Study On Seismic Damage Analysis And Control Of Offshore Long-span Bridges

With the growing demands of transportation networks,the economic development,and the advance of science and technology in China,the numbers and scales of offshore long-span bridges are growing rapidly in recent years.The coast of China is located between the circle Pacific seismic belt and Eurasian seismic belt where strong earthquakes happen frequently.Once the offshore long-span brides fail under strong earthquakes,it will bring significant negative impact upon the economic and social development of China.As the offshore long-span bridges have not experienced extreme seismic actions thus far,it has significant theoretical and practical importance to study their failure process and the methods to control the failure modes to prevent the bridge structural collapse.In this dissertation,focusing on the structural characteristics of the offshore long-span bridge structures,the damage mechanism of the long-span reinforced concrete bridges and the damage control methods of the long-span bridges based on magneto-rheological(MR)dampers are intensively studied,and then the seismic damage analysis and control methods of offshore long-span bridges are proposed.The main conclusions can be summarized as follows:(1)Based on basic principle of continuous damage mechanics,a pressure damage evolution equation of concrete is deduced from the effective stress,and an elastic-plastic damage constitutive model of concrete is proposed according to the stress-strain curve of concrete.The simulation results for uniaxial tension and compression tests of the concrete show that the plastic deformation and stiffness degradation can be well described using the proposed damage model.The proposed model is also validated that it can well simulate the damage evolution of core and cover concrete of the pier under earthquakes through simulating the shaking table test of a reinforced concrete pier.In addition,the model parameters are simple and have definite physical meaning.Therefore,the new developed model is suitable for engineering application.(2)Considering the damage mechanism of reinforced concrete structures under seismic excitations,the stiffness degradation and the free energy degradation of damaged reinforced concrete members are deduced based on the models for steel and concrete.They are further combined to form a damage model for reinforced concrete members.Pseudo-static tests of reinforced concrete columns are simulated.The analytical results indicate that the proposed damage model can describe the damage evolution process of reinforced concrete columns well;also,it can quantitatively determine the damage level of reinforced concrete piers in the orthogonal direction to the unique loading direction.A shaking table test of reinforced concrete piers and a numerical example of a reinforced concrete rigid-frame bridge are also analyzed.The results show that the proposed damage model can describe the degradation of seismic resistance of the reinforced concrete piers and predict failure modes of structures.Thus it can be used to analyze the damage of bridges under seismic excitations.(3)A new method is proposed to define the interface and the detailed area of the multi-scale model for reinforced concrete columns.In the detailed area,a multi-axial damage model is developed for the concrete through the secondary development by using LS-DYNA.To verify the proposed multi-scale model method,quasi-static tests and monotonic pushover numerical tests of reinforced concrete columns are simulated by also comparing with the detailed models.The results show that the new method is accurate and cost effective.Finally,shaking table tests of reinforced concrete columns are analyzed,and the results show that the failure process of reinforced concrete columns such as concrete crushing and steel bar buckling under strong earthquakes can be directly observed.In addition,a continuous reinforced concrete bridge is established using LS-DYNA.The damage of the bridge under strong earthquakes is analyzed,and the numerical results show that the proposed method can capture the seismic induced failure for the bridge.The interaction of different failure modes of the bridge is examined.(4)The method to analyze the seismic damage of bridges under multiple mediums action is established.In this method,the pile-soil interaction is simulated using the lumped mass model,and the hydrodynamic pressure of piles and piers is considered by adopting Morison equation and radiation wave theory.The damage model for bridge structures is defined based on the damage model for reinforced concrete members.The seismic damage analyses of low-pile foundation bridges and high-pile foundation bridges are conducted by considering separate water-structure interaction,pile-soil interaction,different shear wave velocity of soil and multiple mediums action.For the low-pile foundation bridge,the results indicate that the damage index is increased by the water-structure interaction and reduced by the pile-soil interaction.The hydrodynamic pressure hardly affects the bridge damage when the shear wave velocity of soil is small under multiple mediums action.For the high-pile foundation bridge,the results indicate that the influence of the damage index for the structure by the water-structure interaction is relatively small under earthquakes when the water is below the bottom of the cap,the damage index is increased for piles,and reduced for piers by the pile-soil interaction.The depth of the water is found to hardly affect the piers and piles under multiple mediums action.(5)A new real-time semi-active control algorithm based on the damage of bridge members(RTSD),employing seismic-active controllable magneto-rheological(MR)dampers,is proposed to control both the bearing displacements and the pier damages at the same time under earthquakes with various intensities.In the proposed control method,the real-time feedback signals of the bearing displacements and pier displacements are acquired to calculate the displacement indices of bearings and the damage indices of piers,which are the control signals for the output forces of MR dampers.In addition,the damage indices of bearings and piers are introduced as the evaluation criterion for the control method.The performance of the proposed RTSD control is investigated by comparing with two passive controls in the seismic analysis of a three-span reinforced concrete continuous bridge.The results show that the notable advantages of the proposed RTSD control are the reduced damage indices of bearings and piers at the same time under strong earthquakes comparing with the two passive controls.The damper force can be automatically adjusted by the RTSD control to protect both the bearings and piers under various earthquakes.The RTSD control can also control the structure within different prescribed targets under various earthquakes.