Kevin,
You need to examine the mode shapes and decide which are relevant. The software can't make this decision for you. The '80% rule' is very flexible and is a cover all.
You also need to decide what useful bandwidth the exciting signal has. There's no point looking at modes at 1kHz if the if input signal rolls off (half power point) at 100Hz. Get this from the FT of the input half sine function. Try Theory of Vibrations , William Thomson.
An example of irrelevant missing mass : a large number of similar stiffening gussets say. Each one having a response at a higher frequency than anything that is relevant in the input signal. Each one ringing at a slightly different frequency. Because there are many, their cummulative contribution is not negligable. By examination, the engineer could ignore them with justification.
Another, consider a suspended walkway. It's has 1000 uprights holding on the hand rail. They will all ring in a frequency band that is way above the key damaging modes of a few Hz of the suspended platform where the exciting frequencies are a few Hz. (people walking, building movement due to wind loading) It is these key modes that are of interest not the individual ringing modes of each upright.
Every plug, bolt, gusset, bar will have their own mode and contributes modal mass and you can chase these to higher and higher frequencies where the power at these frequencies in the input signal is negligable. They can probably be dismissed for the modes of interest. Remember that their mass does play a part in lower modes so don't remove them without justification!
Ansys is great! Feedback from Ansys is more comprehensive. You get lots of buttons to click , you have many more decisions to make ... but s/w learning curve is steeper and longer, integration with CAD is nowhere near as good as Simulate and answers are, for usual purposes, the same. Both Simulate and Ansys use Lanczos methods to ensure modes are not missed.
The process one has to follow in Ansys is essentially the same as Simulate.
I thoroughly recommend the NAFEMS dynamics couse. The Q&A forum associated with the course can leave one a little frustrated but the content will answer all of your questions and more.
atb
Charles
