Optimal design of multi-span structures with a double cross-section

The results of optimization analyses of structures with locally doubled cross-sections have been introduced. The purpose of this paper is to determine the optimal lengths of strengthened areas ai for two, three, four and five-span systems. The optimal length ai is defined as such a one, for which the structures achieve their maximum resistance at the least consumption of steel. Solving this problem, the variable length ai and stiffness EJi of local strengthenings in areas of internal supports and external bays have been taken into account. Different structural solutions have been analyzed due to the consumption of material and labour. The tested structures have been modelled by beams with changing bending stiffnesses. Statically indeterminate systems have been solved by the integration of differential equation of the IV order, describing the deflection curve. Practical recommendations concerning the optimal ways of local strengthening have been presented.