Abstract: The characteristics of load effect function are studied under full-span and half-span loading conditions. First, the reason for the nonlinear load effect function is discussed. Then a general model of load effect function is proposed through the ultimate load analysis of some examples under full-span and half-span loadings. It is indicated that the function model is accurate at both the structural member level and system level. Finally based on the approximate function model, the nonlinear degree of the load effect curve is discussed. The results show that the curve is a parabola or hyperbola. It provides some references on structural reliability calculation.