Numerical investigation of high-strength steel and ultrahigh-performance concrete for ductile rectangular hollow columns

Ultrahigh-performance concrete (UHPC) has the potential to address seismic performance issues reported for rectangular hollow columns. However, an impediment to the use of UHPC is its limited flexural ductility compared with normal concrete (NC) with the same confinement. This study investigated UHPC rectangular, hollow cross-section bridge columns with HSS as a confinement alternative using flexure (moment-curvature), flexure-shear and finite-element analyses. The flexural analytical model, validated by tests in the literature, was used to perform parametric studies to assess the impact of HSS on column flexural ductility. Analysis results showed that HSS confinement can improve the flexural ductility of UHPC, making UHPC suitable for hollow columns. The flexure-shear interaction model considered the shear strength degradation with increasing displacements to estimate the failure mode. UHPC columns outperformed NC columns in delaying shear-controlled failure. Finite-element analysis was performed on columns with selected design parameters to understand the impact of UHPC on shear damage control. The ratio of flexure to shear displacements was higher in UHPC columns compared with NC columns due to UHPC's higher shear resistance.