Performance of Short and Slender Ultra High Performance Concrete filled Steel Tube and Double Skinned Columns

Abstract

This thesis examines the axial compression load-carrying capacity of ultra-high performance fiber reinforced concrete (UHPC)-filled steel tubes (UHPC-FSTs) and double skin systems, including short and slender columns. The first part of the study investigates experimentally and numerically the axial loading behavior of 135 MPa UHPC-filled double skin steel tubular columns (UHPCFDSTs). A total of 37 short stub columns were tested, including totally filled control tubes and tubes filled with normal- and high-strength (NSC and HSC) concrete. The study examined the effects of outer tube diameter-to-thickness (Do/to) ratio and inner-to-outer tube diameter ratio on the axial capacity of UHPCFDSTs. A nonlinear finite element model using the computer program LS-DYNA was also developed and verified. The experimental results were compared against the Canadian CSA (CAN/CSA) S16:19 code provisions which were found to overestimate the axial capacity by 12-56%. A modification factor was developed and is recommended to be introduced in the code equation.

The second part of the study investigates slenderness effects in UHPC-FST columns. A robust three-dimensional nonlinear finite element model was developed using LS-DYNA to simulate the slender columns under concentric axial compressive loads and was validated using a large experimental database. An extensive parametric study was then performed, varying slenderness ratio (kL/r) based on column length (L) where r is the radius of gyration, diameter-to-thickness ratio (D/t) of the steel tube, effective length factor (k), and steel yield strength (fy). The CAN/CSA S16:19 code provisions grossly underestimated the load-carrying capacity of slender UHPC-FSTs by up to 48% at k= 2.0. A modification to CAN/CSA S16:19 equation was proposed based on multiple regression analysis of the results of the parametric study.