Thank you, Richard.

I have a specified spectra for my system, I input this spectra in the Spectrum definitions. I read the data from a file as follows:
0.1 5.94
0.22 29.67
0.5 59.42
1.0 124.62
6.57 463.00
21.37 463.00
25 118.84
33 17.83
100 17.83

When I check the input, it come out the folloing messages:
33 , 17.83
100 , 17.83
*** ERROR *** Spectrum/Time History case 1 is empty.
*** NOTE *** Input check terminated due to fatal errors

Everytime I start a new spectrum, it automaticaly starts from Case 2 rather than Case 1. I don't know if that is the reason.

Thanks,
Bing