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.
