I'm designing a water processing skid for a high seismic zone (California) and have a question about the horizontal forces. We have a filter that is supported by 4 legs. The legs are 24" tall and the vessel weighs 2500 lbs with the center of mass at 48" (24" above top of legs). The legs are mounted on the upper level of the skid 72" above grade. My question is how to setup the STAAD model. Usually I would only be concerned with gravity loads so my model would consist of 4 legs and the associated vertical loads. I'm thinking for this situation I need to include the horizontal force (V = Cs * 2500) associated to the 2500# then include that force on the legs. If that's true where in the program do I insert that force? Do I put it in the individual seismic load cases (X & Z) or do I add it to the seismic definition? Instead of using the horizontal force V should I calculate the overturning moment due to V and then apply those forces to the legs instead?
Thx