Resistance Of Planar Composite-framed Substructures In Falling-debris-impact Scenario

Within the service life,the structure may encounter unexpected events such as explosions and impacts,leading to the initial failure of some structural elements.This initial damage may cause a chain reaction to expand the scope of damage to the structure,and lead to progressive collapse of structures,having caused heavy casualties and property losses.The falling-debris-impact scenario is one of the reasons for the progressive collapse of structures.It assumes that the structural elements drop down due to accidental load and impact the remaining structures below.The falling-debris-impact scenario occurred in the collapse of the Ronan Point apartment in London,the collapse of the World Trade Center in New York,and the recent collapse of the apartment in Florida.Therefore,it is of great practical significance to investigate the resistance of structures subject to falling-debris impact.The composite structure has many advantages such as high load-carrying capacities,good seismic performance,and convenient construction,and it is also widely used in practical engineering.However,there are relatively few studies on its impact resistance.This thesis focuses on the impact resistance of composite framed structures under the falling-debris-impact scenario,the main work includes:(1)Pre-analysis for the design of impact tests was carried out using ANSYS/LSDYNA software.Based on the existing tests,the finite element model of steel structure and composite structure impact test was established,and the accuracy of the model was verified according to the test results.Design of combined structure specimen for impact test was completed and the dynamic behavior of composite structures with reversechannel connections was investigated by numerical analysis based on the verified finite element model.The simulation shows that when the total impact energy is constant,increasing the impact speed and the number of impacts will help the structure resist impact loads,while the change of impact mass and small-scale load eccentricity and deflection does not affect the impact resistance of the structure.The numerical results provide suggestions for impact test design.(2)Impact tests of three composite framed specimens with different types of connections have been completed.The three connections include Welded Unreinforced Flange-Bolted web connections(WUF-B),Fin Plate connection(FP),and Reverse Channel connection(RCC),which represent rigid connection,hinge connection,and semi-rigid connection,respectively.A typical impact process was divided into 3 stages according to the test data.In stage 1,the specimen resists the impact force mainly through inertial effects,and in stages 2 and 3,the specimen resists the impact force mainly through structural resistance.In the first two stages,the specimen absorbs the impact energy,and in stage 3,the specimen releases the impact energy stored through elastic deformation.The structural resistance of specimens is provided by flexural action(FA),compression arch action(CAA),and catenary action(CA).Among them,the contribution of CAA is limited and can be ignored.FA always provides the most structural resistance contribution in the whole impact process,and CA provides structural resistance under large deformation.The specimen structural resistance increases with increasing deformation before the failure of connections.Favorable flexural resistance and deformation capacity are the key factors for structures to resist impact load.The RCC specimen has the best flexural resistance and deformation capacity,so it has the best impact resistance,followed by the WUF-B specimen,and the FP specimen is the worst.(3)Finite element simulation of the impact test was completed.Numerical analysis proves that a single impact with increased impact mass can be used to simulate the results of multiple impacts,and the finite element simulation of multiple impacts of three specimens has been completed.Parametric analysis was carried out based on the validated finite element model.The results show that compared with bare steel structures,the composite plate can increase the energy absorption capacity of the WUF-B specimen,the FP specimen,and the RCC specimen by 29.5%,17.6%,and 13.0%,respectively.The increase of the strength or thickness of concrete in the composite slab,the thickness of profiled steel plate,the ratio of longitudinal reinforcement,and the strength of shear connection of the composite structure are all conducive to the impact load resistance of the structure.Among them,the impact resistance of WB specimens is greatly influenced by the thickness of concrete and the thickness of profiled steel plate,while FP and RCC specimens are most influenced by the ratio of longitudinal reinforcement.The increase of the Span-Depth Ratio will increase the failure displacement and decrease the maximum structural resistance and energy absorption capacity.Compared with the static loading condition,the energy absorption capacity of WUF-B and FP specimens under impact loading increased by 12.9%and 8.9%,respectively,while the energy absorption capacity of RCC specimens decreased by 6.1%.