Research On Ductility Behavior Of Stiffened Steel Pipe-section Bridge Piers

The viaduct is a very important part of the urban transportation network as a fast road or a dedicated road which connects the important functional areas of the city.When the earthquake occurs,it often becomes the most vulnerable parts of the transportation network due to its special structural type.A large number of investigations on seismic disaster show that the collapse of the viaduct is often caused by the damage near the pier base.The most common type of bridge piers are reinforced concrete piers.However,because of its heavy self-weight and poor ductility performance,they always show a lack of deformation capacity and brittle shear failure under earthquakes.Steel bridge piers are widely used in Japan and other countries due to its light weight,easy construction and good ductility.Steel bridge piers are liable to locally buckle at the pier base,which causes rapid loss of ductility during a severe earthquake.Therefore,it is of great significance to study the ductility behavior of steel bridge piers.In this paper,the ductility of steel pipe-section bridge pier with eight stiffeners is investigated.Firstly,the bearing capacity and ductility performance under a constant axial load and cyclic horizontal loading are parametrically studied.The empirical formulas to predict ultimate strength and ductility ratio are proposed.Then,the mechanical performance of steel stub columns with or without stiffeners under axial load are studied.And a formula is proposed to give the critical height-diameter ratio.On this basis,the ductility of stiffened pipe-section stub columns with stiffeners,subjected to axial load and bending,is studied.And some formulas are put forward to predict the ultimate strain of such type of steel stub columns.The proposed ultimate strain formulas can be employed to judge the failure of the piers in a nonlinear dynamic time-histoty analysis on the basis of beam-column element model.The main research contents and achievements are as follows:(1)Two finite element models of steel bridge piers using a modified two-surface constitutive model(2SM)for steel are established and analyzed.By comparing the horizontal load-displacement hysteretic curves at the pier top and the failure modes at the pier base obtained from the numerical analysis with test results,the finite element modeling method and the modified two-surface constitutive model are verified to predict the ductility behavior and failure modes of such type of steel piers with a satisfactory accuracy.(2)On the basis of experimental verification,a series of finite element models of steel piers with different normalized diameter-thickness ratio,normalized slenderness ratio of stiffeners,normalized slenderness ratio of the piers and axial load ratio is established.Through the parametric studies,the results show that the ultimate strength and ductility ratio of the steel piers are improved with the decrease of normalized diameter-thickness ratio,stiffeners' normalized slenderness ratio,normalized slenderness ratio and axial load ratio.Based on this,some formulas to predict the ultimate strength and ductility ratio of this type of steel piers are proposed.(3)A series of unstiffened stub columns with different normalized diameter-thickness ratio under axial load are analyzed,considering initial geometrical imperfection and residual stresses.A formula for predicting the critical height-diameter ratio of such unstiffened stub columns are proposed.By comparison with the results of Timoshenko theoretical formula and Gao formula,the reliability of research method and the accuracy of the calculation are verified.(4)By setting different normalized diameter-thickness ratio,normalized slenderness ratio of stiffeners and axial load ratio,the critical height-diameter ratio of stiffened stub columns are studied.On the basis,the ductility of the steel stub columns with pipe section under both compression and bending is studied,and some formulas to predict the ultimate strain of such steel stub columns are put forward.The formula can be employed as the failure criterion of steel bridge piers in a dynamic time-history analysis based on beam-column element model.