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[EN] Compressed reinforcement buckling in concrete columns can reduce ductility in structural elements. In order to avoid this, design codes propose maximum required tie spacing. Nonetheless, they do not incorporate the positive effect of concrete fibers in their formulation, whose capability of delaying buckling has been proved. For this reason, recommendations for maximum required tie spacing for elements made with concrete, with or without steel fibers, and with normal or high strength, are proposed in this article. In order to achieve this, the mixed model proposed by Pereiro-Barceló and Bonet was extended to consider elements made of HSC with and without steel fibers thanks to the results of an experimental campaign of HSC columns with and without fibers, under monotonic loading. In these tests, the buckling critical load in compressed reinforcement was experimentally determined in all the columns. In addition, a comparison of the proposed transverse concrete separation with respect to the recommendations proposed by the main existing codes was made., The research presented herein forms part of a research undertaken at the Concrete Science and Technology Institute (ICITECH) of the Universitat Politecnica de Valencia (UPV). The authors are sincerely grateful to the Spanish Ministry of Economy and Competitiveness for the help provided through Project BIA2012-32645, and to the European Union for the financial support obtained from FEDER funds. The authors wish to thank the Spanish Ministry of Education, Culture and Sport for Grant FPU12/01451.

This work is enclosed in a research project on slender columns carried out by Instituto de Ciencia y Tecnologia del Hormigon (ICI-TECH) of the Universitat Polittcnica de Valencia. This research was funded through grants from the Ministerio de Ciencia y Innovacion de Espana with research projects BIA 2006-06429 and European Funds of Regional Development. The experimental program was developed in the Laboratory of Concrete of the Construction Engineering Department of the Universitat Politecnica de Valencia.

This paper focuses on the behavior of high strength reinforced concrete columns subjected to biaxial bending. Experimental research with 56 high strength columns was carried out to achieve this purpose. The analyzed parameters were: the slenderness, the eccentricity, and the skew angle of the applied load. The coupling of the second order effects in each main direction of bending was analyzed. It has been observed that the behavior of the column was strongly influenced by the flexibility of the weak axis. However, the proposed simplified approaches in ACI-318(05) and EC-2 (2004) do not take it into account. The experimental results have been compared with a numerical simulation and with simplified approaches based on moment magnification factors such as that proposed by ACI-318(05) and by EC-2 (2004). Numerical simulation provided good agreement with displacement-load results and failure loads from the tests. In general, the simplified approaches are conservative in the case of the smaller eccentricities and result unsafe in the case of larger eccentricities.