This paper shows a mathematical model, which assists in finding a notion representing interactions between the elements of the cable system. This approach allows to find influence of the load-bearing element stiffness on any of the optimized parameters – the allowable deflection, permissible stress, consumption of materials, structural reliability index. The proposed analytical method gives reasonable initial assumptions about the optimal cable stayed bridge geometry, as well as predictions about the influence of the individual element stiffness on the overall structural behavior. Method presented in this paper shows mathematical and graphical correlation between tension forces in cables and fluctuations of stresses in stiffening girder due to variable loads. Study of uncertainties of variable loads allows to find the influence of stiffness and geometry of cables on structural reliability of stiffening girder, cables and whole construction of the bridge. The optimal lifespan of main elements can be find.