RC rectangular column sections under biaxial eccentric compression - An improved design recommendation

Rectangular column sections, subject to axial compression combined with biaxial bending, are commonly encountered in RC design. As an exact analysis of the strength of such sections involves rigorous calculations, IS 4562 offers a simplified method based on an approximation of the 'load contour' (originally suggested by Bresler4). The normalised load contour is approximated as a straight line at very low axial load levels and as a quarter-circle at very high load levels, and the shape of the curve is linearly interpolated at intermediate load levels. A rigorous analysis on a number of sections with varying aspect ratios and reinforcement patterns reveals significant errors in this codal approximation. It is seen that the resulting designs are either conservative or unconservative to varying degrees at different axial load levels. The present simplified codal procedure can be made considerably more accurate by appropriately modifying the recommended values of the coefficient α (which defines the shape of the normalised load contour), as shown in this paper.