Study On The Failure Mode And Dynamic Response Of Precast Assembled Segmental Beams Under Impact Loading

Segmental precast and assembly construction,with its fast construction speed,easy-to-control quality,less environmental disruption,good durability,and economic and environmentally friendly and many other advantages,the application is increasing in China’s bridge construction in recent years.Compared to traditional monolithic cast bridges,segmental bridges are discontinuous at the joints and have reduced shear capacity.Impact loads,such as those from falling objects and heavy vehicle rollovers,pose a threat to the safety of such bridges and need to be taken seriously.The structural design theory and service safety assessment method of segmental assembled bridges based on impact resistance are technical needs in the engineering community.Currently,there are relatively few studies on the impact resistance performance of precast assembled segmental bridges.This study investigated the failure modes and dynamic responses of segmental assembled beams under impact loading in depth,by means of drop hammer impact tests and numerical simulations in this study.The main research contents and results are as follows:(1)In this study,drop hammer impact tests of segmental assembled beams were carried out,and LS-DYNA finite element numerical models were established to systematically analyze the crack development process,final failure mode,and the response of mid-span deflection,impact force,support force,and acceleration.The results show that the local shear failure of segmental assembled beams under impact loading is prominent,and the typical failure modes are oblique section failure and joint section direct shear failure.The relative position of the impact point and the joint significantly affects the failure mode.When the impact position is in the middle of the segment,the segmental beam will occur oblique section failure,with the main failure crack extending diagonally from the bottom of the joint to the impact position without bending cracks in the mid-span.When the impact position is near the joint,the segmental beam will occur direct shear failure of the joint section,with the main failure crack extending vertically from near the bottom of the joint.The cracks are primarily shear cracks,with few in number and a concentrated distribution.(2)The impact behavior of segmental assembled beams was analyzed in depth based on finite element numerical models,and the influence of impact parameters(impact load position,impact energy)and important design parameters of segmental beams(boundary conditions,concrete strength,prestress level)was investigated.The results show that the displacement response time of the segmental beam is longer than that of the monolithic beam,going through two phases of down and up deflect.The residual deformation is small and cannot be used to measure the degree of damage;segmental beams mainly rely on the deformation of concrete and prestressed tendons to dissipate impact energy.The effect of ordinary reinforcements is smaller,and the shorter the segment,the smaller the effect.When the impact position differs,the damage is only distributed inside the loaded segment and at each joint,with no obvious damage inside the other segments.The displaced profile shows a “broken line” shape without obvious bending characteristics.The shear force of the section near the impact position is the highest,approaching 40% of the peak impact force,leading to local shear failure of the segmental beam.The influence of boundary conditions and span on the initial response and damage characteristics of segmental beams under impact is relatively small.The increase in impact energy will deepen the damage,but will not change the final failure mode.The improvement of concrete strength and prestress level can effectively reduce damage and improve impact resistance.The early response characteristics of impact force are greatly influenced by impact energy and concrete strength,while they are less affected by prestress levels,boundary conditions,and spans.(3)The damage assessment method based on the section damage factor quantified and characterized the damage characteristics of segmental assembled beams under impact loading,taking the damage index of the most severely damaged cross-section as the overall damage index of the beam.Combining the finite element method and response surface analysis method,the vulnerability analysis of segmental assembled beams under impact load was carried out.The results show that concrete strength and confining stress on the joint surface are key design parameters for the impact resistance of segmental assembled beams.The response surface surrogate model can accurately predict the overall damage index and the failure probability of segmental beams under different impact conditions,and the optimal parameters can be designed.The impact resistance of segmental beams is significantly weaker than that of monolithic beams.When the impact energy is in the range of 1600 J to 6400 J,the monolithic beams are mainly slightly and moderately damaged,while segmental beams are mainly moderately and severely damaged.The higher the concrete strength is,the better the effect of confining stress on improving the impact resistance of segmental beams is.and the optimal parameters can be designed.(4)The response characteristics of inertia force,shear force and bending moment in the local response phase and the distribution along the length of the beam were investigated.The mechanical characteristics of the joint section were clarified by the dynamic shear span ratio,and the damage mechanism of segmental assembled beams under impact loading was investigated.The results show that before and after the peak impact force,the distribution of inertia force along the length of the beam is in the shape of “V” and “W”.The higher the impact energy,the more concentrated the inertia force distribution towards the loaded segment;The shear force distribution is in the shape of “N”,with the maximum shear force near the impact position,and the local shear force characteristics are obvious.The bending moment distribution is in the shape of “M”,and the negative bending moment appears at the initial stage far from the mid-span position,causing slight damage to the upper part of the joint.The dynamic shear span ratio can reflect the mechanical characteristics of the joint section in the local response phase,and can be used to evaluate the shear failure behavior of segmental assembled beams under impact loading,The shear failure of the joint section occurs when the dynamic shear span ratio is much less than 1.The span has less influence on the final failure mode of the segmental beam,and the mechanical characteristics of the joint section of the segmental beam in the local response phase are the same for different spans.The impact resistance of the segmental beam depends on the dynamic shear capacity of the joints.