Cost optimization of post - tensioned prestressed concrete I - Girder bridge system

Abstract

Optimum design of a simply supported post-tensioned prestressed concrete I-girder bridge system is presented in the thesis. The objective is to minimize the total cost of the bridge superstructure system considering the cost of materials, fabrication and installation. For a particular girder span and bridge width, the design variables considered for the cost minimization of the bridge superstructure system, are girder spacing, various cross sectional dimensions of the girder, number of strands per tendon, number of tendons, tendon configuration, slab thickness and ordinary reinforcement for deck slab and girder. Explicit constraints on the design variables are considered on the basis of geometric requirements, practical dimension for construction and code restrictions. Implicit constraints for design are considered according to AASHTO Standard Specifications. The present optimization problem is characterized by having mixed continuous, discrete and integer design variables and having multiple local minima. Hence a global optimization algorithm called EVOP, is adopted which is capable of locating directly with high probability the global minimum without any requirement for information on gradient or sub-gradient. A computer program is developed to formulate optimization problem which consists of mathematical expression required for the design and analysis of the bridge system, three functions: an objective function, an implicit constraint function and an explicit constraint function and input control parameters required by the optimization algorithm. To solve the problem the program is then linked to the optimization algorithm. The optimization approach is applied on a real life project which leads to about 35% cost saving while resulting in feasible and acceptable optimum design. As constant design parameters have influence on the optimum design, the optimization approach is performed for various such parameters resulting in considerable cost savings. Parametric studies are performed for various girder spans (30 m, 40 m and 50m), girder concrete strengths (40 MPa and 50 MPa) and three different unit costs of the materials including fabrication and installation. Optimum girder spacing is found higher in smaller span than larger span for both concrete strengths and for all the cost cases considered. Optimum girder depth increases with increase in cost of steel. Top flange thickness and top flange transition thickness remain to their lower limit in all the cases. Optimum web width is found about 150 mm in all of the girder spans and for both concrete strength. Number of strands is about 12.5% more in higher concrete strength than lower concrete strength. Optimum deck slab thickness is higher in shorter span. Girder concrete strength has no effect on optimum deck slab main reinforcement. The cost of bridge superstructure increases about 13% to 14% for 10 m increase in girder span.