Gravity load redistribution and collapse resistance of structures following loss of load bearing elements due to natural and manmade hazards significantly depend on the floor response. Effects of floor system modeling on collapse resistance of damaged structures are studied in this paper. In particular, axial and flexural constraints imposed by the floor slab on joists and beams of RC structures is evaluated and characterized. The axial constraint imposed by the slab leads to additional compressive force, enhancing flexural response and in turn load carrying capacity of beams and joists. The flexural constraint can be captured by proper modeling approaches and can help improve the response of floor joists and beams. It is shown that the method by which the floor system is modeled has considerable effects on the floor response following loss of load bearing elements. The effects of modeling techniques on the structural response are also evaluated from an energy point of view. The effects of flexural-axial, as well as torsional cracking of the floor slab are evaluated and characterized.