User Guide

RISA-3D User’s Guide
Version 5.0
112
For the tutorial we will have RISA-3D automatically detect and use the dominant
mode (the mode with the highest participation factor, discussed later).
Click on the down arrow in the Mode for Signage field and select
“Dominant”.
We will stick with the current combination method and damping ratio which are
CQC and 5% respectively.
So we've specified the spectra be applied in the X direction, and use the dominant
mode for results signs.
Click
and the UBC 97 Spectra dialog is opened.
The default values listed are for Seismic Zone 3, Soil Type “Se” (Soft Soil
Profile).
Click OK.
When complete, you are returned to the Frequencies spreadsheet. We'll go right
back in and perform a RSA for the Z direction.
Again click on the Solve button and click Solve to bring up the Response
Spectra Selection Window. This time change the Spectra Direction to “Z”.
Click and then click OK in the UBC 97 Spectra dialog.
The Frequencies Spreadsheet is presented and looks like this:
Modal Participation
The modal participation percentages are now recorded for the X and Z directions.
About these participation values: Be sure the participation for each direction totals
to 90% or more! For this model, we've met these criteria, but if you're running a
model where the participation total is less than 90%, you must return to the
dynamics dialog, increase the number of modes and redo the dynamic solution
and also the RSA.
OK, so the dynamic analysis and response spectra analyses are done. The final
step is to include these RSA results with the static loads in a set of load
combinations to obtain the final, overall model solution.
Scaling Factors
Now comes the calculation of the spectra scaling factors. We won’t be going
through the actual procedure here, but we’ll give a quick overview. (You can see
an example of the procedure in the Help file. Just search the Help index using the
keywords “scaling factor”, then select “RSA Scaling Factor” from the choices.)