

ANSWER: 

There are two different approaches to combine the RSA responses due to spectra applied along more than one directions. One is to simply create multiple load cases and combine those in the load combinations.

The other approach is to apply the spectra along multiple directions in the same load case, as shown below:

 

This approach is only possible when “Use Dominant Mode” is not used. The reason why that option does not work in multi-direction spectra load is because automatic search for dominant mode is limited to only one direction. For instance, when spectra is applied along x direction, program searches for maximum mass participation among all the modes confined to only along x direction to determine the dominant mode. It is not possible to look for one and only one mode shape where mass participations along multiple directions of application of spectra are maximized at the same time. Normally mass participations maximize in different mode shapes for different directions. That is why there is a restriction not to be able to apply spectra along multiple direction when auto search for dominant mode is turned on.

If you are interested to apply the sign based on certain mode shape when spectra is applied along more than one directions, you still can do it. Only application does not offer any help in automatically finding it. Make use of Signed checkbox to specify the mode shape number to be used to apply the sign. You need to decide which mode to use to apply the sign to the responses. Whatever the sign of any response in that mode shape will be used in the final responses.

In absence of any of these options, auto search of Dominant Mode or manual input of the same, responses are simply a magnitude, a maximum quantity only. Response Spectra method is always to a measure of maximum responses, not necessarily happening at the same time during an earthquake event unlike time history analysis.