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This page contains frequently asked questions related to *[response spectrum analysis|kb:response spectrum analysis]*.

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h1. General FAQ

h2. How is the response spectrum created from acceleration records?

_[See Derivation of response spectrum generation (technical note) page.|doc:Derivation of response spectrum generation]_

h2. Can SAP2000 be used to develop a response for a certain location in the structure?

Yes, please see [Response spectrum generation] page for additional details.

h2. What value of scale factor should be used for the definition of the response spectrum load case?

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.

h2. Why does the deformed shape look for response spectrum analysis look the same negative and positive angle assigned to acceleration loads?

*Extended Question:* I applied U1 [acceleration loads|Acceleration load] 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|doc: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.

h2. Why does the deformed shape for linear add load combination with response spectrum load case does not look correct?

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 shape for 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.

h2. Can I change damping for response spectrum analysis without modifying the response spectrum curve?

*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.

h2. How to obtain the Response Spectrum Response for each mode 

*Extended Question:* Suppose that I run a response spectrum analysis and I want to obtain and display forces for each analyzed mode. How should I do that?


*Answer:* The procedure is the same for both programs. You are able to get the contribution of each mode to the response by using the Display>Show tables… menu command and displaying the Response Spectrum Modal Information table.

The response- spectrum modal amplitudes (U1Acc, U2Acc, U3Acc, U1Amp, U2Amp and U3Amp) give the multipliers of the mode shapes that contribute to the displaced shape of the structure for each direction of Acceleration. For a given Mode and a given direction of acceleration, this is the product of the modal participation factor and the response- spectrum acceleration, divided by the eigen value, w2, of the Mode. This amplitude, multiplied by any modal response quantity (displacement, force, stress, etc.), gives the contribution of that mode to the value of the same response quantity reported for the response spectrum load case.

Please note that mass-normalization for modes is always done based on the database units (These are the units that you choose when you create a new model using templates. These are also the units that the GUI environment is set to in the unit’s box on the bottom right corner, called "present units"). So, modal amplitudes should be always used in units that are the same as the database units to interpret the results.