R/C Column Earthquake Response in Two Dimensions

The earthquake responses of a spirally reinforced concrete column carrying a concentrated mass and subjected to the simultaneous action (biaxial response) of both horizontal components of normalized 1940 El Centro, 1962 Taft, and 1971 Pacoima Dam accelerograms were studied. Two different masses, corresponding to initial elastic periods of 0.65 and 1.5 sec, were considered. The column force-displacement characteristics were synthesized from idealized stress-strain properties for steel and confined and unconfined concrete, using a discretization of the column cross section combined with finite element techniques. Maximum displacements were significantly larger in biaxial response when excitation was sufficiently intense in relation to the column strength to cause appreciable inelastic deformation. If planar analysis indicates inelastic response in a column resisting earthquake effects in both horizontal directions, the effects of the axial load (P-Δ effect) may be more critical and the required ductility may be larger than that based on planar analysis.