Untersuchungen und theoretische Modelle von druckbeanspruchten Stahlstützen in Verbundbauweise

Experimental and analytical modelling of steel columns reinforced by concrete encasement. Experimental investigations of steel‐concrete axially compressed columns that were tested in the Laboratory of the Institute of Building Structures of the Warsaw University of Technology are presented. The results of conducted experimental work created a basis for the detailed investigations into the appraisal of the influence of different factors associated with the behaviour of tested elements. Analytical modelling was done with use of the finite element method and the ABAQUS software. In the analysis, the emphasis was put on the influence of those factors that have the most important effect on strength and deformability of composite columns. As to the authors' knowledge, the investigated factors have not been accounted for accurately enough in current design codes. The factors such as the load application to the composite column end and the restraining conditions at the supports are among those most important from the design point of view. The investigations have a great practical aspect at this particular moment when in Poland there is an ongoing discussion associated with the proposed new national code on design of composite steel‐concrete structures. The said codification proposal is an attempt to harmonize the national design rules with those applied in the Eurocode 4. The experimental part was concerned with testing of 16 composite columns that were made as completely encased steel I‐type elements (HEA160 section used). The main sectional dimensions were 260 mm × 260 mm, and the column length – 2500 mm. In addition, the reference steel column of HEA160 section was tested. The parameters that were being changed during experiments consisted of the following ones: the way of the load application to the column end face, compressive strength of the concrete encasement and the restraining effect at the supports that reproduced that of assumed in the numerical studies. The behaviour of composite columns with fibre‐reinforced concrete heads was also investigated. The comparison of load deflection curves from the experiments and those from the computer modelling was carried out. In addition, failure loads from the laboratory tests were compared with those calculated according to some chosen design codes. The analysis carried out proven that both the way at which the load is applied to the element end face and the type of end restraints had a great influence on the strength of composite columns. It is clear from the experiments that in case of load application directly through the steel section, the attainment of the limit state of cracking was much earlier than the strength ultimate limit state, so that it controlled the design of composite columns. In this case, an increase in strain of steel sections was observed. In addition, the rigid restraining conditions at the column ends resulted in a decrease of the load bearing capacity because of the stress concentration in the concrete encasement within the support zones. The failure of composite columns was controlled by the combined stress state in the concrete encasement. The study carried out has shown that the load bearing capacity of composite columns can be substantially increased by the applications of fibre‐reinforced column heads at their end supports. The strength of such columns increased by 30–48% if compared with the same composite columns but made without fibre‐reinforced ends.