Response Spectrum Analysis

In a response spectrum analysis, the results of a modal analysis are used in terms of a known spectrum to calculate displacements and stresses in the model. For each mode, a response is read from a design spectrum based on the modal frequency and a given damping ratio. All modal responses are then combined to provide an estimate of the total response of the structure.

You can use a response spectrum analysis rather than a time history analysis to estimate the response of structures to random or time-dependent loading environments such as earthquakes, wind loads, ocean wave loads, jet engine thrust or rocket motor vibrations.

Response Spectrum

Plots peak responses over time for a range of single-degree-of-freedom (SDOF) systems subjected to a particular base motion as a function of their natural frequency ωI, or vibration period TI. Each response spectrum curve corresponds to a particular modal damping ratio ξI.

Typical quantities plotted in response spectra are:
  • Maximum displacement SdI, ξI)
  • Maximum pseudo-velocity SvI, ξI) = ωI*Sd
  • Maximum pseudo-acceleration SaI, ξI) = ωI*Sv = ω2I*Sd
The response spectrum is used as base excitation input for a response spectrum analysis.

Relative displacement response spectrum for a particular damping ratio ξ1.

equals the maximum absolute value of the relative displacement time history response for each SDOF oscillator.

Natural circular frequency of vibration for each SDOF oscillator.