The Study On Flexural Behavior Of Steel-Concrete Composite Continuous Box Girder

With the rapid progress in composite structure,the steel-concrete composite box girder structure has become a development trend in long-span bridges.The conventional composite box girder is formed by connecting open steel box with concrete deck through shear connectors,which in general has relatively thick concrete bridge deck.If the concrete bridge deck adopts the composite slab form,the bridge deck weight can be reduced and the performance and ultimate bearing capacity of it can be improved.In this thesis,experimental and finite element studies were used for exploring flexural behavior of steel concrete composite box girder which integrates a composite slab instead of conventional concrete slab.A total of six composite box girder specimens namely: CBG1,CBG3,CBG4,CBG5 and CBG6(two span continuous)and CBG2(simply supported)were constructed and tested under static loading condition.Basically a four bending test(two point loads on each span)was used until failure.Failure mode,displacement,strain data were then acquired for comparative study.Along with that nonlinear FE models for the six steel-concrete composite box girders were established and analysis were conducted to investigate the flexural capacity and the result was verified with experiment data.Thus using the FE model consisting basic parameters of CBG3,different parametric study was conducted.Based on the static testing and nonlinear FE analysis,the simplified calculation model of the steelconcrete composite beam was established with consideration of the full shear connection and comparison was conducted between the proposed model and the ultimate plastic theory.Some important conclusion was extracted from the research and are listed as follows:(1)From experimental study,the specimen showed good elastic at initial stage and inelastic behavior after yield.It was noticeable that by using a composite slab instead of normal reinforced concrete slab there is substantial reduction in crack formation and propagation in concrete.Moreover,the combined effect of prestressing and composite plate action in negative moment region can enhance the crack control as compared to prestressing in normal reinforced concrete slab.(2)The finite element model established using ABAQUS can predict the elastic,inelastic and ultimate behavior precisely.The existence of steel plate for composite slab can make the distribution of concrete strain in tension zone more uniform,improves the mechanical properties of concrete which is further verified by the FE model.(3)From parametric analysis,the influence of thickness of steel web,concrete depth and grade of steel is greater in the ultimate load carrying capacity.As the ratio stiffness of concrete slab to steel box girder decrease from the initially designed/constructed value,the ductility of the structure increases.So for better ductility performance,lower ratio of stiffness of concrete slab to steel box girder is expected as long as there is full shear connection.The action of shear connector is apparent for lower concrete depth provided steel section remains same but has less effect in full shear connection.(4)Finally from the proposed simplified method to calculate bending moment,the values are compared to experimentally obtained result which shows values at yielding stage is close to each other but it under-predicts the capacity in ultimate stage.Further research needs to be carried out to for the effect of partial action of shear connection before bringing it to engineering design and practice.