Behavior of hybrid FRP-concrete-steel double-skin tubular columns subjected to cyclic axial compression

Hybrid FRP-concrete-steel double-skin tubular columns (hybrid DSTCs) are a new form of hybrid columns which consists of a layer of concrete sandwiched between an inner steel tube and an outer FRP tube. While a large amount of research has been conducted on the monotonic behavior of this novel form of columns, only a limited amount of work has been conducted on their behavior under cyclic loading. This paper presents the first ever study on the behavior of circular hybrid DSTCs under cyclic axial compression. Results from a series of stub column tests, where the hybrid DSTCs were subjected to cyclic axial compression, are first presented and discussed. The test results show that hybrid DSTCs are very ductile under cyclic axial compression, with the envelope axial load-strain curve being almost the same as the axial load-strain curve of a corresponding DSTC under monotonic compression. It is also shown that repeated unloading/reloading cycles have a cumulative effect on the permanent strain and the stress deterioration of the confined concrete in hybrid DSTCs. The experimental stress-strain curves of the confined concrete in hybrid DSTCs are then compared with predictions from two existing models: (1) a monotonic stress-strain model for the confined concrete in hybrid DSTCs; and (2) a cyclic stress-strain model for the concrete in FRP-confined solid columns. The comparison suggests that the combined use of the two models can give reasonably accurate predictions of the test results.