Chapter 6: Spectrum Analysis
A spectrum analysis is one in which the results of a modal analysis are used with a known spectrum to
calculate displacements and stresses in the model. It is mainly used in place of a time-history analysis
to determine the response of structures to random or time-dependent loading conditions such as
earthquakes, wind loads, ocean wave loads, jet engine thrust, rocket motor vibrations, and so on.
The following spectrum analysis topics are available:
6.1. Understanding Spectrum Analysis
6.2. Single-Point Response Spectrum (SPRS) Analysis Process
6.3. Example Spectrum Analysis (GUI Method)
6.4. Example Spectrum Analysis (Command or Batch Method)
6.5. Where to Find Other Examples
6.6. Performing a Random Vibration (PSD) Analysis
6.7. DDAM Spectrum Analysis Process
6.8. Performing a Multi-Point Response Spectrum (MPRS) Analysis
6.9. Example Multi-Point Response Spectrum (MPRS) Analysis (Command or Batch Method)6.1. Understanding Spectrum Analysis
The spectrum is a graph of spectral value versus frequency that captures the intensity and frequency
content of time-history loads. Three types of spectra are available for a spectrum analysis:
- Response Spectrum
- Single-Point Response Spectrum (SPRS)
- Multi-Point Response Spectrum (MPRS)
- Dynamic Design Analysis Method (DDAM)
- Power Spectral Density (PSD)
SPRS is the only method available in the ANSYS Professional program.
6.1.1. Response Spectrum
A response spectrum input represents the maximum response of single-DOF systems to a time-history
loading function. It is a graph of response versus frequency, where the response might be displacement,
velocity, acceleration, or force. Two types of response spectrum analysis are possible: single-point response
spectrum and multi-point response spectrum. The output of a response spectrum analysis is the max-
imum response of each mode to the input spectrum. While the maximum response of each mode is
known, the relative phase of each mode is unknown. To account for this, various mode combination
methods are used (rather than simply summing these maximum modal responses).
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