Study On The Stiffness Degradation Limit Of RC Members Under Long Duration Seismic Loading

It is essential to define the function of the structure by the damage quantification index(model)in the performance-based structural design methods.The rationality of the damage quantification index(model)depends on the oneness between the damage description and the structure damage mechanism.That is to say,whether it can synthetically reflect the destructive potential of the earthquake motion intensity,duration and spectral characteristics to the structure.Compared with the design method based on bearing capacity,the adoption of the peak deformation of a structure(component)as the standard index of performance design is a significant progress in design theory.But it is undeniable that this design method is still based on the peak response of structure,which can consider the influence of the earthquake motion intensity and spectral characteristics,but it still can not effectively consider the influence of duration on the structure.In recent years,several violent earthquakes have shown the remarkable long duration feature such as the Fukushima earthquake in Japan in 2011,the 2010 Chile earthquake and the 2008 Wenchuan earthquake in China,which lasted 300 seconds,200 seconds and 180 seconds,causing serious damage and collapse of the structure.Thus,how to reasonably consider the influence of earthquake duration in performance evaluation of structures is of great significance for the enrichment and development of performance-based seismic design.This project,supported by the National Natural Science Fund(50908022).Based on the different variable amplitude loading tests of 9 RC component specimen,the influence of the duration of earthquake,peak displacement,longitudinal ratio and stirrup ratio on the damage of RC component were investigated.Besides,the possibility of giving the performance division standard of damage process under different variable amplitudes according to the stiffness reduction of beam was discussed.The experiment shows that under the condition of same peak displacement,the apparent damage of RC component becomes larger as the duration becomes longer or the peak displacement arises earlier.The increase of reinforcement can restrain the development of apparent damage,but the loss restrain effect of increasing longitudinal bar is more effective than increasing stirrup.Moreover,the development of stiffness degradation of RC component is synchronous with the development of apparent damage.It is found in the research that,stiffness damage index proposed in this paper can provide unified standard of performance division of the damage process under different variable amplitudes.Therefore,it is proposed that reinforced concrete component is corresponding to the stiffness damage index with a range of 5 performance states.Through comparing stiffness damage index with peak deformation index and Park-Ang damage index,it is found that stiffness damage index is in accord with peak deformation index on defining the light damage of RC component,while stiffness damage index is in good agreement with Park-Ang damage index on defining the moderate and severe damage of RC component.