Thanks very much for your reply, you really help me a lot.

1. It seems that when I was doing the mode superposition by using the modal and transient structural, there is a limit for the time steps, I could not set it to 20000, instead the maximum I could put is 10000, do you have any idea why it will happen?

2. when I use the mode superposition with both the modal and harmonic response, there is a problem that I could not change the position where the acceleration applied from the whole body to the two surfaces on the two sides or corners, do you have any idea that how could I apply the acceleration on the two surfaces on the sides?

3. After I have done the full transient analysis, just insert the data I got into the matlab and deal the data with the FFT in matlab to get the frequency response. am I right?

4. one last thing is that: As for the BCs, we do have two choices like you stated before, either fixed support with acceleration or fixed rotation with displacement.

it does not matter which choice you pick but still will get to the same result at last. am I correct?

Best Regards

thanks very much again since you never feel impatient to answer my questions

xiaofu