Dynamic Analysis - Question on reaction results

For my platform model, I have performed Dynamic Analysis using the response spectra tab. Since there is no diaphragm, I have manually calculated the COM and applied the same as nodal loads.
I have reviewed my results and am satisfied with the model in terms of reviewing the mass participation, period, drifts etc. My only concern is the end reactions. the reactions in the X & Z directions look fine, but I am concerned about the reactions in the Y-Direction. The total Dead load (including the self weight is 170 kips), but my end reactions are being published greater than the dead load (about 300 kips.) I have attached the model here too for your review. I would appreciate if some one can catch my mistake and advise me.