Higher compression strength concrete as infill of slender steel circular columns

Results presented in this paper are based on the research programme dealing with use of higher strength materials for columns made of steel circular tubes filled with concrete. Benefits of the combination of steel tube and a concrete infill are reflected in convenient load response, better fire resistance and also in constructional solution. Increasing in material strength is consequent upon effort of minimization material consumption and it is also discussed in case of concrete filled steel tubes. The previous results indicated that a use of high strength steel is economically disadvantageous, while a use of higher strength concrete has sense but only to certain degree. In case of an assessment of slender columns at the ultimate limit state (ULS), a buckling resistance decides. In estimation of buckling resistance theoretical value, slenderness of columns plays important role. And slenderness is inversely proportional to concrete compression strength, when calculating according Eurocode 4. Within the research programme, experimental analysis of axially compressed circular tubes filled by concrete was conducted for columns created by a combination of these parameters: concrete of cylindrical strength 55 MPa and 80 MPa, steel of strength class S235 and S355 and tubes of diameter 133 mm and 168 mm. There is steel tube thickness 4.5 mm and tube length 3.00 m in every case. First, the paper deals with evaluation the results and their comparison with expected values determined by a standard calculation algorithm. The second part is focused on monitoring the influence of concrete material characteristics on column buckling resistance. The aim of the paper is a verification of theoretical method of determination buckling resistance and an appraisal of efficiency higher strength concrete in these types of composite members. It can be stated based on the result presented in this paper, that buckling resistance of slender column from concrete filled steel tube is influenced more by modulus of elasticity than by concrete compression strength. The use of higher strength concrete is irrelevant if the modulus of elasticity does not increase at the same time.