Experimental investigation on concentrically loaded square concrete-filled steel tubular columns

Abstract

Concrete filled steel tube (CFST) column consists of a hollow steel tube filled with concrete. This composite section offers numerous structural benefits over reinforced concrete and steel only sections, including high strength, high ductility and large energy absorption capacities. Extensive experimental and numerical studies have been carried out by several researchers on concentrically and eccentrically loaded CFST columns with various geometric and material properties. Most of this research work has been performed on CFST columns constructed with available standard tube shapes. However, limited research has been found on CFST columns in built-up steel sections. Current design rules for CFST columns are specified in AISC-LRFD, ACI 318, EC-4, British standard BS 5400 and Canadian Standard Association CSA. In the upcoming version of Bangladesh National Building Code (BNBC 2016) the design guidelines for CFST columns are included which is adopted from AISC (2005) specifications. The applicability of these design provisions in the construction environment of Bangladesh needs to be explored. To this end, an attempt has been made in this study to investigate the behaviour and strength of the CFST columns constructed with built-up steel section and locally available materials. This paper presents an experimental investigation on the behavior of CFST columns regarding three parameters: concrete compressive strength, cross sectional slenderness ratio and global slenderness ratio. Total nine CFST columns with square cross section were tested under concentric loading. The tested columns were filled by concrete with compressive strength of 27 MPa to 44 MPa, cross-sectional slenderness ratio of 25 to 42 and global slenderness ratio of 3 to 10. The influence of these parameters on the failure mode, load-strain response, ultimate load and performance indexes of the square CFST column is discussed. Finally, the design approaches adopted in (Eurocode 4, AISC-LRFD 2010, ACI 2014 and Wang et al. 2016) are reviewed and applied to calculate the ultimate strength of the tests columns. Subsequently, the predicted values are compared with the experimental results obtained from the experiments. Based on the results, it was determined that concrete compressive strength, cross sectional slenderness ratio and global slenderness ratio have significant effect on the fundamental behavior of CFST column. Increasing the concrete compressive strength improved the ultimate capacity and concrete contribution ratio of the column but decreased the peak strain because of its less ductile behavior. On the other hand, columns with higher global slenderness ratio showed lower ultimate capacity and less ductile behavior with global buckling failure. However, columns with lower cross sectional slenderness ratio exhibited better column performance for its higher steel contribution and columns with higher cross sectional slenderness ratio showed outward local buckling failure. Moreover, all the codes somewhat overestimated the capacities except AISC-LRFD (2010). AISC-LRFD (2010) presented best prediction with a mean of 0.99 and Standard deviation of 0.04. EC4 and ACI (2014) predicted higher capacity than the experimental results about 8% and 2% respectively; whilst Wang et al. (2016) predicted highest 12% higher capacity of all the methods analyzed. In general, all the codes showed good agreement with the experimental results.