Experimental study of concrete-encased composite columns with GFRP-steel tubes filled with sea-sand concrete under cyclic loading

Long-term durability and seismic performance are critical for marine structures. To improve their performance, a novel concrete-encased, concrete-filled composite (CECFC) column has been developed, which incorporates a glass fiber reinforced polymer (GFRP)-steel tube filled with sea-sand concrete. This paper presents an experimental study on the seismic performance of CECFC columns with GFRP-steel tube. The cyclic test program and results are described on seven CECFC columns and one control specimen, constructed with steel/GFRP composite bars embedded in sea-sand concrete (SGFCB). The effects of axial compression ratio, composite tube diameter, and stirrup spacing on the seismic responses of CECFC columns are investigated. The results indicate that CECFC columns predominantly exhibit bending failure. However, the confinement provided by the internal composite tube and surrounding concrete effectively delays crack propagation and concrete crushing. Compared with the SGFCB column, the CECFC design achieves a 10%-15% increase in ultimate load, a 20%-40% improvement in ductility, and a 20%-30% enhancement in energy dissipation capacity, and a 30%- 85% increase in equivalent damping ratio. While increasing the axial compression ratio enhances the initial load-carrying capacity of the column, it also reduces ductility by approximately 30% and raises the load degradation rate by 10%-15%. Similarly, increasing the tube diameter improves the lateral confinement and enhances the ultimate load by 8%-15%; however, excessive enlargement may lead to localized concrete failure. Finally, a design model is proposed for predicting the flexural capacity of CECFC columns with GFRP-steel tube. • Tests on seismic performance of CECFC columns with GFRP-steel tube are described. • The columns incorporate circular GFRP-steel tube filled with sea-sand concrete. • Circular GFRP-steel tube and external concrete markedly enhances confinement effect. • A design method is proposed for CECFC columns using an iterative numerical technique.