Seismic behaviour and modelling of rectangular FRP-concrete-steel tubular columns under axial compression and cyclic lateral loading

Fibre-reinforced polymer (FRP) composites, which have been commonly used to retrofit existing structures, have found increasing applications for new structures. FRP-concrete-steel double-skin tubular columns (DSTCs), which consist of an external layer of FRP, an inner steel tube, and sandwich concrete, are gaining popularity in newly constructed bridges. Previous studies focused on the structural performance of DSTCs in circular and square cross-sections. This paper investigated the cyclic behaviour of rectangular DSTCs under a combined loading condition of axial compression and cyclic lateral loading. The particular focus was placed on the influence of rectangular aspect ratio. In addition, both effects of FRP thicknesses and lateral loading directions (bending around its strong axis or weak axis) were also assessed experimentally. Test results demonstrated that (1) rectangular DSTCs had excellent seismic behaviour (i.e., for the rectangular aspect ratio from 1.0 to 2.0, the ductility index increased monotonically from 3.58 to 5.72), (2) the increase of FRP thickness could improve the peak lateral load of specimens (i.e., the peak lateral load for the specimen with 2.10 mm FRP was 95.3 kN, while that of the corresponding specimen with 1.05 mm FRP was 88.3 kN), and (3) the specimen bending around the strong axis had superior performance compared with the identical specimen bending around the weak axis. The cyclic behaviour of DSTCs was simulated in Open System for Earthquake Engineering Simulation (OpenSees) by the developed models which was capable of providing predictions with reasonable accuracy. In practical engineering, it was recommended that, (1) rectangular DSTCs should be designed with a reasonable aspect ratio to match the specific loading conditions of the two symmetry axes; (2) the FRP thickness along the column height could be optimized in accordance with the bending moment gradient to achieve the efficient use of the FRP strength.