Experimental Study On The Falling Layer Impact Resistance Of Concrete-filled Steel Tube Composite Frame After The Failure Of Key Components

Building structures may suffer from accidental disasters during the whole service life,such as fire,explosion,vehicle impact,terrorist attacks,etc.The accidental load caused by these accidental disasters may cause the failure of a vertical load-bearing component of the structure,resulting in the disproportionate large-scale damage of the building structure to the initial damage,which will bring serious casualties and huge property losses.At present,the research on the impact resistance of concrete-filled steel tubular(CFST)composite frame and CFST composite bolt-welded hybrid joint is not yet complete.Therefore,the impact resistance of CFST composite frame under two different failure conditions is studied in this paper:(1)Two CFST column-composite beam plane frames with two stories and two spans were fabricated.Under the failure conditions of middle column and corner column,the drop hammer impact test was carried out based on the quasi-static loading test.The global and local failure modes of the frame during the impact process were observed,and the collapse mechanism of the structure was analyzed according to the strain of the key sections of each component.It is found that after the CFST composite frame is subjected to three consecutive impacts,cracks develop in the steel beams and slabs near the failure column.The concrete in the local area of the first-story failure column is seriously damaged,and the composite slab and steel beam in the local area of the second-story failure column are completely separated.The trend of the time history curves of impact force is similar.With the increase of the mass and drop height of steel ball,the impact force increases,and the curves conform to the three stages of typical impact.The strain time history curves of each component are analyzed.It is found that the web and upper flange of the steel beam section near the non-failure column are in tension,and the strain value is large and accompanied by obvious oscillation.The steel beams on both sides of the failed column are first compressed and then tensile,after that oscillated slightly.(2)According to the test,it is found that under the vertical load,the weld is easy to crack and the steel beam is prone to out-of-plane buckling.Based on the analysis of the impact resistance of traditional bolt-welded hybrid joint,a local reinforcement method by configurating the cover plate on the bottom flange and using the corrugated web was proposed to delay the cracking of the weld and resist out-of-plane buckling.The finite element(FE)software ABAQUS/Explicit was employed to study the impact resistance of the joint.The results show that the cover plate increases the bending resistance of the steel beam and delays the failure of the joint.The folding effect of the corrugated web delays the fracture of the web.Through parameter analysis,it is found that to reach the optimum impact resistance,it is recommended that the long diameter of the long bolt of the cover plate is 0.08-0.09 times the height of steel beam,and the thickness of the cover plate is in the range of 10-12 mm.Compared with the model with corrugated web thickness of 2mm,the maximum resistance of the model increased by 122.8% when the thickness was6 mm.Furthermore,a simplified calculation method of impact resistance and energy dissipation considering the bending effect and catenary effect is proposed.The tensile process of the corrugated web is equivalent to the spring,and the catenary effect considers the bending bearing capacity and axial bearing capacity of the cover plate.Compared with the experimental and numerical results,the relative errors of the impact resistance predicted by the theoretical formulas is within +10%.(3)The proposed new joint with good impact resistance was applied to the composite frame,and different impact conditions were designed to study the impact resistance of the frame.It is found that under the impact load,the bottom flange weld of the original frame produces cracks and extends to the web.At the same position,the cover plate delays the cracking of the bottom flange of the new frame.The corrugated web has obvious strengthening effect,and the steel beam does not occur out-of-plane buckling.The internal force curves of the specimen are extracted,and it is found that the cover plate and the corrugated web increase the axial force of the steel beam,thereby increasing the resistance of the specimen.The analysis of the interface friction between the cover plate and the contact surface shows that increasing the interface friction within the specification range can effectively improve the resistance of the frame.The resistance of the plane frame can be improved by increasing the bolt preload properly.