I referred example 28 of American examples to calculate frequency. I gave cut off mode shape 100. In the output, I found that the frequency increases with each mode. I am confused which mode should be considered for obtaining natural frequency? I need to restrict the natural frequency to keep it away from RPM of the equipment.
There are many natural frequencies for any structure, and in theory there is no end (3x number of mass points). The first mode, with the longest period, is generally the most important for seismic loads, but with vibrating equipment a higher mode that matches the machine frequency could be trouble.