Optimization of continuous post-tensioned concrete bridge girders with variable depth

Post-tensioned single-cell concrete box girders of variable depth are used in continuous bridges to achieve both economy and aesthetics. To determine the minimum prestressing for a given bridge deck profile, the minimum cost design of a continuous single box girder of non-uniform depth is achieved through a constrained optimization procedure. This study considers both two and three span continuous bridge girders subjected to American Association of State Highway and Transportation Officials (AASHTO-96) HS Bridge loading. Both short and long tendons are used in order to obtain the optimum prestressing force. The nonlinear problem is solved by transforming it into a linear one by using a new design variable, which proportions long and short tendons.;A computer code (PCPCBGND) is developed by using a standard FORTRAN for analysis and optimum structural design of bridge girders where a gradient search technique is used to solve iteratively the optimization problem. The design variables include: interior to exterior span ratio for three-span, the depth profile of the cross-section, tendon eccentricities, and prestressing force. The total cost of the member considered is the cost of structural materials (concrete and prestressing steel), excluding the formwork. The design constraints are the prescribed limits of working stresses, the strength, and serviceability requirements. Several examples are solved by using the design required by the American Institute (ACI-343R-95) to demonstrate applications and some important findings. The study shows that optimum prestressing is attained only at an optimum proportion of long and short tension. Furthermore, for an economical design, the ratio of interior to exterior span for a three-span continuous girder should be from 1.30 to 1.40.