Flexural Behavior of Hybrid FRP-Concrete-Steel Double-Skin Tubular Members

TLDR

The hybrid member is a double‑skin tube with a steel inner tube, FRP outer tube, and concrete infill, used as columns or beams, and differences between test and predicted results arise from unmodeled strain gradients, crack concentrations, and slip between the concrete and tubes. The study experimentally investigates the flexural behavior of a hybrid FRP‑concrete‑steel double‑skin tube and develops a corresponding theoretical model using the plane‑section assumption and fiber‑element approach. The investigation varied section configuration, concrete strength, and steel and FRP tube thicknesses, and employed a plane‑section assumption with a fiber‑element model to predict flexural response. The hybrid beams exhibit a highly ductile response, with flexural stiffness, ultimate load, and cracking behavior substantially improved by relocating the steel tube toward the tension zone or adding FRP bars, and the theoretical predictions closely match the experimental results.

Abstract

This paper presents the results of an experimental study on the flexural behavior of a new type of hybrid FRP-concrete-steel member as well as results from a corresponding theoretical model based on the plane section assumption and the fiber element approach. This new type of hybrid member is in the form of a double-skin tube, composed of a steel inner tube and an FRP outer tube with a concrete infill between the two tubes, and may be employed as columns or beams. The parameters examined in this study include the section configuration, the concrete strength, and the thicknesses of the steel tube and the FRP tube, respectively. The results presented in this paper show that these hybrid beams have a very ductile response because the compressive concrete is confined by the FRP tube and the steel tube provides ductile longitudinal reinforcement. The beams' flexural response, including the flexural stiffness, ultimate load, and cracking, can be substantially improved by shifting the inner steel tube toward the tension zone or by providing FRP bars as additional longitudinal reinforcement. The predictions from the theoretical model are in reasonably close agreement with the test results. Differences between the test and predicted results arise from factors not considered in the theoretical model, including the existence of a strain gradient in the confined concrete, concentrations of cracks and the slips between the concrete and the two tubes; these are issues to be accounted for in the development of a more accurate model in the future.