Rectangular FRP-concrete-steel hybrid multi-tube concrete columns: Eccentric compressive behavior, confinement mechanism, and eccentricity-dependent model

This study addresses the insufficient understanding of the eccentric compressive behavior of rectangular FRP-concrete-steel hybrid multi-tube concrete columns (MTCCs) through integrated experimental and theoretical investigations. Twenty-four specimens were systematically tested to examine the individual and coupling effects of aspect ratio, confinement level, and loading eccentricity. Eccentricity drove a substantial 44%–151% ultimate strain amplification at the extreme compression fiber (ECF), while the sensitivity of this enhancement to additional FRP thickness diminished with increasing eccentricity. Rectangular columns exhibited superior load retention to their square counterparts due to a “localized confinement enhancement effect” arising from the coupling of geometry and eccentricity. The eccentric confinement mechanism was elucidated by decoupling the highly non-uniform confinement distribution into three distinct contributions through equivalent confining stress and three-dimensional loading paths. The eccentricity-induced strain gradient enhanced FRP confinement near ECF but progressively weakened it toward the neutral axis. The geometry-induced corner-to-flat-side differentiation was partially offset at small eccentricities. The internal steel tubes provided additional confinement that elevated the load-bearing capacity. Building upon these mechanisms, a generalized eccentricity-dependent (EccD) modeling framework was developed and validated against the present MTCC test results and independent literature results for rectangular FRP-confined unreinforced and reinforced concrete columns, demonstrating reasonable predictive accuracy across all three column types.