Experimental Investigation on Behaviour of Concrete Filled Tubes

Abstract

Concrete filled steel tube (CFST) columns is a structural system based on filling steel tubes with concrete and both the materials acts compositely to resist the loads because of their synergetic interaction. The CFST columns have excellent structural characteristics like strength, deformation capacity and stiffness, which allows them to be used as load carrying compression members in high-rise buildings, bridges, deep underground tunnels and towers. These composite columns are designed mainly for axial load due to gravity and uniaxial or biaxial moments. As the analytical design approach for these columns is little bit complex and cumbersome, these columns are mostly designed using force-moment (P-M) interaction curve given by various international design codes like Eurocode – 4, AISC – 2010, ACI-381-05, AIJ – 2008. Amongst all these specifications and codal provisions, Eurocode 4 provides a most simplified method for the development of P-M interaction curve using plastic stress method and hence, it can be used as a main guideline for the design of composite columns. In India, the use of Concrete filled steel tube columns is still growing in infrastructure sector. As there is no dedicated code available in India to design the composite columns, the designers have to heavily rely on EN 1994-1-1 (2004) for the design of composite columns and hence there is a need to develop design guidelines for the design of the composite columns. So several experiments are required to be done on composite columns as per the steel sections available in India to study the basic behaviour of composite sections and compare the experimental results with the derived analytical results. In Major Project work, derivation for development of P-M interaction curve for different shapes of Concrete Filled Steel Tubes (Circular, Square and Rectangular) using Eurocode 4 is studied followed by an experimental study which is performed to investigate the axial-flexural load carrying capacity of concrete-filled tube columns. The parameters in the study include different lengths (300 mm – short column and 800 mm – slender column) and shapes of columns (Circular – O.D. = 88.9 mm x th = 4 mm, Square - 91.5 mm x 91.5 mm x 4 mm and Rectangular - 96 mm x 48 mm x 4mm). Maximum axial load carrying capacity of various shapes and various lengths of Concrete Filled Steel Tubes is worked out as per the steel sections used in the experimental work prior to the final tests and also various checks including stability check and local buckling check are performed as per the codal provisions of Eurocode 4. 18 tests are conducted each for slender (800 mm) and short (300 mm) Concrete Filled Steel Tubes. Four eccentrically loaded columns with eccentricities 10 mm, 20 mm, 30 mm and 35 mm and one concentrically loaded column is to be tested for each shape and each length under monotonically increasing axial and eccentric load and loading end conditions acting as hinged support. The CFST specimens were filled with 40MPa Concrete. Also hollow stub column tests with both ends acting as fixed support are conducted for each parameters adopted in the test. The percentage increase in strength offered by the concrete in case of CFST is observed as compared to virgin steel specimen. Experimental results for slender columns indicate that the moment capacity of CFST is reached as inelastic behaviour of the steel tube and concrete infill is combined with global buckling and crushing of the concrete due to local bulging. It has been observed that all slender specimens fails in global buckling mode and short columns fail in local bulging predominantly due to crushing of concrete followed by local buckling after reaching the peak load. Also all the hollow stub columns showed elephant foot buckling as its post-peak failure behaviour .The Pu-Mu Interaction curve obtained experimentally reasonably agrees with factored analytical strengths predicted by Eurocode 4 and E-Tabs 16.2.1. The experimental P-M values are 19.39 % higher than analytical factored P-M values. Based on experimental results, Circular columns is recommended over square and rectangular columns as the load carrying capacity is enhanced due to confinement effects and composite action developed as compared to all other shapes having the same area and same length.