See Derivation of response spectrum generation (technical note) page.
Yes, please see Response spectrum generation page for additional details.
When you define the response spectrum curve, the acceleration is typically entered as a fraction of gravitational acceleration (the gravitational acceleration is 9.81 m/sec/sec or 32.2 ft/sec/sec). The scale factor should be entered such that the response spectra values multiplied by the scale factor will be equal to the desired acceleration in current units.
In other words, if the acceleration for your response spectrum function is entered as a fraction of gravitational acceleration and the current length units are ft, then scale factor of 32.2 should be used for U1, U2 and U3.
Extended Question: I applied U1 acceleration loads with 10 degree angle and U1 with -10 degree angle to my structure. However the deformed shapes look exactly the same. Would you please confirm a negative angle is understood by the program? I also tried U1 with 45 degree angle and U1 with 135 degree angle, and the deformed shapes look the same. Why?
Answer: The output of a response spectrum analysis is the likely maximum response to a given loading. This is described in detail in CSI Analysis Reference Manual, chapter "Response Spectrum Analysis", section "Overview". If your structure is symmetrical about a plane of y = constant, then the response you are getting is expected, since both the structure and the loading are symmetrical about the same plane and a single positive result is produced for each joint displacement.
First, it is important to remember that the output of a response spectrum analysis is the likely maximum response to a given loading and the structure can experience this response in "both directions".
Deformed for shape linear add load combination that contains a response spectrum load case shows the displacements, either minimum or maximum, whose absolute value governs the response. Depending on the sign of the other loads involved in the load combination, the resulting displacement can be either in positive or negative direction, even for adjacent joints.
Extended Question: We are running a response spectrum analysis for a structure and wanted to increase the structure damping. Currently we only have a 5% damping response spectrum from the geotech. Is there a way to increase the structure damping without changing the response spectrum?
Answer: Yes, this is possible. The program enables you to specify response spectrum function damping ratio and modal damping ratio. Increasing the modal damping ratio can be used to increase the damping without the need to change the response spectrum function. See Damping in response spectrum analysis page for additional information.