Abstract
Prediction of damage is important in evaluating the earthquake response of reinforced concrete (RC) structures. Damage prediction is usually measured in term of ductility factor. However, concerning reinforced concrete structures subjected to a lot of cycles of severe loads, a combination of maximum displacement response and cyclic deterioration of stiffness and load carring capacity may cause RC structures to collapse. Damage due to maximum displacement response and cyclic deteriolation can be made over to ductility factor and cumulative fatigue damage, respectively. So, from viewpoint of collapse, it must be investigated the properties of lowcycle fatigue for RC structures. In order to investigate low-cycle fatigue damage in RC columns, seven reinforced concrete frames with rigid beams have been tested. As the results, restoring force characteristics, experimental equation of stiffness deterioration, △δ-N_f curve and cumulative fatigue damage hypothesis have been formulated. Then, the dynamic response analyses have been carried out by applying the restoring force characteristics obtained from the tests and the results of the dynamic response analyses have been evaluated by the concept of cumulative fatigue damage formulated from the tests. From the results of the study reported in this work, it is concluded that : (1) The Wohler curve (△δ-N_f curve) can be shown as Fig. (5). (2) As shown in Fig. (7), the linear cumulative fatigue damage hypothesis can not be applied for low-cycle fatigue damage of reinforced concrete columns. (3) The non-linear cumulative fatigue damage factor for reinforced concrete columns can be suggested in the form of Eq. (7) and Eq. (8). (4) In case that the maximum displacement response is in the work-hardening region of skeleton curve, the seismic damage of RC columns depends on only the ratio of the maximum displacement response vs. the collapse displacement of the column and the damage caused by cyclic deterioration (cumulative fatigue damage) can be neglected, i.e., only ductility factor is important for prediction of damage in this domain. (5) In case that the maximum displacement response is the falling branch of the skeleton curve, the seismic damage of RC columns must be evaluated by a combination of the maximum displacement response and the cumulative fatigue damage. The seismic damage caused by low-cycle fntigue becomes 20-35% of the full damage at that time, i.e., not only ductility factor but also cumulative fatigue damage is important for evaluating seismic damage in this domain. (6) The seismic damage factor decreases hyperbolically as the period T increases. (7) When T≤0.1sec, the seismic damage increases rapidly as the ratioβ=mz_<max>/Q_y increases a little, and the structure collapses suddenly, just in case β=1.0. (8) Though the structures were subjected to various kinds of eathquake motions, cumulative hysteretic absorbed energy has the tendancy to be constant for each period at collapse of the structures.
