Test And Analysis Of The Flexural Behavior Of The Concrete Beam Encased Grouted-Filled Square Steel Tube Truss

The concrete beam encased grouted-filled square steel tube truss is a new type of steel-concrete composite member,which uses steel truss chord instead of longitudinal bar and web bar instead of stirrup.Compared with ordinary reinforced concrete beams,the concrete beam encased grouted-filled square steel tube truss have the advantages of high bearing capacity,high stiffness and good integrity and can be applied to long-span and super-tall buildings.Moreover,the concrete beam encased grouted-filled square steel tube truss can also be used as the primary and secondary beam in the built-up layer structure to realize the self-bearing at the construction stage and solve the problem of weak bearing capacity of the original building roof which unable to support the formwork.In addition,compared with the internal solid web type steel-concrete beam and Angle steel truss concrete beam,the concrete beam encased grouted-filled square steel tube truss is connected to the reinforced concrete column,which solves the problem of difficult construction of beam-column joints.This paper designed five the concrete beam encased grouted-filled square steel tube truss which were tested under the action of static loads,its failure characteristics?the distribution and development of cracks?curve of load-deflection at mid span?curve of steel tube strain curve?strain distribution?cracking load and ultimate load were obtained;In addition,a finite element numerical model was established based on the finite element software ABAQUS for nonlinear analysis of the bending performance,and the results were compared with the test results.Based on the test and finite element analysis results,the formulas for calculating the flexural capacity,cracking moment crack width and stiffness of the normal section of the concrete beam encased grouted-filled square steel tube truss are presented.The results show that:(1)The lower chord steel tube in the pure bending section of the concrete beam encased grouted-filled square steel tube truss is first subjected to tension and yield,and then the concrete in the compression area is crushed under continuous loading,and the member is declared to be destroyed,presenting ductile failure;the stiffness of the concrete beam encased grouted-filled square steel tube truss is large,and the cracking has little influence on the stiffness of the weakened composite beam;The ultimate bearing capacity of the test beam increases with the increase of the area of the lower chord steel tube.The test beam generally conforms to the assumption of plane section.(2)based on the nonlinear analysis of ABAQUS,the whole process of the finite element numerical model under stress,failure mode,load-mid-span deflection curve and load-mid-span steel pipe strain curve were obtained.The results were in good agreement with the test results,which verified the accuracy of the finite element numerical model.According to the extended test parameters,the flexural bearing capacity of The concrete beam encased grouted-filled square steel tube truss increases with the increase of the area of the lower chord steel tube,while the influence of concrete strength,the area of the upper chord steel tube and the node spacing on the flexural capacity is not obvious,the calculated results by the calculation formula are in good agreement with the test results,which can be used for calculation and analysis.