Time history analysis of an elevated water tank

Question

Hey! 
 For a frame structure, the self weight and floor loads (acting along gravity/global Y) which are defined in the dead and live load cases are defined again in the global directions(positive X, Y and Z directions) as dynamic masses for a time history load case. How will the dynamic masses be defined for an elevated water tank? 
 I have modelled an elevated intze tank where the container is modeled using plates. The loads defined are - selfweight and pressures (hydrostatic and hydrodynamic) which act on the wall and base of the container. These (trapezoidal) pressures act along the local z axis of the plates. So while defining the dynamic masses for the time history load case how should the pressures be defined?

Sye · Answer

Modeling the mass of water for dynamic analysis can be tricky and I don&rsquo;t think you should use the hydrostatic pressure loads applied on the walls/floor of the tank for that purpose. I would rather calculate the total load due to the water and apply it at the cg of the tank. The load would be applied in global X, Y and Z. The software would internally calculate the masses from this load and use it in the dynamic analysis. Since the cg would be at a certain height above the top of the structure, you would have to connect the cg with the rest of the structure. One option is to define a node at the cg of the tank, declare it a master node and make all the other nodes, in the periphery of the tank at that level, slave nodes. The load would be applied at the master node. Needless to say that this would be a simplistic approach to solving a problem that is far more complex. If you would like to model this more accurately you may refer to text books or literature that deal with dynamic analysis of elevated tanks. To the best of my knowledge these books offer more insight into how a mass spring system can be used for modeling the dynamic masses. Portion of the mass can be considered to be rigidly attached to the tank as considered above while some proportion of it would have to be considered as oscillating. They also provide guidelines on how to calculate these masses which are essentially functions of the geometry of the tank and the height of water and also how to calculate the associated spring constants. I hope this helps.