Experimental And Theoretical Studv On Mechanical Behavior Of RC Box Girder Subjected To Combined Actions

In-service reinforced concrete (RC) box-girder bridges are generally subjected to the combined actions of bending, shear and torsion. According to the National Natural Science Foundation of China (51178416), the main objective of this study is to explore the mechanical characteristics of RC box girder subjected to bending, shear and torsion. Details of this study are mainly summarized as follows:(1) Based on the truss model, an iterative reinforcement design approach of RC box girder subjected to combined actions was presented. The detailed calculation flowchart was given.(2) Experimental study of four RC continuous box girders with single-column support was conducted by considering different design parameters, in which the proposed reinforcement design approach was used. The elastic mechanical properties of test model under different condition boundaries and loading cases were analyzed. The crack patterns and failure modes of four test models were observed and compared. Also, the effect of different design parameters on the strength and rigidity of box girders were discussed. Finally, the proposed reinforcement design approach was verified by the test results.(3) Based on the elastic-plastic and fracture concrete model in the software of MSC. Marc, finite element (FE) models of the test models were developed, in which the bearing contact simulation and Rots shear crack model were considered. The results show that the whole loading process can be effectively simulated using the presented FE model. The crack patterns and the degradation curve of flexural (torsional) rigidity are in good agreement with the test results.(4) The transverse stability of RC box-girder bridge using single-column support was discussed. Theoretical analysis result shows that the factors of span, cross section and reinforcement arrangement should be considered when checking the transverse stability of RC box-girder bridge with single-column support. Parameter analysis by FE method shows that tensile bearing can obviously improve the stability performance. However, the supporting way of fixed pier and girder has little effect on overturning-resistance performance.(5) Based on the calculation method of flexural bearing capacity, the formula of bending-shear-torsion strength interaction of RC box girder was derived by introducing the truss model and shear-torsion strength interaction curve of RC member. The applicability of the proposed formula was discussed. Then a simplified formula was fitted and given by considering the parameters of height-width ratio, top slab width, reinforcement ratio in top and bottom slabs, concrete strength, and stirrup ratio. The results show that, for coupled bending and torsion member, the proposed simplified formula can be well verified when comprising with literature and FE results. Comparing with model tests of RC box girder subjected to combined bending, shear, and torsion, the predicted interaction curves are conservative.(6) A modified plate-truss model used for the nonlinear whole-process analysis of RC box girder subjected to bending, shear and torsion was proposed. In the proposed model, the truss theory was satisfied in the top and bottom slabs of box girder, the flange slabs were assumed to be non-crack compressive slabs with varying thickness, and the webs were considered to be several truss elements with different equivalent shearing thickness. The result shows that the torque-twist curves, bending moment-curvature curves, and even the load-reinforcement strain curves can be well predicted by the proposed model. The angles of principal stress in different walls of box girder are also well agreement with the test results.(7) Based on the modified plate-truss model, a crack estimation approach of RC box girder subjected to bending, shear and torsion was presented. After introducing the achievements of crack estimation for plane-stress RC member (e.g. shear wall), the parameters of principal strain and equivalent shear thickness were extracted in the whole-process nonlinear analysis. The concrete average crack spacing and width of RC box girder were then analyzed. Comparing with previous model tests of RC box girder, the results show that the predicted average crack spacing for the web of the loading side is in good agreement with the observed value. The load-crack width curves in crack propagation stage by the proposed method conform well to the test results.(8) Based on the modified plate-truss model, the time-varying ultimate capacity of RC box girder subjected to bending, shear and torsion were studied. An average constitutional relationship of steel bar was developed by considering both the bonding-slipping effect and corrosion ratio of steel bars. The calculation flowchart of time-varying bearing capacity was given by the introducing material degradation models in general environment. It is concluded that the predicted mechanical behavior conforms well to the test result for pure torsional and coupled torsional RC members. The torsional strength will decrease by15%after100years in the assumed environment and loading conditions.