The details and fidelity of the simulation would depend on the questions you are trying to answer.
"are the chair legs strong enough to support a large person?" could use a very simple model
"is the seat pan going to bend excessively?" might need a more detailed weight distribution
" Will the pressure on a persons buttocks exceed xx PSI ?" might need a detailed generic model of bones and muscles and fat, and calibration studies of force vs. deflection to adjust material properties. (or research to find a database of that info). I'm pretty sure the human butt is not a linear elastic system, but approximating it as elastic might be close enough for your purposes. May be a hard cylinder inside a softer cylinder might be sufficient.