Investigation On The Failure Modes And Seismic Performance Of Coastal Bridges In Condition Of Non-uniform Corrosion Along The Elevation

In the marine environment,coastal bridges are usually divided int o five corrosion zones,i.e.,atmospheric,splash,tidal,submerged and sea-mud zone,which will result in the non-uniform corrosion phenomenon along the elevation of the structures.Among them,the bridge column in the splash and tidal zone suffers from more serious corrosion-induced damage due to the dry-wet cycles and sufficient air.At present,the unique corrosion characteristic of the coastal bridges and its influence on structural seismic performance are seldom addressed by the researchers.Understanding the non-uniform corrosion phenomenon and the resulting change of the failure mode and mechanical behavior,determining structural seismic performance variation tendency,are important for the earthquake disaster prediction and disaster risk management.Based on the pseudo-static test,shaking table test and numerical simulation method,the non-uniform corrosion phenomenon is investigated in th is paper,the main research contents,methods and results are as follows:(1)The pseudo-static test of the coastal bridge columns in the marine environment was conducted to investigate structural performance degradation characteristics and seismic performance variation when the splash and tidal zone was located at the middle region.Six cantilever test specimens with a circular section were designed and constructed,and all of them were with the identical geometric size and material properties.The electrochemical accelerated corrosion method was adopted to achieve the corrosion degree from zero to severe at the splash and tidal zone.The damage phenomenon in the cyclic loading test and experimental results indicated that: severe performance deterioration difference between the submerged zone and the splash and tidal zone would induce the shift up phenomenon of the plastic hinge,which resulted in the remarkably unconventional failure mode.Taking the corrosion rate that caused the plastic hinge shift as the turning point,structural seismic performance exhibited a small variation in the left interval and an obvious reduction was observed in the right interval.(2)The time-dependent seismic failure mode of the coastal bridge piers in the whole service life,and the way to distinguish the seismic failure mode and determine the equivalent plastic hinge length were investigated.First,according to the condition of the plastic hinge occurred and formed,five possible seismic failure modes of the bridge columns with non-uniform corrosion were analyzed.Then,the dynamic diffusion process of the chloride ions in different corrosion zone was revealed based on COMSOL multi-physical field analysis software.After that,time-dependent finite element models of the coastal bridge piers were built by Open Sees software,the way to distinguish the number and location of plastic hinge was determined.According to the curvature distribution characteristics along the elevation,the equivalent plastic hinge length calculation method was analyzed.At last,the equations to calculate failure mode transfer moment and plastic hinge length were proposed by fitting the multi-parameters analysis results of the finite element models of costal bridge piers.(3)Based on the developed failure mode distinguish method,the scaled corrosion-damaged bridge columns were designed and subjected to shaking table test.Four test specimens with circular cross section were built,the splash and tidal zone was located at the middle region.The amplitude of the original earthquake record was modulated to different levels for the base input,structural damage evolution characteristics and time-varying seismic response law during earthquake excitation were observed.Test results indicated that the increase of corrosion degree would lead to the increase in the dynamic characteristic parameters and displacement response,and decrease in the acceleration response.In addition,the variation of deformation and curvature distribution form of the test specimens validated the plastic hinge shift phenomenon and the possibility of co-existence of two plastic hinges.The accuracy of the finite element modeling method of the coastal bridge piers was also proved by the dynamic response data.(4)Based on Open Sees software package,time-dependent finite element models of a coastal bridge were established,the quantitative relationship between component’s damage index and seismic intensity was determined according to the incremental dynamic analysis results of a number of seismic records,the fragility analysis and assessment of the coastal bridge components and system in the whole service life was conducted,meanwhile,time-dependent law of the seismic failure probabilities of such type of structures was investigated.The results indicated that: under the earthquake excitation,the damage difference among different components was significant and the bridge pier exhibited the most serious damage.The failure probabilities of slight,moderate,severe and collapse limit states of the bridge system in the whole service life presented the trend of decreas ing firstly and then increasing.