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

The assembled monolithic concrete frame structure has been widely applied in the engineering field due to its good economic and environmental benefits.The existing research is mainly aimed at its seismic performance,but few literature paid attention to its progressive collapse potential.Therefore,experimental research and theoretical analysis on assembled monolithic concrete frame structures subjected to the loss of a middle column are carried out 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:Four 1/2 scale beam-column substructure specimens were designed and manufactured,including 1 castin-situ beam specimens,2 assembled monolithic beam specimens and 1 assembly monolithic frame specimens.Four specimens were statically loaded under vertical displacement control load without additional constraints on the failure corner node.The tests results show that,after the failure of the concrete frame corner column,the failure processes of the remaining structure are: cross section cracking,forming plastic hinge under the yield of tension longitudinal reinforcement,concrete crushing,bar breaking.The failure of the frame beam connected with the failure corner column is concentrated on the position of the beam near the side column and the truncation of the longitudinal reinforcement at the top of the beam near the side column.There is no obvious cracking or deformation between the middle span and corner column section.According to the quasi-static test results,the whole loading process of the structure consisted of three distinct stages: elasto-plastic stage,plastic-hinge stage,and and collapse stage.After the failure of the corner column of concrete frame,there is no obvious pressure arch mechanism and suspension mechanism in the speciments due to the lack of horizontal constraints,the main working mechanism of structure collapse resistance is vierendeel action and beam mechanism.The internal force redistribution effect of the vierendeel action reaches the maximum at the end of the elastic-plastic stage,while the function of vierendeel action gradually weakened during the plastic-hinge stage,the behavior of the beams is close to the cantilever beam.The following conclusions are obtained by comparing the test results of cast-in-situ beam specimen and the assembled monolithic beam specimen,encrypt stirrup speciment and uneccrypt stirrup speciment,beam specimen and frame specimen.The failure mode and load displacement curve of assembled monolithic specimens are not significantly different from those of cast-in-place specimens,so that the collapse resistance of assembled monolithic frame structure can be considered equivalent to cast-in-place.The stirrup encryption at the end of the beam near the side column and in the truncated position of the longitudinal bar near the side column,can avoid the buckling of the reinforced bar at the bottom of the beam and effectively improve the anti collapse bearing capacity of the structure.Because of the lack of horizontal constraints in the substructure of the corner region,the anti collapse ability of the structure is not sensitive to the change of the stiffness of the support,and there is no obvious difference in the anti collapse ability of the beam specimen and the frame specimen.This paper focuses on the theoretical analysis of the collapse resistance of the structure before the collapse stage,because there are no second load peaks in the collapse stage.On the basis of experiments,a simplified model of double plastic hinge considering vierendeel action is put forward based on several reasonable assumption.The formulas for calculating the bearing capacity and displacement of 2 characteristic points(the yield point of steel bar and the critical point of bearing capacity)on the load displacement curve are derived.Using this model to calculate the bearing capacity and displacement of the yield point of steel bar and the critical point of bearing capacity are both of high accuracy.The broken-line consist of those 2 characteristic points as the turning point is in good agreement with the test load displacement curve.On the basis of the model,the influence of longitudinal reinforcement ratio and longitudinal truncation position on collapse resistance is studied,the results show that increasing the reinforcement ratio of the top longitudinal reinforcement can improve the anti collapse capacity of the structure,and lengthen the truncation position of the longitudinal bar near the top of the edge column to the middle cross section,and the anti collapse bearing capacity of the structure can be fully played.