Experimental and Theoretical Studies on Elastoplastic of Reinforced Concrete Short Columns Doubly Confined in Square Steel Tube and Hoops

The structural members which satisfy the following mechanical properties are desirable ones for buildings from the viewpoint of preventing the earthquake damages and improving the serviceability of buildings. 1) The large load-carrying capacity can be developed within strain levelas lower as possible. 2) The ductility is high. 3) The energy absorption capacity is large. 4) The stiffness is large. R/C short hooped columns were hardly prevented from brittle failure such as shear failure. The transversally reinforcing method by using a steel tube is quite effective to make strong and ductile short columns. Especially, if the R/C circular columns were confined in circular steel tubes, it was experimentally proved that excellent mechanical characteristics could be expected even for the short columns with large flexural capacity. However, for the R/C short square columns confined in the square steel tube, as the width to wall thickness ratio of square steel tube or the amount of total longitudinal bars became gradually larger, a pinching effect and degradation of the lateral load capacity were observed. Consequently, R/C short columns confined in bellows square steel tube were proposed by Tomii in order to increase the flexural stiffness and strength of the steel plates. As another method, R/C short columns doubly confined in square steel tube and hoops are proposed in this paper. The objectives of this paper are as follows. 1) To examine the effects of double confinement. 2) To investigate experimentally the possibility of arrangements for longitudinal bars with large diameter at the corner in square section in order to improve the efficiency of flexural strength and stiffness of R/C columns. 3) To discuss mechanical characteristics of the R/C short columns doubly confined in square steel tube and hoops by comparing the experimental results with the theoretical results taking into account of bond strength between longitudinal bars and concrete. In order to study the strength and behavior of R/C short columns doubly confined in steel tube and hoops, a total of nine specimens were tested. Four specimens were loaded axially and concentrically in a 500-ton capacity testing machine. The other five specimens were tested under a constant axial load and alternately repeated lateral loads. The paper presents test results and discusses the strength and the ductility of R/C short columns doubly confined in steel tube and hoops by comparing the test results with the theoretical results considering bond strength between longitudinal bars and concrete. The following conclusions are reached on the basis of these experimental and theoretical studies. 1) The compressive and the flexural strength of reinforced concrete square sections doubly confined in steel tube and hoops are larger than those of the sections singly confined in steel tube or hoops. 2) The ultimate moments of all the tubed columns are smaller than theoretical predictions ignoring bond strength. However, taking into account of bond strength between longitudinal bars and concrete, experimental values on flexural strength agree well with the theoretical results. One of the reasons for this is the effects of bond deterioration due to large amount of longitudinal bars and their large diameters. 3) The maximum bending moments at the flexural strength of reinforced confined concrete sections arc reached beyond $\varepsilon _c\fallingdotseq 0.01$ at the extreme compression fiber