Hi Hongzhou,
Good to hear that you made it work!
Yes exactly, as you say, taking the mean is a smoothing operation and therefore it is much easier to fit than other quantities like e.g. maxima.
It sounds a bit like your model could be a stochastic simulator, since if I understand correctly, the structural response is varying for the same structure properties (inputs) if different seismic records are applied. If you are only interested in the mean response, a PCE is enough. But if you are interested in more properties of the response, e.g. quantiles or the distribution, you might want to consider methods for stochastic simulators.