Study On Dynamic Performance And Damage Evaluation Of Reinforced Concrete Deep Flexural Members Under Impact Loading

In recent years,frequent traffic accidents and natural disasters such as falling rocks in mountainous area have made the engineering structure in service period inevitably suffer from extreme impact loads.Extreme impact loads are highly likely to cause structural damage or even collapse.Deeply bent members are widely used in bridges,high-rise buildings and super high-rise buildings due to their high stiffness and strong bearing capacity and they are important bearing members in engineering structures.However,at present,the research on deep flexural members is still concentrated in the field of static load,and there are few studies on dynamic response and damage assessment of deep flexural members subjected to dynamic load,especially impact load.Therefore,based on the finite element software ANSYS/LS-DYNA,the numerical simulation method is used to study the dynamic response of reinforced concrete deep flexural members under impact load and the influence of different parameters on the impact resistance of deep flexural members.At the same time,the damage development process and damage assessment methods of deep-flexural members after impact loads were also studied.The specific research contents and main results are as follows:(1)The key points in the process of establishing finite element model of reinforced concrete deep flexural members under impact load by ANSYS/LS-DYNA are described in detail,including reasonable selection of steel bar and concrete material model,determination of element type,contact type,grid size division and data processing method.The finite element model of reinforced concrete deep flexural members and reinforced concrete shallow beams subjected to impact load is established,respectively.The rationality and accuracy of the numerical model establishment method are verified by comparison with the experimental results in the same working conditions in the literature.(2)Based on the validated model of reinforced concrete deep flexural member,the impact contact process of reinforced concrete deep flexural member under impact load,the stress process of deep flexural member,the energy dissipation mechanism and the distribution of cracks are analyzed.The results show that under impact load,the deep flexural members show typical shear failure characteristics.In the peak stage of impact force,the inertia force is larger,the component deforms locally while the impact force in the descending section is close to the support reaction force.During the impact process,the initial impact kinetic energy is mainly converted into the internal energy of the component,and the bottom longitudinal reinforcement and concrete are the main energy-consuming materials.The yield strength and ultimate strength of longitudinal reinforcement increase with the increase of strain rate.The peak value of impact force and the peak value of bearing reaction can not truly reflect the shear capacity of deep flexural members.(3)Control-variable method was used to analyze the effects of the design parameters such as concrete strength,longitudinal reinforcement ratio,boundary condition and shear span ratio,as well as impact parameters such as impact position,impact mass,impact shape and impact stiffness,on the impact resistance of reinforced concrete deep flexural members.At the same time,orthogonal test was used to analyze the sensitivity of impact mass,impact velocity and concrete strength to maximum impact force and maximum displacement.The results show that the change of concrete strength grade,impact thing shape and impact stiffness has obvious influence on impact force.The variation of the shear span ratio,the longitudinal reinforcement ratio and the impact mass mainly affect the vertical deformation of the deep flexural members.The smaller the shear span ratio,the smaller the damage to the deep flexural members and the stronger the energy consumption of the concrete.Increasing the axial and rotational constraints of the end of the deep flexural members can change the energy dissipation mechanism of the deep flexural members and improve the impact bearing capacity.The change of impact position will affect the damage distribution of deep flexural members and the location where deep bending members are destroyed.The parameter sensitivity analysis shows that the concrete strength has the most significant influences on the impact force peak,while the impact mass has the most significant influences on the displacement peak.(4)The damage formation process of deep flexural members under impact loading is analyzed.Based on the maximum displacement damage criterion,taking impact mass and impact velocity as control variables,the damage assessment curve of reinforced concrete deep flexural members under impact load m-v curve is established by numerical trial calculation method,and the mathematical expression of m-v curve is fitted.