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This page contains frequently asked questions related to *[response spectrum analysis|kb:response spectrum analysis]*. {on-this-page} 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:* 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.page is devoted to frequently asked questions (FAQ) related to response-spectrum analysis. |
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RSA definition
What value of scale factor should be entered in the definition of a response spectrum load case?
Answer: When the response-spectrum curve is defined, acceleration is typically entered as a fraction of gravitational acceleration (9.81 m/sec2 or 32.2 ft/sec2). Scale factor should be specified such that its product with response-spectrum values generates the acceleration units desired.
For example, if acceleration for the response-spectrum function is plotted as a fraction of gravitational acceleration, and if length units are in feet, then U1, U2, and U3 should be scaled by 32.2 units.
How are multiple response-spectrum curves defined?
Answer: Multiple response-spectrum curves may be defined using Named Set and interactive database editing to generate results in tabular format. The steps to do so are as follows:
- Define one response-spectrum curve, then save the definition by selecting Save Named Set.
- Use the interactive database editor to create the remaining Named Sets, one for each plot, by editing the tables within Model Definition > Other Definitions > Named Sets.
- Display the output table by selecting Analysis Results > Structure Output > Named Set Data.
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Can damping be changed without modifying the response-spectrum function?
Answer: Yes, the response-spectrum function damping ratio and modal damping ratio may be specified. Modal damping ratio may be increased such that damping increases without the need to change the response-spectrum function. Additional information is available in the Damping in response-spectrum analysis article.
RSA application
How is response-spectrum analysis applied only above grade?
Answer: During response-spectrum analysis, loading is applied to the entire structure, though sub-grade modes will have much higher frequencies, providing minimal contribution to modal response. However, the mass participation of subgrade levels may be subtracted from the total mass.
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RSA deflection
Why is deflected configuration identical for acceleration loads applied in either direction?
Extended Question: Acceleration loads are applied along U1 first with a 10° angle, then with a -10° angle. Why does response-spectrum analysis generate identical deflected shapes for either orientation?
Answer: For each response quantity and realization of structural period, response-spectrum analysis generates a single positive result for the likely maximum response of a structure subjected to a loading condition. If the structure is symmetric, results should be identical regardless of loading orientation because absolute values are taken during numerical formulation.
Additional information is available in the CSI Analysis Reference Manual (Response Spectrum Analysis, page 339).
Why does the deflected shape for a linear-add load combination not look correct?
Answer: Response-spectrum analysis generates the likely maximum response that a structure may experience in either direction. The deflected shape for a linear-add load combination which contains a response-spectrum load case will display either the minimum or maximum displacements, depending on which has the greater absolute value. Depending on the sign of loads within the load combination, displacement may be in the positive or negative direction, even for adjacent joints.
See also Deformed shape for moving load analysis.
RSA output
How are response spectra and resultant forces obtained for each mode?
Answer: The response contribution from each mode is available through Display > Show Tables > Analysis Results > Structure Output > Modal Information > Table: Response Spectrum Modal Information.
Response-spectrum modal amplitudes (U1Acc, U2Acc, U3Acc, U1Amp, U2Amp and U3Amp) give the mode-shape multipliers which contribute to structural displacement for each direction of Acceleration. For a given mode and direction of acceleration, amplitude is the product of modal participation factor and response-spectrum acceleration, then divided by the Eigen value (ω2) of the mode. When amplitude is multiplied by any modal response quantity (displacement, velocity, acceleration, force, stress, etc.), this value represents the contribution of a given mode to the same response-quantity value reported for the response-spectrum load case.
Please note that mass is always normalized for modes, based on the database units chosen when a new model is created from templates. The GUI environment is also set to these units, as shown in the Units Box in the bottom-right corner of the display window. While interpreting results, modal amplitudes should be consistent with these database units.
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How are response-spectrum curves generated from an acceleration record?
Answer: For response, please see the Derivation of response-spectrum equations article.
Can SAP2000 develop response spectra for a given location within the structure?
Answer: Yes, please see the Generating response-spectrum curves article for response.
Why are Von Mises stresses not computed and plotted during response-spectrum analysis?
Answer: Von Mises stresses are computed only when results have correspondence. Response-spectrum forces do not have correspondence when modal combinations are performed. As a result, design will proceed according to the force envelope, which is a function of the maximum values for P, M2, and M3. In reality, these maximum forces do not occur simultaneously, therefore no plot is available. If necessary, post-processing is available for modal results.
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