Long-Term Effects on Response of Reinforced Concrete Columns to Cyclic Loading

Abstract

An experimental research program was undertaken to define the effects of long-term deformations on the seismic performance of reinforced concrete columns. Thirteen beam.column specimens were subjected to simulated seismic drift histories after the columns sustained axial load for a predefined duration. The experiments are supplemented by computer analyses that simulate: (1) the changes in column internal stresses due to long-term deformation of concrete during the sustained load period, and (2) stress and strain demands from nonlinear flexural response to short-term lateral loading. Columns which sustained load for longer durations were observed to develop inclined cracks at earlier stages of the lateral load tests due to a combination of (1) reduced concrete compression stresses from axial stress redistribution, and (2) transverse tension stresses in the column core due to restrained shrinkage of the concrete. This damage produced decreases in (1) lateral stiffness prior to column hinging, (2) flexural strengths at first yield and ultimate, and (3) toughness (as indicated by energy dissipated through hysteresis). However, the observed reductions in flexural strength did not exceed 15%, and, in most cases, column drift capacity exhibited improvement with sustained load duration.