Evaluation Of Approximate Design Procedures For Biaxially Loaded Rectangular Reinforced Concrete Columns

Abstract

Many building codes including the Ethiopian Building Code Standard, EBCS- 2 [l], recommend different approximate procedures for the design of biaxially loaded reinforced concrete columns. The Ethiopian code and other codes such as the British Standard, BS 8110 [2], recommend the design of biaxially loaded column for uniaxial bending using an "equivalent" uniaxial eccentricity of load along the axis parallel to the larger relative eccentricity. Another commonly used approximate design
procedure adopted by many codes [2,3] is based on the use of simplified expressions for the normal load contour of the failure surface. Such interaction curves have at least two rigorously determined points
corresponding to the design values of the ultimate uniaxial moment capacities of the cross-sections under different levels of normal forces. The approximation according to the ACI [4,5], can also be categorized in this group. However it involves the determination of an additional point on the actual interaction diagram where the magnitudes of the moment components related to the respective uniaxial capacities are equal.

Although one or the other approximate procedure is recommended by the different building code standards, the extent to which such procedures may lie on the safe or the unsafe side or relative merits of
the different approaches is lacking in the literature. The aim of the paper is to evaluate the different approximate procedures by comparing the results with the more rigorous solution for biaxially loaded columns [7, 11 ].

The comparative result of the investigation shows that the ACI's approach [4, 5] represents the most accurate approximation for biaxial bending. The approximation according to the Ethiopian Building Code Standard, EBCS - 2 gave mostly conservative results. Based on the investigation, improvements on the y- factors have been suggested to give less conservative results.