Yes, I agree that .003 s is a good step size based on it being adequate to resolve 17 Hz motion in the time history results.

1. Since the earthquake data is only applied in the horizontal X direction, I only looked at that. Also, since the geometry is large in the X direction and small in the vertical Y direction, there was very little Mass Participation in the Y direction.

2. I did an FFT on the acceleration-time history and plotted the result in matlab. There is a nice set of scripts called vibrationdata.

3. A different acceleration load, sampled at a higher rate, say 2000 Hz, might show a large amount of energy at 50 Hz in the FFT. If there were some modes in the model that were near 50 Hz, then you would want to run the Transient structural simulation with time steps at 1000 Hz or 0.001 s time step.

4. Under Analysis Settings, with Current Step 1, change Time Integration from On to Off.

5. Under the Solution Information folder, look at the Solution Output. You can search the text for the word ERROR. Look at what it says there and look above there at the convergence and look for information on why it stopped converging.  You can search this site using the details of the error.

The website is a bit broken now so you can't Insert Image or Attach files to replies. Sorry.