Using thin-walled box columns in reinforced concrete buildings is very useful regarding the minimization of the building weight which results in decreasing the loading effect, particularly the effect of earthquake forces. Designing such columns has to be considered from different points of view such as local buckling of the compression flange. section capacity, overall buckling of the column, beam to column connection, column to foundation connection and so on.

In this paper, the behavior of the thin-walled reinforced concrete box columns under axial load and biaxial bending is investigated. The two following major subjects are considered in this work:

1) local buckling of the compression flange: the thin walls of the concrete box section behave as the orthotropic plates that are under compression load. The main assumptions are: a) reinforced concrete is an orthotropic material. b) the plates arc perfectly plane with no initial deformation, c) the plates have rectangular shape. d) the plates are thin, which means their thickness is small comparing to their other dimensions. e) the short sides of the plates are simply supported while the long sides are cantilever. Assuming that the small deformation results when the box column buckles, differential equation governing the buckling of such plates is obtained using small deformation theory for thin plates. Solving the differential equation, the critical load under which the plate buckles is determined.

2) the section capacity assuming that the reinforcement is distributed uniformly in the section. the section capacity is calculated based upon strength design method using failure surface theory and Bresler reciprocal load procedure. Based upon the derived formulation, a computer program is developed for analyzing thin-walled box sections. Also, the interaction curves are prepared for analyzing such column sections.

Finally, considering both above mentioned factors, non-dimensional design diagrams are plotted. The diagrams help to find the safe loading that can be applied on the column resulting from one of the factors that prevails.

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