How would I create a response spectrum for a time-history? I have run a transient analysis and am trying to use the RESP command to generate the equivalent response spectrum. The input is a sine wave with an amplitude of 0.025 inches. In POST26, the node where the loading was input has the correct displacement versus time, a sine wave with an amplitude of 0.025. I then use the RESP command to generate a displacement response spectrum. When I plot displacement versus frequency, I would expect to see one vertical line with an amplitude of 0.025 at a value of 1 Hz, but I do not see anything at all on the graph.
No, other frequencies will be excited by the 1 hz excitation. I have written below how you would create a response spectrum for this case. The response spectrum created shows that a structure with a 1-Hz natural frequency will have a peak displacement of 6.8e-2 in response to the transient input. This corresponds to a dynamic load factor of 2.75 and seems reasonable.
plvar,33 ! response spectrum