Experimental Study On The Progressive Collapse Resisitance Of Assembled Monolithic Concrete Frame Structures Under A Middle Column Removal Scenario

The assembled monolithic concrete frame structure is a kind of assembled concrete structure which is widely used in China at present.At present,the research on this kind of structure is mainly focused on the seismic performance,but few literature paid attention to its progressive collapse potential.Thus,experimental research and theoretical analysis on assembled monolithic concrete frame structures subjected to the loss of a middle column are investigated to further study the collapse-resistance performance and failure mechanism in the collapse process.The detailed work and conclusions of this paper are stated as follows:Five 1:2 scale specimens were designed and manufactured,including 1 cast-in-situ beam specimens,2 assembled monolithic beam specimens and 2 assembly monolithic frame specimens,and were statically loaded under vertical displacement control load to study the failure mechanism of progressive collapse.According to the test results,under a middle column removal scenario,The failure process of two cross beam above the failure column in turn is cross section cracking,reinforcement yield,concrete crushing,reinforcing bar fracture,local plastic hinge formation,full section tension and collapse failure.The sign of the collapse limit is the fracture of the top reinforcement near the side joint or the arriaval of displacement limitation.The two section of the beam ends and the steel cutting section near the side column is the most serious section.During the collapse process,compression arch action stage,transitional stage and catenary action stage successively experienced.The compression arch action and the catenary action greatly improve the collapse resistance and deformation capacity of the structure.The compression arch action can increase the bearing capacity by 26% and the catenary action can increase the bearing capacity by 80%.The rotation of the plastic hinge at the end of the beam and the local plastic hinge at the truncated position of the top bar near the side column provides a far higher deformation capacity than the standard.By comparing the test results of the cast-in-place specimens and the assembled monolithic specimens,the similarities and differences in the anti collapse performance of the assembled monolithic specimens and the cast-in-place specimens are studied.The results show that the collapse resistance of the assembled monolithic concrete frame structure can be considered as the same as the cast-in-place structure.The two common bottom reinforcement connections,the bending anchor connection and the welding connection will not occur the anchoring failure of the node area.By comparing the test results of two cross beam specimens and frame specimens,the influence of different boundary conditions on the collapse resistance of the structure is studied.According to the test results,the failure of the frame specimen is also concentrated on the two cross beam above the failure column and no damage occurs at the side column and the joint.Due to the small rotation constraint of the frame specimen relative to the beam specimen,the compression arch effect is weaker than that of the beam specimen.,the peak load platform segment is longer.In catenary action stage,the bearing capacity of specimens without external beams is weaker than that of beam specimens while the bearing capacity of specimens with external beams is greater than that of beams.Based on the test,a reasonable assumption is made and the displacement and bearing capacity calculation formulas of 4 characteristic points(steel yield point,peak point of compression arch action stage,steel fracturation points and peak point of catenary action stage)on the load displacement curve are given.The displacement and bearing capacity of the yield point can be calculated accurately by the beam mechanism.Considering only the beam mechanism,the peak load capacity is 20% lower than the experimental value.Load displacement model considering the effect of compression arch action is established by displacement increment method.The have high accuracy to calculate the peak load carrying capacity in the compression arch stage.Based on the proposed model,the effects of the horizontal confining stiffness and the reinforcement ratio on the collapse resistance are studied.According to the results,the level of restraint stiffness is bigger,the bearing capacity is higher.Increasing the reinforcement ratio can also improve the bearing capacity of the structure,and increasing the reinforcement near the middle joints and in top position is more effective.The axial force of the structure before and after the fracture of the steel bar is small,so it can be neglected to calculate the bearing capacity by the flexural member.The displacement can be calculated by calculating the rotation of the plastic hinge of the beam end.At present,no literature proposes methods to calculate displacement at peak point of catenary action stage.The existing methods to calculate the bearing capacity is under the specified deformation and is not suitable for beam specimens with local plastic hinge.Based on the proposed assumption,this paper gives a calculation method for the displacement and bearing capacity of the peak point at the stage of the structural suspension stage,which is suitable for the beam specimens with truncation of steel bars.The arrival of penetrating top bars and all top bars are respectively the upper and lower limits of the bearing capacity.The broken line model with the above four characteristic points as the turning point is established,which is in good agreement with the experimental curve.