Experimental Study On Seismic Behavior Of Recycled Aggregate Concrete Filled Square Steel Tube Columns Under Composite Loading

The use of recycled aggregate concrete can not only reduce the environmental pollution and land waste caused by construction waste to a certain extent,but also reduce the amount of gravel mining,in line with the concept of Ecological Civilization Construction in China.Recycled aggregate concrete filled steel tube structure not only does well in mechanical properties and economic benefits,but also can effectively solve the shortcomings of recycled aggregate concrete mechanical properties and durability worse than natural concrete.Recycled aggregate concrete filled steel tube as a load-bearing member,in a single stress state is rarely.A large number of seismic damage investigations show that the vertical bearing members are often in a complex stress state composed of axial force,bending moment,shear force and torque.In order to improve the recycling rate of waste concrete and increase the application of recycled aggregate concrete filled steel tubular members in practical engineering,it is necessary to study the seismic performance of recycled aggregate concrete filled steel tube members under composite stress state.The work and main conclusions obtained in this paper are as follows:The quasi-static tests of 12 specimens of recycled aggregate concrete filled square steel tube columns under bending-shear-torsion and compression-bending-shear-torsion coupling loading were carried out.The influencing factors included recycled coarse aggregate replacement rate,steel tube wall thickness and axial compression ratio.Through the test,the failure characteristics and the variation law of the surface strain of the steel pipe of the specimens during the whole loading process are observed and analyzed.The load-displacement hysteresis curves and torque-torsion angle hysteresis curves of the specimens at the top of the columns are obtained.The seismic performance indexes of the specimens such as skeleton curves,ductility performance,stiffness and strength degradation,energy dissipation capacity are analyzed.The experimental results show that:(1)Under the bending-shear-torsion coupling loading,the specimens generate 45° oblique “bulge” above the bottom stiffener.With the increase of axial load,“bulge” gradually develops from oblique to transverse,under the coupling loading of compression-bending-shear-torsion.(2)Under the bending-shear-torsion coupling loading,the hysteresis loop of the specimens were gradually full and spindle-shaped.Under the compression-bending-shear-torsion coupling loading,the hysteresis loop of the specimens appear the phenomenon of“pinching”,which is arched.(3)The replacement rate of recycled coarse aggregate has no obvious influence on the failure process and failure characteristics of the specimens,and has little influence on the seismic performance indexes of the specimens under bending-shear-torsion and compression-bending-shear-torsion coupling loading.(4)Increasing the wall thickness of the steel tube can effectively improve the initial stiffness,horizontal peak bearing capacity,deformation performance and energy dissipation capacity of the specimens under bending-shear-torsion composite loading;However,the improvement effect of the horizontal peak bearing capacity and deformation performance of the specimens under the compression-bending-shear-torsion composite loading is not obvious.(5)With the increase of axial compression ratio,the “pinching” phenomenon of hysteresis curves is more obvious,the failure of specimens is earlier,and the horizontal bearing capacity and deformation capacity are reduced.