Abstract: Elastic finite element analyses are conducted toinvestigate the factors influencing the membrane behavior of steel-concretecomposite floor slabs. Parameters include concrete slab thickness,lateral stiffness of frames and the arrangement of the lateral stiffnesssystem. The results show that the membrane behavior of composite floor slabsis influenced not also by the length-width ratio but also its in-planestiffness and the lateral system. A new method ofevaluating the membrane behavior of composite floor slabs is established basedon the parameter studies. The method is proved to be valid by comparisonwith the test results. The method mentioned in this paper provides auseful tool in composite floor slab design.