Life-cycle Seismic Fragility Analyses Of Offshore Bridge Structures Subjected To Spatially Varying Seafloor Motions

To optimize the traffic network and promote the economy development in the coastal areas,China has constructed a certain number of large-scale sea-crossing bridges in its southeast coast,which locates at the Circum Pacific seismic belt.The structural safety of offshore bridges can be seriously threatened by the frequent earthquake events.Offshore bridge structures possess the characteristics of long service life and high investment cost,their damages under earthquakes may cause huge economic loss and significant social influence.How to solve the seismic safety issues confronted by the offshore bridges during their life-cycles has become a great challenge for the researchers in the bridge seismic field.Offshore bridge structures locate at complex marine geological sites.The propagation of earthquake waves can be significantly affected by seawater and subsea soil layers,which can result in obvious characteristic differences between the onshore and seafloor seismic motions.Additionally,the seismic motions at multiple supports of offshore bridges can be very different due to the ground motion wave passage effect,coherency loss effect and local site effect.Therefore,spatially varying seismic motions on the offshore sites should be used as inputs in the seismic analysis of offshore bridge structures.Moreover,the chloride induced corrosion effect on the offshore bridges located in marine environments can lead to serious structural performance degradation.It is of great significance to propose a life-cycle seismic performance evaluation method for offshore bridge structures considering the effect of chloride induced corrosion.Based on this background,the simulation method of spatially varying seafloor motions and the life-cycle seismic fragility analysis method of offshore bridge structures are studied in this paper.The major contents are listed as follows:(1)The dynamic stiffness matrix of seawater layer subjected to seismic P-wave excitation is theoretically derived based on one-dimensional wave propagation theory and fundamental hydrodynamic equations.By considering the effect of pore water on the P-wave velocity and Poisson's ratio of subsea soil layers,the dynamic stiffness matrix of an offshore site is assembled and the ground motion transfer function is calculated.Three-component seafloor seismic motions are stochastically synthesized using the spectral representation method.The simulation results show that the characteristics of the synthesized seafloor motions are in line with actual seafloor earthquake recordings.The vertical seafloor seismic motions are significantly suppressed near the P-wave resonant frequencies of overlaying seawater layer due to the destructive interference effect.Therefore,the vertical-to-horizontal PGA ratios of seafloor motions are much lower than those of onshore motions.(2)Based on the calculation method of offshore site transfer functions,two simulation methods of spatially varying seafloor motions are proposed by respectively using base rock seismic spectral model and actual onshore earthquake recordings.The effects of wave passage,coherency loss and local site are all considered in the simulation.The results show that the power spectral density functions of the simulated spatially varying seafloor motions are compatible with respective target values.Moreover,the lagged coherency losses between spatial vertical seafloor motions are much lower than those between spatial base rock vertical motions,especially at the P-wave resonant frequencies of overlaying seawater layer.(3)A life-cycle seismic fragility analysis method for offshore bridge structures is proposed by considering the effect of chloride induced corrosion.Based on the time-varying model of chloride corrosion current density,an equation for the corrosion degree of reinforcements in offshore bridge structures is derived.By considering the uncertainties regarding the structural,material and corrosion parameters,the finite element models of offshore bridges at different time steps are constructed.The life-cycle seismic fragility of an offshore reinforcement concrete continuous rigid frame bridge subjected to spatially varying ground motion's is analyzed.The numerical results show that the seismic fragility of the example offshore bridge increases during its life-cycle.At the 90th year after corrosion initiation,the seismic fragility mean PGA required to reach a same damage limit state is nearly 40%lower than that of the intact bridge.Moreover,the life-cycle seismic fragility of the example bridge can be significantly affected by the ground motion wave passage effect,coherency loss effect and local site effect.It is necessary to use spatially varying ground motions as inputs in the life-cycle seismic fragility analysis of offshore RC bridges.(4)By considering the effects of soil-structure interaction(SSI),hydrodynamic added mass and chloride induced corrosion,the finite element model for a sea-crossing cable-stayed bridge is constructed using OpenSees.Soil spring elements based on the P-y,T-z and Q-z materials are employed to connect the bridge piles to the foundation.Life-cycle seismic fragility of the example cable-stayed bridge is analyzed by using offshore site spatially varying seismic motions as inputs.The numerical results indicate that the offshore seismic motion inputs,ground motion spatial variation,SSI,hydrodynamic added mass and chloride induced corrosion can all influence the seismic fragility of the example sea-crossing cable-stayed bridge with different extents.The methodology proposed in this paper can yield more reasonable seismic fragility predictions for the sea-crossing cable-stayed bridges during their life-cycles.