Experimental Investigation On Axial Compressive Behavior Of Concrete Columns With Rectangular Cross-Section Reinforced By Bundled-Tube Stirrups

One of the typical failure characteristics of conventional reinforced concrete (RC) columns with rectangular cross-section in the severe earthquake is the crush of concrete and the buckling of longitudinal reinforcements at the ends of columns, which leads to the rapid decrease of residual capacity of the columns, and then the building easily tends to lean or even collapse causing casualties and economy losses. To improve the bearing capacity and ductility of RC columns with rectangular cross-section, the bundled-tube stirrup confinement to enhance the ends of rectangular cross-section columns is proposed. Against this background, an experimental investigation and the response analyses of the axial compressive behavior of RC columns with rectangular cross-section reinforced by bundled-tube stirrups were presented in this paper.In this paper,8 RC columns with rectangular cross-section was fabricated and tested under the axial compression, including 4 RC columns reinforced by bundled-tube stirrups and 4 conventional RC columns as reference. The results show that the ductility of RC columns reinforced by bundled-tube stirrups is better than that of the conventional RC columns, and the load-displacement curves of RC columns can be divided into four stages, i.e., elastic stage, elastic plastic stage, decline stage and failure residual stage; the cross-section of rectangular columns reinforced by bundled-tube stirrups under the axial compression load can be divided into four zones based on the lateral confinement degree of concrete, namely highly confined region, partially confined region ?, partially confined region ? and unconfined region. In the process of loading, the concrete of unconfined region, partially confined region ? and partially confined region ? were gradually deactivated for carrying load in turn. Under the same ratio of volumetric stirrups, the peak load of RC columns with rectangular cross-section reinforced by bundled-tube stirrups may increase or decrease when compared with that of conventional RC columns, which depends on the ratio (?) of the volumetric ratio of circular spiral stirrups to the rectangular hoops. When ? is less than 3,the column with bundled-tube stirrups exceeds; vice versa, the conventional column exceeds. The displacement at the peak point of RC columns with rectangular cross-section reinforced by bundled-tube stirrups is greater than that of the conventional RC columns, and the descending branch of the load-displacement curve is gentler, which indicates that the residual capacity of RC columns reinforced by bundled-tube stirrups is relatively higher. When the axial strain of RC columns with rectangular cross-section reinforced by bundled-tube stirrups is 0.02 and 0.03, the ratio of residual capacity (P0.02, P0.03) to the corresponding peak loads (P) are 2.0 and 1.7-1.8 times of those of the conventional RC columns, respectively. With the decrease of the volumetric ratio of circular spiral stirrups (2.502%,1.810%,1.474%), P0.02/P decreases from 62.2% to 37.3% and P0.03/P reduce from the 47.6% to 27.3%, respectively.On the basis of the experimental results, Mander et al.'model for the concrete reinforced by stirrups is used to propose the calculation formulas of peak load and the corresponding axial strain for RC columns with rectangular cross-section reinforced by bundled-tube stirrups. The theoretical values agree well with the experimental results.Finally, the finite element analysis model of RC columns with rectangular cross-section reinforced by bundled-tube stirrups is established, and two parameters of the strength grade of concrete and steel bar are analyzed, respectively. The results indicate that the peak load of RC columns with rectangular cross-section reinforced by bundled-tube stirrups increases with the increase of concrete strength grade, but the axial strain at the peak point presents decreasing tendency; with the increase of steel strength, both the peak load and the corresponding axial strain present an increasing tendency. In addition, the finite element simulations results are used to further verify the rationality of the above theoretical calculation formulas.