Limit State Interaction Charts for RC Square Columns subjected to combined axial loads and Biaxial Bending

Abstract

Abstract Designing columns undergoing biaxial bending is a cumbersome process. Researchers tried to give approximate methods to simplify the biaxial design. But still, the exact relation between the resultant eccentricity and the angle of the neutral axis is not known. Consequently, a trial and error method has to be used in deciding the angle and depth of the neutral axis for columns subjected to biaxial bending. Indian standard code for design IS 456 (2000) uses a method replicated from Bresler’s approximate design method which gives conservative results at very low axial load levels and a little unsafe results at higher axial load levels. To simplify the problem of biaxial design weber produced charts for columns bending along a diagonal. Similarly, equilibrium equations can be used to generate Interaction equations for moment acting at an angle of 45° and an interpolation procedure can be developed based on the limit state method to design columns. The generation of design charts for biaxial case thus becomes a topic of research. This paper presents a simplified Interpolation method for designing R.C. square section columns subjected to combined axial compression and biaxial bending complying with the limit state method. A computer-based approach was used to generate design charts with an arrangement of 20 bars symmetrically reinforced with Fe415 and Fe500 and for d’/D ratios of 0.05, 0.10, 0.15, and 0.20. Necessary equations for preparing interaction charts were derived by modifying existing uniaxial equations. The derived equations were used to prepare an Excel sheet to get the coordinates of interaction curves. MATLAB code was used to generate smooth curves for each case of p/fck. The Comparison Charts indicated that the axial load capacity predicted by the interaction charts is in excellent agreement with the theoretical axial load capacity.