I see, it is like a tuning fork.  You should definitely run the modal solution first.  What is the first natural frequency?  How does that compare with the frequency of your half sine, which at 13 ms is a 38.5 Hz forcing frequency (among others).  If you have a Modal analysis and link the solution of the Modal into the Setup cell of the Transient Structural, you will be making it a linear analysis, which is good to do for a first evaluation.