Numerical Analysis of Concrete Filled Steel Tube Under Axial-Flexural Loading

Abstract

A Concrete Filled Steel Column (CFT) composite column comprises of Hollow Steel Tube (HST) infill with concrete to support heavy loads. Experimental investigations on the CFT column have proves that it has better strength, deformation and stiffness as compared to Reinforced Concrete (RC) columns. This is attributed to core concrete which delays or even prevents local buckling of the outer steel tube, while the inner concrete core is confined by a hollow steel tube. The CFT columns are commonly used in Tall buildings, Bridges, Heavy industrial structures, Deep foundations, etc. now-a-days. Behaviour of the CFT column under pure compression has been studied well through numerous experimental investigations with varying geometrical and material properties leading to the design P-M interaction chart. Various design codes; EC-4; ACI-381-05; AISC 360-2010 and AIJ-2001 have been given design procedure for the CFT column out of which EC-4 provides the most simplified design method using four points based P-M interaction curve. However, the study on the CFT column under axial-flexural loading is limited to a circular section and in a centre area of research. Since experimental investigation requires capital investment, time and effort subjected to practical limitations, numerical investigation of CFT column to axial-flexural loading becomes important and evident.

The major objective of the present study is to carry out numerical investigations on CFT columns; circular, Square and rectangular under axial flexural loading for which experimental results were available. In order to develop Finite Element (FE) based computational models, an experimental program consist of coupon testing and testing of CFT and HST columns; circular and square have been designed. FE package ABAQUS is used for the development of FE models of CFT and HST columns subjected to axial compression loading as well as axial flexural loading. FE model of CFT column to axial compression load was validated with results reported in the literature. Numerous FE models have been generated for CFT column to axial-flexural loading to evaluate the ultimate capacity, ductility and percentage confinement of concrete. At last, a parametric study in terms of; geometric shape, slenderness (L/i) ratio, material strength, Diameter to thickness (D/t) ratio and relative slenderness is performed. Liang et al[] biaxial model was used for modeling steel material for CFT column, Ramberg-Osgood model used for modeling steel material for HST column, and Mander model of confined concrete was considered for concrete material in FE model of CFT column. Dynamic explicit analysis was performed for nonlinear analysis of FE models which reduced the computational time by four times as compared to a static method. It has been found from experimental results of CFT and HST column that, it fails due to local brooming at one of the ends of the test specimen under axial compression. However, the CFT test specimen under axial-flexural loading shows global buckling with multi-fold wrinkling of the steel tube on the compression side. FE models developed for HST and CFT column yields good agreement for ultimate load, and failure patterns with experimental results. P-M interaction curve developed for circular and square CFT column using FE models shows good agreement with one derived using EC-4. While it shows over estimation compared to P-M charts obtained through ETABS software. Parametric studies indicate that the circular section provides the best confinement followed by square and rectangle CFT columns. Confinement effects were found to increase with an increase in material strength but decrease with increases in L/I ratio, D/t ratio and relative slenderness.