Investigation On The Seismic Performance Of Non-uniform Corroded Coastal Bridge Piers Under Bidirectional Earthquake Excitation

In the long-term service period,coastal bridges sustain continuous performance deterioration due to the high humidity and chloride concentration.As the key structural element of coastal bridges,pier columns suffer more serious non-uniform corrosion damage than that of the superstructures.As a result,the performance deterioration of these columns are non-uniform along their elevation.In earthquake-prone regions,coastal bridge structures with corrosion-induced damage face a more severe challenge due to the multiple hazard actions of earthquake and corrosion.At present,the studies on seismic behavior of coastal bridges are mainly focused on unidirectional earthquake excitation.However,few studies were carried out for coastal bridge columns under bidirectional earthquake excitation.Revealing the damage modes of non-uniform corroded bridge columns under bidirectional earthquake excitations and understanding the load paths effects on their mechanical behavior is of great significance to the life-cycle risk assessment of columns and the whole bridge structures.Based on the biaxial pseudo-static test,theoretical analysis and numerical simulation method,the biaxial seismic performance of corroded bridge columns are investigated in this study.The main research contents,methods and results are as follows:(1)Biaxial pseudo-static test on the cyclic behavior of non-uniform corroded bridge piers was carried out.Seven identical cantilever test specimens with a rectangular section were designed and fabricated.These specimens were corroded in the designed splash and tidal zone with different levels by using the electrochemical accelerated corrosion method.Low-cycle pseudo-static tests were carried out under unidirectional,rectangular and cross-shaped loading paths.The test results showed that:under the same biaxial load protocol,the strength,deformation,and energy dissipation capacity decreased with the increase in corrosion level.With the same level of corrosion,the seismic performance of a bridge pier under uniaxial loading was better than that under biaxial loading.The coupling effect of the rectangular load action was observed from the increasing tendency of the phase lag between the displacement and resultant horizontal force with the increase in the lateral deformation acting on these specimens.(2)The method to calculate the sectional strength of non-uniform corroded pier columns subjected to unidirectional and bi-directional earthquake excitations was proposed based on a discretization approach.First,the simulation methods of material properties and geometric dimension degradation for steel and concrete in different service periods were given.Second,the cross-section of a bridge pier was discretized into a series of steel fibers and concrete fibers.Based on the internal force equilibrium method,the M-N interaction curve,M_x-M_y-N failure surface and the shear strength formula of non-uniform corroded bridge pier were established.At last,the proposed sectional strength calculation method was validated by using two groups of test data and an example of coastal bridge pier was analyzed.(3)The numerical simulation method for seismic performance of coastal bridge piers considering bond property deterioration between reinforcement and concrete was improved.Based on the simplified bond stress-slip constitutive model of un-corroded and corroded reinforcing bars,second-order ordinary coefficient differential equations of slip were established by the method of internal force equilibrium.By solving these equations with the given boundary conditions,the axial stress-slip curves of un-corroded and corroded reinforcing bars were established.After that,the elastic modulus of corroded reinforcing bar was modified which could include bond slip effect indirectly.This numerical simulation method was proved by experiment data and then the parametric study was conducted.The simulation results showed that the seismic performance of locally corroded pier specimens could be better modeled with the inclusion of bond slip effect.With the increase of corrosion region length and plastic hinge length,the influence of bond slip on the seismic performance of specimens such as energy dissipation,initial stiffness and unloading stiffness increased.(4)The loading path effects on seismic performance of non-uniform corroded pier specimens was investigated by means of finite element method.The hysteretic curves of specimens with five different loading paths and three different corrosion degrees were simulated.The strength,deformation,stiffness and energy dissipation of these specimens were analyzed,and the biaxial damage index formula based on Park-Ang model was proposed.The modeling results indicated that:the strength and deformation capacity was lower for column specimens subjected to biaxial loading when compared with unaixal loading.Bidirectional loading led to higher energy dissipation and larger damage index than unidirectional loading.Under the bi-directional loading,non-linear deformation caused the difference between displacement and load vectors,which reflected the coupling effect of specimens.