We have a building with most of the structure rotated to the primary x and y axes. When using RAM Frame we have used the "Rotate Axes" in the dynamic response spectra load case. Our understanding is that this adjusts the direction that the seismic force is applied, but that the story forces reported in the Frame output are on the primary X and Y axes. Is this correct?
If so, is there guideance on how the X and Y scale factors are to be determined in this case. The FAQ indicates that for a primary axes that the base shear in the perpendicular direction is ignored when determining the scale factor, but it does not appear that this can be the case when the axes are rotated.
You have the right idea about the rotate axes behavior on dynamic response spectrum loads in Ram Frame. When we calculate summations of things we do tend to do this along the global axis however, so the reported building story shear is along X and Y. There is no special way to convert that into your axis system for the purpose of scaling. I suppose you could work with basic trigonometry to convert the angles, but you are then working with two CQC sums, not the raw modal data.You could also scale based on the sum of reactions. I would recommend using dynamic analysis with consideration of sign in either case. You could also rotate the model, but that's more work.
Thank you for the clarifications. Our model has most lateral elements in line with the rotated axes, but there is one brace each oriented with the global axes. Is there a disadvantage to not rotating the load case, so that we could then scale off the global axes?
I would tend to run it both ways and base the scale factor off the more conservative (larger) ratio of dynamic to static base shear, but that's my nature.