STRENGTH AND STIFFNESS OF REINFORCED CONCRETE COLUMNS UNDER BIAXIAL BENDING

Abstract

The strength and stiffness behavior of reinforced concrete columns subjected to biaxial bending and compression was investigated. In order to provide data points on biaxial interactions surfaces, none rectangular cross section columns and fifteen partial circular cross section columns were loaded to failure as moments were increased while thrust was maintained at a constant value. Test data included longitudinal strain profiles as well as transverse displacement measurements at all levels of load. Test measurements were compared with a discrete element analytical model that was modified to provide results that were in favorable agreement with measured response. Test results from other investigators were included in the data against which analytical results were checked. The reciprocal thrust equation was selected as the simplest analytical expression that provided approximations of strength that were consistent with test results. The influence of slenderness effects in skew bending was examined. The moment magnifier method was used for approximate analysis of slenderness. Member stiffness relationships recommended by the ACI Building Code were studied in addition to alternate approximations of EI. The ACI procedures were found to five safe results at low thrust levels only if ACI Eq (10-7) was used. Moment magnification used separately for each principal axis of bending appeared to give rational reflections of biaxial behavior. The discrete element analytical model was also used to produce more analytical data for comparison with the approximate methods of analysis. Simple approximations of EI were not found to be consistent with test data when thrust levels varied and when slenderness ratios were large.