Experimental verification of global reliability of slender reinforced concrete columns from the regular concrete C45/55 and high performance concrete C70/85 was realized within the applied research of the Faculty of Civil Engineering at Slovak University of Technology in Bratislava (SUT). Production of test columns and samples was in cooperation with ZIPP Bratislava Ltd. Columns are designed in the way to collapse due to stability before the resistance of the critical cross-section is reached. The relative compression of concrete was scheduled about 1,5 ‰. For performed experimentally verified slender columns, the reliability of simplified and nonlinear design methods according to European standards was compared.

The European standard EN1992-1-1 shows a significant lack of an overall reliability in cases when loss of stability of concrete columns precedes reaching of resistance in a critical cross-section. The columns, subjected to axial force and bending moment, experimentally tested in the laboratory of Faculty of Civil Engineering SUT in Bratislava, were designed so that the loss of stability occurs at the concrete strain about 1.5 ‰ on the compressed edge of a critical cross-section.

The European Standard [1] for design of concrete compressed slender members shows a significant deficit in global reliability for the design method based on non-linear analysis. The experimental investigation at the Faculty of Civil Engineering SUT in Bratislava was planned for slender concrete columns made of different concrete strength classes - C45/55, C70/85 and C100/115. A basic aim of the analysis was to design concrete columns subjected to bending moment and axial force where the stability failure proceeds at the compression strain 1.5 ‰. The first of three series of experimental and numerical analyzed columns is presented in the paper. The experiments were realized at the Faculty of Civil Engineering SUT in Bratislava with cooperation of ZIPP Bratislava LTD Company.

Load capacity of slender compression members Part 1: Component tests and round robin test of the nonlinear numerical prediction of system behavior Non-linear calculation methods allow a realistic description of the physically caused complex load-deformation behavior of reinforced concrete structures and suggest in comparison of proven approximate methods a more accurate design in both ultimate and serviceability limit states. Especially the geometrically and materially nonlinear analysis of slender compression members in the ultimate limit state is, however, still controversial because of the known inconsistencies in the design concept, which is why, despite the in EN 1992-1-1 explicitly granted ability to use non-linear methods, further research is needed. The following paper compares an a priori carried out round robin test of numerical simulations to predict the load capacity of a slender single column with an a posteriori conducted test series. The results show that the numerical calculations overestimate the load capacity in some cases significantly but, however, the widely accepted and normatively governed nominal curvature method yields in these cases to too conservative results. It will be necessary to develop universal and consistent safety formats for non-linear analysis and provide "best practice" guidelines to ensure the safe use of the now widely available software packages.

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