David Gill

Random Uniform Base Excitation Units Problem

Discussion created by David Gill on Mar 10, 2008
I am running a Linear Dynamic Study with a Random input. I am using a Uniform Base Excitation as an input to the system so that I can create a power spectral density (PSD) output. I need to enter acceleration, and so the dropdown box under "PSD Acceleration" on the "Uniform Base Excitation" menu gives the units of g^2/Hz. This is confusing. The imput excitation should be an acceleration with units of "g" or "m^2/s". The output of a PSD can be g^2/Hz, but this unit assumes that I know what the width of my discretization is (for example, if I solve every 10Hz, then my output is g^2/10Hz which averages the g^2 value across the 10Hz window). When I'm entering the input acceleration, I don't know what the frequency discretization width is.

So, is the acceleration input on this page really in g's and it is just labeled for what the output of the PSD is going to be? Am I supposed to run the whole analysis, see what the discretization width is, and then come back to change my input correctly? What's going on here?
Thanks for the help.