Slenderness limit for hybrid FRP-concrete columns

Due to confinement effects, concrete-filled fiber-reinforced polymer (FRP) tubes have an enhanced performance comparable to conventionally reinforced concrete members. However, the strength and ductility enhancements may result in slender sections, which may be susceptible to instability or geometric and loading imperfections. Because stiffness of FRP is less than that of steel, designers cannot directly use current specifications to size compression members. Uniaxial compression tests on a total of seven hybrid columns, with slenderness ratios up to 36, showed that strength is reduced by as much as 71% of the equivalent short column, and the axial and hoop strains are reduced by as much as 85 and 87%, respectively, These significant reductions result in underutilization of confinement in slender columns. An analytical tool with an incremental approach was developed and compared favorably with test results. A parametric study was carried out, which showed the modulus of elasticity of the tube to have a pronounced effect on the slenderness limit of the column. A new equation was proposed for the slenderness limit of hybrid columns based on a maximum of 5% strength reduction.