Research On Dynamic Response And Residual Axial Capacity Of Square Recycled Aggregate Concrete-Filled Steel Tube Columns Subjected To Blast Loads

Numerous old buildings have been demolished with the advancement of modernization,resulting in hundreds of millions of tons of waste concrete,polluting the environment and occupying land resources,and the recycling of waste concrete has become an urgent problem to be solved.The recycled aggregate concrete-filled steel tube（RACFST）column inherits the advantages of CFST column,including high bearing capacity,high ductility,and convenient construction,becoming an effective way to recycle waste concrete.Additionally,many accidental or man-made explosion loads endanger people’s safety and seriously affect the reliability of the structure.As the main load-bearing component,the failure of structural column under explosion load inevitably increases the possibility of continuous collapse of the structure.RACFST columns may also suffer from accidental loads such as explosion during their service.This paper experimentally and numerically investigated the dynamic response and residual axial capacity of square RACFST columns subjected to explosion load.The following are primary conclusions:（1）Close-range explosion tests were performed on 15 square recycled aggregate concrete-filled steel tube columns,and the effects of various factors on the failure modes and dynamic responses were investigated.The results show that the columns suffered localized damage.The recycled aggregate replacement ratio has no significant effect on the failure mode.Shear failure occurs on the columns under contact explosion,and the failure mode exhibits the decreasing crater size and depth with the increasing blast center distance.The columns have a tendency to change from local failure to bending failure with increasing TNT charge.With increasing steel tube thickness,the failure mode of the columns exhibit the decreasing crater size and depth.The strains and residual deflection of the columns are positively correlated with the recycled aggregate replacement ratio and TNT equivalent,while negatively correlated with the blast center distance and the steel tube thickness.（2）Numerical simulation was performed to further explore the dynamic response under blast loads,the validity of the established model was verified and performed parametric analyses.The results show that the stress distribution of the column is symmetric about the middle cross-section.The area with large stress concentrates on the column middle and ends,while the transition area between them is small,indicating that the middle and end of the column are vulnerable parts.The concrete absorbs approximately 80% of the total energy,implying the dominant role in the energy consumption of explosion.The mid-span deflection of columns is positively correlated with the column height and the replacement ratio of recycled aggregate,while it is negatively correlated with the steel tube thickness and steel strength.Under the identical amount of material,the blast resistance performance of circular column is better than that of square column.Under the identical explosion surface,the square column exhibits adequate blast resistance performance than that of the circular column.（3）Axial compression tests were performed on 9 blast-damaged columns and 3 intact columns,and damage assessment was performed based on residual bearing capacity indexes.It indicated that both blast-damaged and intact columns exhibit shear failure under axial loads,resulting in obvious shear failure lines.The ultimate bearing capacity of blast-damaged columns is obviously lower than that of intact columns,and the damage degree is positively correlated with the replacement ratio of recycled aggregate and TNT equivalent,while negatively correlated with the blast center distance and steel tube thickness.It is found by damage evaluation that the load capacity loss range of explosion column is about 1.9% – 32.1%.The columns with a higher recycled aggregate replacement ratio than70% would suffer moderate damage,while the columns with recycled aggregate replacement ratio lower than 70% only suffered slight damage.Further,the residual axial capacity of the column does not improve remarkably when the hoop coefficient is greater than 1.3,while increases the material cost,and there are certain limitations.