Ground motion duration effects on hysteretic behavior of reinforced concrete bridge columns

This study examined seismic behavior under long-duration ground motion in flexural-dominated reinforced concrete bridge columns designed per modern seismic design codes. Two column specimens with identical design parameters were tested. The first column (named CLC) was tested using a long-duration loading protocol developed to represent the number of response cycles expected under long-duration ground motions. The second column (named COC) was tested using a baseline loading protocol with one cycle for each drift loading to obtain baseline behavior for comparison with the behavior of the CLC column. Test results showed that the CLC column had a similar peak strength but a lower ductility capacity compared with the COC column. When drift was 3% or less, the columns showed a similar hysteretic envelop response. However, degradation of stiffness was greater in the CLC column. When drift exceeded 3%, the CLC column started to show greater strength degradation. The relationship between the damage index and damage condition was established. Experimental observations of hysteretic behavior in the two columns revealed that strength degradation is related to maximum displacement and energy dissipation. Stiffness degradation is related to energy dissipation,whereas pinching is related to maximum displacement. The experimental results were used to construct a hysteretic model, which was calibrated with experimental data obtained for the columns under one set of hysteretic parameters. The proposed hysteretic model was used to carry out constant-R and constant-ductility analyses. The constant-R analysis results showed that ductility demand is not necessarily higher during long-duration ground motions than during short-duration ground motions. Ductility demand obtained by the proposed model is generally higher than that obtained by the modified Clough model. The difference between the two models increases as the duration of ground motion and R factor increase and as the period decreases. Constant-ductility plots show that the difference in the ductility factor between the two models is generally within 10% under short-duration ground motions. Under the long-duration ground motions, the ductility factor obtained by the proposed model is up to 20% higher than that in the modified Clough model.