If you have a free hand, not only should you be considering seismic as discussed above, but an isolated 30' platform is likely to cause some dynamic amplification. A while ago I modelled a vessel of about 8' dia on a 35'high skirt. It had a top platform with some large valves. When modelled from the ground up, the calc gave a natural frequency of about 2hz with and amplified peak acceleration of well over 1 g even though the ZPA was only 0.2 g or thereabouts. This was the wrong answer. Somebody else modelled the vessel as a weightless rigid element and got the right answer. Best policy is to go with the flow.