ABSTRACT
Many variables that needs to be considered in the design of different types of columns with the concrete confinement phenomenon included enforces testing and analysing each type separately. Experimental research as well as analytical calculations most often aim at assessing quantitative and qualitative influence of the mentioned parameters on the effective concrete strength in different types of composite columns cross sections. In (Cheng & Lin (2006)) a comprehensive analysis of the effect of the shape and layout of steel sections have on stress distribution in concrete and the resultant load capacity of columns was presented. The confinement of concrete in CFST rectangular cross section columns was presented in (Ge & Usami (1994)) and (Cai & He (2006)). Additionally, research has been carried out on CFST rectangular cross sections, with a view to deriving benefits from the complex character of stress distribution in concrete. These can be achieved through seeking solutions at the stage of construction design (Hu et al. (2003)). In (Susantha et al. (2001)) a method of predicting how the stress-strain relationship changes, taking into account the triaxial character of stress in concrete in tubular columns of various cross section shapes was proposed. The effect of concrete confinement has also been studied in
1 INTRODUCTION
Reaching critical values of load-bearing capacity in stub steel-concrete columns is accompanied in most cases with cracking and crushing of concrete. Destruction of such kind was noticed in ConcreteFilled-Steel-Sections (CFST) columns made of tubes of relatively thin sidewalls, designed with respect to seismic threats. Similar observations were made in research conducted by the authors of this paper on columns fully encased in concrete, as well as on two-chord battened columns filled with concrete. Hence, analysis of effective loadbearing capacity of concrete in composite columns appears to be an extremely important issue. Consequently, research has been done on possible methods of enhancing the load-bearing capacity of concrete subjected to axial compressive stress in conditions where the risk of lateral buckling was substantially reduced. In the 60ies of the previous century, In (Rüsch & Stöckl (1969)) a relationship between using spiral steel reinforcement and subsequent improvement of confined concrete strength was suggested. Furthermore, in (Mander et al. (1988)) a thorough analysis of a complex character of stress distribution in concrete was provided, taking into account different types of reinforced concrete columns. Similar research has been conducted in recent decades with regard to composite steel-coNCRete columns (El-Tawil (1999)). It is well known that restraining deformation of concrete is related to pressure exerted by lateral reinforcement, a steel tube or steel section. In the effect of confinement, compressive strength of concrete increases. That is conditional on numerous factors, which can be classified as the following:
CFST elliptical cross section columns (Yang et al. (2008)), as well as in columns of the FRP type (Yuan et al. (2008)). Analysing the problem of concrete confinement in columns consisting of rectangular steel tubes sections often aims at assessing how to account for the confinement effect in calculating the load-bearing capacity of such columns so as to comply with current standards. For safety reasons, the effect that the complex character of stress distribution in concrete has on enhancement of load-bearing capacity in rectangular tubular columns is most often disregarded, and accounted for only in circular cross sections, as in Eurocode 4. That problem was explored by many researchers. A method of computing load-bearing capacity for rectangular cross section CFST columns was suggested in (Huang et al. (2008)), and the results were compared with computations according to numerous standards. In (Kupina (2015)) the ultimate load of rectangular short CFST columns was analysed and author stated that the increase in compressive strength of concrete due to its confinement is approximately 16%. In case of analysed in this paper two chord steel-concrete columns the confinement effect of concrete is hard to estimate due to the complex non-prismatic geometry of specimens. These types of columns can be analysed either as concrete encased sections or as CFST columns. In (Szmigiera (2012)) the analysis of the stress state of concrete filling the steel cross-section using experimentally obtained horizontal and vertical strains of steel and concrete was conducted. The analysis showed that the concrete fill in this type of columns is in the complex stress state and associated with this fact increase in the ultimate bearing capacity of columns was approximately 8%. In this article two other approaches on estimating concrete confinement was presented. First one is based on the comparison between ultimate load measured experimentally and computed according to the formulated by Emperger (1908) addition law that is placed in the Eurocode 4. Second one is based on the measured in the experiment longitudinal axial strains of steel and concrete.
