Optimization of Prestressed Concrete Girders for Bridge Design, Master’s Thesis

Abstract

Structural concrete bridge girders forms almost major portions of total cost of superstructure and they appear deep particularly for large span bridges as compared to ordinary beams to meet the required stiffness and stability. Enlarged size of the girders makes the overall cost of the bridge be costlier and to cope with this, great cost saving was possible to achieve through the use of structural design optimization. In this research, design optimization was carried out by taking total material cost of girders as an objective function and all requirements of strength, stability, serviceability, fatigue and geometric restrictions as constraint functions. A straight girder system bridge with a total width of 9.9m and supporting dual lanes of traffic with standard width of 3.65m each and 1.3m wide overhang both sides was used. It was subjected to three main load cases, the action of dead, live and prestressing loads. Dead load includes self-weight of bridge deck components, railings, girders, diaphragms and wearing surface. Live load was the design vehicular live load of AASHTO LRFD HL-93. Prestressing force was based on maximum tensile prestress at the top fiber and minimum compressive prestress at the bottom fiber. Other load effects like impact factor and multiple presence factor were also taken into account. Linear static method of analysis was used. A program was developed for design optimization of prestressed concrete girders in MATLAB R2017a software. In this study, effects of construction materials, grades of concrete, girder spacing, bridge length on the optimum cost were investigated. The results of optimization indicates that reinforced concrete (RC) T girder was economical up to a span of 40m and for a span longer than 40m prestressed concrete (PC) box girder was better. It was observed that as grades of concrete increases depth of the girders reduces, for bridge supporting dual lane of traffic, an optimum girder spacing was found to be 2.5m. Optimum design of prestressed bridge girders could reduce cost of material with 38% for prestressed concrete T girder and 25% for prestressed concrete box girder as compared to the cost of conventional design approach.