International Concrete Abstracts Portal

Based on the success of authors’ previous study, the seismic behavior of a 1/3 scale model of two-bay and three-story reinforced concrete frame with short columns being replaced by split columns at lower two stories is experimentally investigated under cyclic loads to present the seismic behavior of reinforced concrete frames by using the technology of split column. At first, a test model of two-bay and three-story reinforced concrete frame scaled to 1/3 of actual frame is designed, in which the original columns at lower two stories of the model frame are short columns and are replaced by the split columns, and is applied to constant vertical loads and cyclic horizontal loads at the top of the frame. The hysteresis curves between the cyclic horizontal load and the lateral displacement at the top of the model frame is obtained, from which it is seen that under the excitation of cyclic load, the model frame underwent the process of cracking, yielding, and maximum loading, and was destroyed under the ultimate load finally. It is also seen that the model frame with split columns represents better ductility, and the ductility factor, defined as the ratio of ultimate displacement by yielding displacement, of the model frame reaches 6.0. The yielding process of the model frame is obtained from the strain values of the longitudinal bars of beams and columns, from which it is seen that the frame with split columns can realize that the plastic hinges are generated at the ends of beams at first and then the columns begin yielding while the frame has still the load and deformation capacity. When the cyclic load reaches the maximum load, the columns begin yielding, but the deformation of frame may increase continually. It is demonstrated that splitting the short columns can change the failure mode from shear to flexure, thus enabling a frame to have much better ductility.

Four concrete columns using prestressing strand as transverse reinforcement were tested to establish design parameters for the use of high-strength transverse reinforcement under seismic loading. Two tests were dynamic. The confinement of flexural hinges was satisfactory at reinforcement levels below that called for by code, provided that the spiral pitch was small enough to prevent buckling of the longitudinal reinforcement. Dynamic loading did not have any unanticipated effect on flexural performance.

A dry column confining method using high-strength steel bars proposed by one of the authors is applied to a total of 24 reinforced concrete columns. Two series of tests were conducted on these columns. Hoop steel ratio of the columns was 0.1%, typical of RC columns designed before the Japanese Building Code in 1981. These RC columns were laterally reinforced using 19 sets of four high-strength steel bars and L-shaped steel blocks that were attached around the column by tightening the high-strength steel bars. Five specimens were exposed to concentric compression. Uniform bending test was conducted on 19 specimens under constant axial load. High-strength steel bars were pre-tensioned before testing. The amount of pre-tension was controlled by strain in the bars. It was found that the new confining method is effective to increase ductility of the RC columns. Strength of a RC column increases in a proportional manner to the amount of pre-stress in high-strength steel bars. Strength estimation method is proposed. The measured strength varied between 93% to 116% of the calculated strength.

In this study, FRP jacketed short reinforced concrete columns with low concrete strength and inadequate transverse reinforcement were tested under uniaxial compression. The diameter of the longitudinal bars and spacing of the transverse bars were designed to allow buckling of longitudinal bars. The effects of the jacket thickness, pre-damage, cross-section shape and the usage of FRP jackets either continuous or as straps like hoops or spirals were investigated. The test results showed that external confinement of this type of columns was very effective in terms of deformability and strength enhancement. The buckling of the longitudinal bars was delayed significantly by the FRP jackets. The pre-damage did not have an adverse effect on the performance of the jacketed members. It was also observed that for equivalent amount of FRP, continuous jackets and straps provided similar performances. The compressive strengths and the corresponding axial deformations of the columns were also predicted by the empirical equations proposed by the authors before. It was seen that these predictions were in reasonable agreement with experimental results.

It has been widely known that concrete filled steel tubular (CFT) columns have much higher strength and deformation capacities than common reinforced concrete (RC) columns because of beneficent interactive confinement effect between the filled concrete and the steel tube. The confinement effect by steel tube furthermore contributes to improving ductility of high-strength concrete, and enables application of the CFT columns in high-rise buildings located on seismic regions. This paper describes the axial and the flexural behaviors of CFT columns with circular and square sections based on many experimental researches conducted in Japan. The emphasis of this paper will be placed on the stress-strain curve models for concrete in CFT columns, which play the fundamental role in assessing both of the axial and the flexural behaviors of the CFT columns.