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RSA definition

What value of scale factor

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should be entered in the definition of a response

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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.

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1. Define one response-spectrum curve, then save the definition by selecting Save Named Set.
2. 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.
3. Display the output table by selecting Analysis Results > Structure Output > Named Set Data.

Can damping be changed without modifying the response-spectrum function?

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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.

RSA deflection

Why is deflected configuration identical for acceleration loads applied in either direction?

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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?

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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 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.

How are response-spectrum curves generated from an acceleration record?

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Answer: Von Mises stresses are computed only when results have correspondence. Response-spectrum forces do not have correspondnece 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.