The ultimate capacity and load–deformation relationship of concrete-filled steel tube (CFT) columns are the two primary concerns in practical engineering, to determine which a well-established compressive strength model of confined concrete is important. Compressive strength models of confined concrete developed based on the experimental tests of actively confined concrete, e.g., the models of Richard and Mander, are widely used in the researches of CFT columns. For the tests of actively confined concrete, specimens are subjected to a constant hydrostatic lateral pressure. However, when a CFT column is compressed axially, the lateral stress of the concrete is not constant but increases with compressive stress. The relationship between the lateral stress and compressive stress of confined concrete is known as stress path. Different stress paths between actively-confined concrete and the concrete in CFT columns could lead to totally different behaviors of concrete, which, however, have not been investigated till now.
Recently, compressive tests of CFT stub columns, aiming to clarifying the stress paths and corresponding effects on the compressive strength of confined concrete, were conducted by Dr. Siqi Lin and Professor Yan-gang Zhao at the Kanagawa University, and assistant Professor Liusheng HE at Tongji University. They investigated the stress paths of confined concrete in CFT specimens with different column parameters, such as concrete strength, steel strength and D/t ratio. Furthermore, they discussed the relationship between stress paths and compressive strength of confined concrete, which were characterized via the indices proposed by their team, i.e., the lateral stress domination index and effect index. Their research work is published in the journal, Engineering structures.
They found that the stress path is influenced by the column parameters. The strengths of concrete and steel tube influence the slope of the stress path in the post-fracture stage of core concrete, the length of which, however, is affected by the D/t ratio. Also, their results suggest that the compressive strengths of confined concrete in CFT columns are generally path-dependent. Based on the investigation, a compressive strength incorporating the influence of stress paths was proposed. Their study for the first time investigated the stress paths and discussed the corresponding effects on the compressive strength of confined concrete, which gave an interesting new understanding of confinement effects of concrete. Also, according to Dr. Lin, Professor Zhao and assistant Professor HE, the newly-proposed method may provide a potential way to develop a unified compressive strength model for different confined concrete structures.
Lin, S., Zhao, Y., & He, L. (2018). Stress paths of confined concrete in axially loaded circular concrete-filled steel tube stub columns. Engineering Structures, 173, 1019-1028.Go To Engineering Structures