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.
