Study On Axially-loading Behaviors And Fire Performance Of Thin-walled Circular Steel Tubular Columns With Built-in Lateral Confinement Reinforcements And Filled With Demolished Concrete Lumps And Fresh Concrete

Concrete-filled steel tubular column is a kind of member with excellent comprehensive performance and has been used widely in practical engineering. In order to further improve its mechanical properties in the case of invariable amout of steel, combining with the thought of our research team about efficiently recycling of waste concrete, a new kind of structural member named thin-walled circular steel tubular columns with built-in lateral confinement reinforcements and filled with demolished concrete lumps(DCLs) and fresh concrete(FC) was presented in this paper. Furtherly, the preliminary investigation of the axially-loading behaviors and fire performance of this member has been carried on. The main research works as follows:1. Twenty thin-walled circular steel tubular stub columns with built-in lateral confinement reinforcements and filled with DCLs and FC were tested under axial loading. The influences of mix ratio of DCLs, strength of flesh concrete, build-in stirrups and their arrangements on axial-loading behaviors of specimens were investigated. The test results show that:(1) in the case of the same amount of steel, axial load bearing capacity and ductility index of the stub column can be enhanced simultaneously by reducing the tube thickness while setting the inner lateral stirrups, and the latter enhanced more significantly;(2) when the strength of FC is close to that of DCLs, DCLs do not result in the decrease of the column’s axial load bearing capacity, but when the strength of FC is much higher than that of DCLs, axial load bearing capacity decreases and ductility index improves evidently;(3) the formula with better prediction accuracy for calculating the axial load bearing capacity was suggested for the short column.2. ABAQUS software was employed to establish the finite element model of thin-walled circular steel tubular stub columns with built-in lateral confinement reinforcements and filled with DCLs and FC. The influences of strength of compound concrete made of DCLs and FC, distance between center line of stirrup and inner wall of steel tube, the ratio of stirrup’s weight to total steel weight, initial diameter-thickness ratio and outer diameter of steel tube on stub column’s axial-loading behaviors were analyzed. The simulation results show that:(1) distance between center line of stirrup and inner wall of steel tube within a range of 20~50 mm has little effect on the load-deformation curve of the stub column;(2) when the ratio of stirrup’s weight to total steel weight is within a range of 10%~30%, the axial load bearing capacity of the stub column is gradually increasing with the increase of the ratio;(3) when initial diameter-thickness ratio is within range of 80~120, compared with the stub column without build-in stirrups in the case of the same amount of steel, the range of load capacity improvement of the stub column with build-in stirrups is a little descreaseing with the increase of inital diameter-thickness ratio.3. Three thin-walled circular steel tubular slender columns with built-in lateral confinement reinforcements and filled with DCLs and FC were tested in fire. The influences of build-in stirrups and their arrangements on fire performance of specimens were investigated. The test results show that:(1) when loading rate is equal to 0.4 and diameter-thickness ratio is equal to 125~130 approximately, the column can meet first grade fire resistance requirements, whose outer wall of steel tube is daubed 30 mm thickness of cement mortar and set a layer of steel wire mesh overall length(set two layers of steel wire mesh in the middle locally);(2) in the case of the same amount of steel, fire resistance of the columns can be improved significantly by reducing the tube thickness while setting the inner lateral stirrups.