Reinforced Concrete Pier Considering Pile-Frozen Soil Interaction Seismic Vulnerability Analysis

Permafrost in China is mainly distributed on the Qinghai-Tibet Plateau.Many projects,including the Qinghai-Tibet Highway,are located in the "Qinghai-Tibet Project Corridor" with a fragile geological environment.In August 2017,the Qinghai Gongyu Highway,the first highway in the permafrost region of the Qinghai-Tibet Plateau,was officially opened to traffic.Permafrost is sensitive to temperature,and its physical and mechanical properties and engineering stability are significantly affected by temperature.In addition,the Qinghai-Tibet Plateau has strong geological structural activity and high probability of earthquake,which poses a great threat to the safety of transportation infrastructure in the region,including Bridges.The most typical is the 2021 Qinghai Maduo 7.4 earthquake,which caused serious damage to the Tongyu Highway Mustangtan Bridge.Because there are few researches on seismic resistance of Bridges in permafrost areas at home and abroad,and the researches on pile-frozen soil interaction theory are relatively weak.Therefore,taking the four-span continuous girder bridge of a highway in our country as an example,the pile foundation pier is selected as the research object.The seismic vulnerability analysis is carried out,and the effects of the changes of four different parameters,the thickness of permafrost layer,the length of free section of pile foundation,axial compression ratio and shear span ratio on the earthquake vulnerability of pile foundation pier are studied.The main research work of this paper is as follows:(1)The theory and method of seismic vulnerability analysis of bridge are introduced in detail,the selection method of seismic wave and ground motion parameters are introduced,the earthquake damage of pile foundation pier in frozen soil area is analyzed,the earthquake damage of pile foundation pier is analyzed,and the failure criteria of bridge structure described from different aspects such as strength and deformation are introduced.Some damage indexes suitable for describing the failure state of pier column are summarized.Finally,through the comparative analysis of the selected damage indexes,the damage indexes of pier and pile foundation are further determined,and the detailed calculation methods are listed.(2)The possible plastic hinge locations of pier and pile foundation members,the constitutive models of concrete and reinforcement,the simulation method of pile-frozen soil interaction,the introduction of soil spring and the determination method of soil quality around pile are discussed,and the three-dimensional finite element model of pile foundation pier is established.On this basis,the moment-curvature analysis of pier section and pile foundation section is carried out,and the calculation process of damage index of pier and pile foundation is introduced.(3)On the basis of obtaining the probabilistic demand model of bridge pier and pile foundation,the log-linear regression analysis is carried out on the time-history analysis results of bridge pier and pile foundation in permafrost area,and the seismic vulnerability curves of bridge pier and pile foundation are established respectively.The finite element calculation results show that the failure probability of bridge pier and pile foundation increases significantly with the increase of ground peak acceleration PGA.Under the action of earthquake,the pier will be destroyed first to a large extent.The pier is a vulnerable component with low seismic capacity,which acts as a protective component in the seismic design of bridge.When the PGA is small,the pier and pile foundation are prone to slight and moderate damage.With the increase of PGA,the pier and pile foundation are prone to serious damage,but the collapse failure is not easy to happen.The pile foundation has better seismic performance.Based on the exceedance probability values of pier and pile under different damage states,the vulnerability curves of pile foundation pier system are established by using the first-order boundary estimation method.(4)Seismic vulnerability analysis of pile-frozen soil interaction system under different influencing factors is carried out,and seismic vulnerability analysis of pile-foundation pier and pile-foundation pier system is carried out for M2～M9 pile-frozen soil interaction model.The effects of the thickness of the thawing soil layer,the length of free section of pile foundation,axial compression ratio and shear span ratio on the seismic vulnerability evaluation results of pile foundation pier and pile foundation pier system are studied.