It is possible to model fluid-structure interaction (Sloshing) in SAP2000 by following one of these steps:

1) A SIMPLIFIED SEISMIC ANALYSIS OFCIRCULAR LIQUID STORAGE TANKS

http://e-collection.library.ethz.ch/eserv/eth:1068/eth-1068-01.pdf

2) If you assume that fluid is incompressible, you can model the water using special joints with added mass, then connect these joints to the structure using gap links

3) If you want to explicitly model the water; the structural part can be modeled using frame, area, and solid elements made up of isotropic materials. You should use gap links to connect the water to the structure. The fluid would need to be modeled using solid elements made of a material that has a small shear modulus compared to the bulk modulus. If the shear modulus is small compared to bulk modulus, Poisson’s ration is approximately equal to 0.5. The material property for the fluid elements can be defined on these lines. Based on the published literature, following approximate properties are recommended to be used to model water as a solid element:

Young’s Modulus = 0.9 ksi
Poisson’s Ratio = 0.4995
Shear Modulus = 0.3 ksi
Bulk Modulus = 300 ksi
Weight Density = 0.036 lb/in^3

More details on fluid-structure interaction can be found in the literature. For example, chapter 23 of Dr. Wilson's book "Static and Dynamic Analysis of Structures is dedicated to modeling Fluid-structure interaction

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