## General Mechanical

#### Response Spectrum vs Random vibration

• Autonewbie
Subscriber

Hi, after reading some notes... I still not able to determine when to use response spectrum or random vib. Anyone can share your tips? Thanks!

• peteroznewman
Subscriber
Look at the input data you have to excite the structure.
If the input is a table of values of acceleration PSD values in the units of G^2/Hz vs frequency in Hz, that is the type of input you use in a Random Vibration analysis. The output of the Random Vibration analysis is the 1-sigma value of stress, which means the response will be less than that value 68% of the time. You can multiply that value of stress by a factor of 2 to get a 2-sigma value and the response will be less than that value 95.45% of the time. You can multiply that value of stress by a factor of 3 to get a 3-sigma value and the response will be less than that value 99.73% of the time.
If the input is a table of values of acceleration in units of G vs frequency in Hz, that is the type of input you use in a Response Spectrum analysis. The output of the Response Spectrum analysis is the predicted maximum response the structure will have to that input.
The above examples mention acceleration, but tables using velocity or displacement are also possible, though less common.
Both Random Vibe and Response Spectrum have in common a requirement for a Modal Analysis with a Fixed Support to create the Base Excitation where the input goes in.
If you have acceleration-time data, and not a short table of values vs frequency, you can compute either a PSD table or a Shock Response Spectrum table to use as input.
If the acceleration-time data is a sample from a vibration source that may run for a long time, like an engine, and each time you record a sample of the vibration using an accelerometer, the statistical values of the recording are the same, then you would want to analyze a structure subject to that vibration using Random Vibration.
If the acceleration-time data is the complete record of an event, such as an earthquake, and you want to know if the structure will survive that particular event, then you want to analyze the structure using Response Spectrum.