Study On Ductility Behavior Of Partially Concrete-filled Steel Box-section Bridge Piers

In recent years, large earthquakes often occur. Considering viaducts asan important part of the transportation network, its serious damage duringan earthquake may lead to paralysis of the transport network, thus seriouslyaffect post-disaster relief work. Many earthquakesurveys show that seriousdamage or even collapse of bridges during an earthquake is mainly due tothe destruction of the piers. Compared with conventional reinforcedconcrete piers, steel piers have many advantages: lightweight, convenientconstruction, urban space saving, shape diversity, attractive appearance,simple post-earthquake retrofit measures, higher seismic performance, andso on. Filling concrete into pure steel piers hasthe following two advantages:On one hand, it can improve its ability to resist vehicle impact. On the otherhand, it can improve energy absorption capability and enhance the ductilitybehavior of steel piers.The object of this study is partially concrete-filled steel box-sectionbridge piers subjected toa constant axial load and cyclic horizontal loadingat the pier top. By using the finite element analysis software packageDIANA, three-dimensional and two-dimensional elasto-plastic analyticalmodels are employed to investigate the ductility behavior of partiallyconcrete-filled steel piers. The main contents of this thesis are as follows:(1) Experimental verification of the finite element formulation of steelpiers. Based on the experimental data, three-dimensional elasto-plasticfinite element formulation is employed to predict the lateral load-lateral displacement hysteretic curves and failure modes of steel piers. It isobserved that the analytical results are in good agreement with theexperimental results.(2) Study on the failure modes and damage mechanisms of steel bridgepiers. A series of three-dimensional elasto-plastic analytical models areestablished with four variable parameters, i.e., the flange platewidth-thickness ratio, the column slenderness ratio, the filled-in concreteratio and axial load ratio. A summary of possible failure modes is made asfollows, for the pure steel piers, local buckling occurs at the bottom of thespecimen. For the partially concrete-filled steel piers, when the concretefilling rate is less, local buckling usually occurs at the upper plate adjacentto filled-in concrete. When the concrete filling rate increases to someextent, local buckling will appear near the bottom of the specimen. Inaddition, damage mechanisms of the pure steel piers and theconcrete-filled steel piers are illustrated according to strain contours atmain loading steps.(3) Study on the ductility behavior and stiffness degradation of steelbridge piers under constant axial load and cyclic horizontal loading. Theductility behavior and stiffness degradation of partially concrete-filled steelbridge piers are investigated with different variable parameters, based onthe lateral load-lateral displacement hysteretic curves as well as skeletoncurves.(4) Proposal of simplified numerical method to determine the ductilitybehavior of partially concrete-filled steel box-section bridge piers. Thegeometrical and material properties of two-dimensional beam elementmodels are consistent with the three-dimensional models. By comparingthe numerical results with3-D accurate results, a method to modify theultimate strain of the steel plate near the pier base is proposed. Theductility index of2-D beam element model is determined by multiplyingthe ultimate strain of the bottom plate with a correction coefficient. Thecorrection coefficient is determined to be1.6by fitting method. It is found that the modified2-D numerical results are in line with theexperimental results, which indicates that the proposed simplifiednumerical method and the correction coefficient are accurate and validenough. Finally, the solution of how to design a steel bridge pier withoptimum concrete filling rate is proposed and the optimum concrete fillingrate of the specimens are obtained, which can provide theoretical basis forthe optimum seismic design of concrete-filled steel piers.