Effect of Concrete Cost on Optimum Design of I-Steel Girder Bridges

Abstract

Composite Reinforced Concrete (RC) Slab on steel I-girder bridges are widely used in the world and the optimum design of these bridges is always raised as a big question. There are many factors effect on the cost of optimum design like length, width of the bridge and else. This paper presents the effect of varying concrete cost, with keeping the steel cost constant, on the optimum design of steel I-girder bridges. The orthotropic plate theory for RC deck slab analysis was used in this study, considering span as simply supported. The problem of optimum cost of steel bridges is formulated as that of minimization of initial cost (IC) for all the bridge, which consists of substructure cost and superstructure cost. The performance constraints in the forms of flexural failure, deflection failure are based on the American Association of State Highway and Transportation Officials (AASHTO) 1989 [14th Edition]. The Sequential Unconstrained Minimization Technique (SUMT) is used to produce required optimizations for costs. A factor called Cost Ratio* which is representing the relation between concrete cost and steel cost was introduced, to show the varying concrete cost comparing with steel cost. The computer program called CPSAO is written with FORTRAN has been built to carry out computation, consisting of the analysis, design and optimization subroutines based on the feasible direction method. It has been considered that the RC deck slab and steel girders as an equivalent orthotropic plate, to find the optimum design for a bridge. To demonstrate the effect of concrete cost on the optimum design of steel I-girder bridges, different lengths of bridges and different values of cost ratio were used. The effect of these parameters on the total cost and others parameters has been presented in graphical forms. It is found that increasing concrete cost (i.e. decreasing cost ratio) leads to increase bridge cost, but meantime makes cost of superstructure and substructure closer.