Polyethylene terephthalate (PET) fibre-reinforced polymer (FRP) composites are an emerging form of composites based on recycled PET fibres and have a large rupture strain. A novel form of hybrid columns with PET FRP tubes, i.e., square PET FRP-confined concrete encased high-strength steel (HSS) hybrid columns (referred to as PET FRP-confined concrete-steel columns or PFCCSCs), was recently proposed. The compressive behaviour of circular FCCSCs has been investigated, but there is no research on the compressive behaviour of square PFCCSCs (SPFCCSCs). In SPFCCSCs, the inner HSS tube is expected to be effective in decreasing the confinement nonuniformity in FRP-confined square columns and enhancing the ultimate axial strain and compressive strength of the core concrete. Furthermore, the axial strength of the inner HSS tube is expected to be sufficiently utilized because its buckling is delayed due to confinement from the surrounding concrete. In total, fourteen columnar specimens were tested under monotonic or cyclic axial compression, and the effects of key parameters (i.e., the steel tube yield stress and loading patterns) were investigated. The test results demonstrate that the inward and outward buckling of the inner steel tube in SPFCCSCs can be effectively suppressed or delayed by the surrounding concrete, which was well confined by the PET FRP tube. The compressive strength of a hybrid column is approximately 20% higher than the simple summation of the corresponding strengths of an FRP-confined concrete section and an HSS tube section due to the optimal combination of the three components in the hybrid columns. The SPFCCSCs with a PET FRP tube exhibit a monotonically ascending load-strain response with excellent ductility, which is of particular interest for applications in seismic engineering and underground engineering. The loading scheme has a marginal effect on the load-strain envelope response of SPFCCSCs.

聚对苯二甲酸乙二酯（PET）纤维增强聚合物（FRP）复合材料是一种基于回收PET纤维的复合材料的新兴形式，并且具有较大的断裂应变。最近，提出了一种新型的带有PET FRP管的混合柱，即方形PET FRP约束的混凝土包裹高强度钢（HSS）混合柱（称为PET FRP约束的混凝土-钢柱或PFCCSC）。已经研究了圆形FCCSC的压缩行为，但是还没有关于方形PFCCSC（SPFCCSC）的压缩行为的研究。在SPFCCSC中，内部HSS管有望有效减少FRP约束方柱的约束不均匀性，并增强核心混凝土的极限轴向应变和抗压强度。此外，HSS内管的轴向强度可望得到充分利用，因为其屈曲由于周围混凝土的限制而延迟了。总共，在单调或循环轴向压缩下测试了14个柱状试样，并研究了关键参数（即钢管的屈服应力和载荷模式）的影响。测试结果表明，SPFCCSCs内钢管的向内和向外屈曲可以被周围的混凝土有效地抑制或延迟，而这是由PET FRP管很好地限制的。由于混合柱中三种成分的最佳组合，混合柱的抗压强度比FRP约束混凝土截面和HSS管截面的相应强度的简单总和高出约20％。带有PET FRP管的SPFCCSC表现出单调上升的载荷应变响应，并具有出色的延展性，这在地震工程和地下工程中尤为重要。加载方案对SPFCCSC的载荷应变包络响应有边际影响。