Dynamic Response Of Offshore Bridges Under Seabed Ground Motions

Mostly built in southeastern coastal regions,frequent earthquakes of China's offshore bridge projects have exerted serious threat onto offshore bridge structure safety.With large investment and long construction period,the main academic research focuses on how to improve the seismic performance of offshore bridge structure.As for offshore sites,seawater layers and submarine soft soil layers affect ground motion propagation.Thus,there exists difference between seafloor ground motion and land-ground vibration;Meanwhile,offshore bridges are mostly multi-span and long-span structures.Due to ground motion's influence from its traveling wave effect,local field effect and coherence effect,ground motion of different pier bottom varies from each other as well.Besides,mostly built with reinforced concrete structure,the mechanical properties of offshore bridge structure will produce corresponding deterioration if the structure suffers long-term immersion in corrosive seawater;Moreover,since both steel and concrete are strain rate sensitive materials,the mechanical properties of material,the collapse resistance and failure mode of the structure will also change accordingly when earthquake occurs.This paper comprehensively considered types and spatial effects of ground motion,and corrosion effect and strain rate effect of reinforced concrete.Dynamic time history analysis method and incremental dynamic analysis method applied,it studied the seismic performance of offshore bridge structure under various above influences.The details are listed as follows:(1)An offshore bridge taken as research object,ABAQUS finite element software established as software model,and Lanczos eigenvalue method applied as method,it figured out the shape and frequency of bridge structure,and analyzed its dynamic characteristics;It also applied simulated seafloor ground motion and land motion relevant to spatial effect to conduct input,and analyzed the influence of their spatial effect onto offshore bridge structure's dynamic response.The results are like:Spatial effects of seafloor ground motion and ground motion increase dynamic response of offshore bridge structure to different extent,during which the bridge seismic performance would be overestimated if using land-based ground motion input or ignoring ground motion 's spatial effect.(2)It applied reinforced concrete dynamic fiber beam element relevant to material strain rate effect based on ABAQUS software,and analyzed dynamic response of strain rate effect onto bridge structure.The result showed that the influence that strain rate effect of material exerted onto the displacement element of pier top was little,while which became relatively larger onto pier's maximum pier bottom shear force and bending moment,and which increased along with the increase of ground motion intensity onto the pier's top drift rate and bearing shear strain.Hence,under strong earthquake actions,the seismic performance of bridge could be enhanced if taking the strain rate effect of material into consideration.(3)When operating corrosion formulas of reinforced material,it obtained mechanical properties of material after degradation.By changing corresponding mechanical parameters in the model,it studied dynamic response of bridge structure after material degradation,and the results shows:The material corrosion effect amplified maximum pier top displacement,pier top drift rate and bearing shear strain;While,it reduced maximum pier bottom shear force and bending moment value.If corrosion effect ignored,the seismic performance of bridge would be seriously overestimated.The impact of corrosion factors needed to be concerned in seismic design of bridge.