Journal of Constructional Steel Research 63 (2007) 182–193 www.elsevier.com/locate/jcsr Experimental and computational study of concrete ﬁlled steel tubular columns under axial loads P.K. Gupta ∗ , S.M. Sarda, M.S. Kumar Civil Engineering Group, Birla Institute of Technology and Science Pilani, Rajasthan 333031, India Received 2 November 2005; accepted 11 April 2006 Abstract The paper presents an experimental and computational study on the behaviour of circular concentrically loaded concrete ﬁlled steel tube columns till failure. Eighty-one specimens were tested to investigate the effect of diameter and D/ t ratio of a steel tube on the load carrying capacity of the concrete ﬁlled tubular columns. The effect of the grade of concrete and volume of ﬂyash in concrete was also investigated. The effect of these parameters on the conﬁnement of the concrete core was also studied. Diameter to wall thickness ratio between 25 < D/ t < 39, and the length to tube diameter ratio of 3 < L / D < 8 was investigated. Strength results of Concrete Filled Tubular columns were compared with the corresponding ﬁndings of the available literature. Also a nonlinear ﬁnite element model was developed to study the load carrying mechanism of CFTs using the Finite Element code ANSYS. This model was validated by comparison of the experimental and computational results of load–deformation curves and their corresponding modes of collapse. From the experimental and computational study it was found that for both modes of collapse of concrete ﬁlled tubular columns at a given deﬂection the load carrying capacity decreases with the increase in % volume of ﬂyash up to 20% but it again increases at 25% ﬂyash volume in concrete. c  2006 Elsevier Ltd. All rights reserved. Keywords: Concrete ﬁlled steel tubes; Concrete; Concentric loading; Columns; Conﬁnement; Circular hollow sections; Finite element method 1. Introduction Composite concrete-ﬁlled steel tubes (CFTs) have been used increasingly as columns and beam–columns in braced and unbraced frame structures. Their use worldwide has ranged from compression members in low-rise, open-ﬂoor plan construction, using cold-formed steel circular or rectangular tubes ﬁlled with precast or cast-in-situ concrete, to large- diameter cast-in-situ members used as the primary lateral- resistance columns in multi-story braced and unbraced frames. Concrete ﬁlled steel box columns, fabricated from four welded steel plates, and concrete-ﬁlled steel fabricated circular pipes have been used in some of the world’s tallest structures. In addition, concrete-ﬁlled steel box columns are commonly used as bridge piers. Concrete-ﬁlled steel tubular structural members have a number of distinct advantages over equivalent steel, reinforced ∗ Corresponding address: Civil Engineering Group, Indian Institute of Technology, Roorkee, Uttaranchal, India. Tel.: +91 1596 245073x254; fax: +91 1596 244183. E-mail address: spramod 3@yahoo.com (P.K. Gupta). concrete, or steel-reinforced concrete members. Steel members have the advantages of high tensile strength and ductility, while concrete members have the advantages of high compressive strength and stiffness. Composite members combine steel and concrete, resulting in a member that has the beneﬁcial qualities of both materials. The steel tube serves as a form for casting the concrete, which reduces construction cost. No other reinforcement is needed since the tube acts as longitudinal and lateral reinforcement for the concrete core. In addition, the placement of longitudinal steel at the perimeter of the section is the most efﬁcient use of the material since it provides the highest contribution of the steel to the section moment of inertia and ﬂexural capacity. The continuous conﬁnement provided to the concrete core by the steel tube enhances the core’s strength and ductility. The concrete core delays local buckling of the steel tube by preventing inward buckling, while the steel tube prevents the concrete from spalling. Experimental research on concrete ﬁlled tubular metallic columns has been ongoing throughout the world for many years, with signiﬁcant contributions having been made 0143-974X/$ - see front matter c  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jcsr.2006.04.004