In the Direct Analysis Method wiki, step 3 says when you view mode shapes, the first and second mode shapes will usually be a translational mode in each orthogonal direction and the third mode will usually be a rotational mode. In my model, the first mode is translational in the x-direction, the second mode is rotational, and the third is translational in the y-direction. Should the translation only be in mode shapes 1 and 2 and rotation only in mode shape 3 or can mode shapes 2 and 3 switch as mentioned above? What might this indicate about the proportioning of my lateral frames?
My building is u-shaped and I have even frame distribution in the x-direction but because of the u-shape, they frame spacing is a bit different in the y-direction.
There is nothing wrong with the principal X or Y mode shape being mode 3. it just means that the period of the first rotational mode is longer. This tends to happen for models that are rotationally soft. A classis example is a structure where the frames in plan make a cross + shape for example. The program will automatically use the period of the X and Y principal mode shapes, but you can force the seismic ELF load case period value (T=) if it's ever required.
Answer Verified By: Jon Bergren