Research On Seismic Strengthening Design Method For Existing RC Frame Based On Viscous Damper

Higher demands on seismic safety of building structures are required with the development of society.Especially with the implementation of the new mandatory national standard "Seismic ground motion parameters zonation map of China",a large number of existing buildings face the urgent need for seismic strengthening.Among various retrofitting techniques,seismic retrofitting scheme based on viscous dampers has been widely adopted since it has advantages of simple concept,clear mechanism,and significant effect compared with the traditional retrofitting methods.In China,damping retrofitting are mostly based on minor earthquake,which may not be able to control the specific performance level under major earthquake or meet the needs of performance-based design.So it is necessary to propose a relatively more reasonable damping design method if a certain performance level under medium or even major earthquake is required for some key buildings(like hospitals).Reinforced concrete(RC)frame,which is the most widely used building structure in practical engineering applications,was selected as the research object in this thesis.Besides,this thesis adopted the method based on response spectrum and the elasto-plastic time history method for analysis.Seismic performance before and after anti-seismic reinforcement was analyzed in depth,and the rules of damping effect influenced by relevant design parameters was discussed.Finally,a set of performance-based damping design method based on displacement control of major earthquake was proposed and verified by examples.The main research work and achievements are as follows:(1)Firstly,single-story,three-story,and six-story RC anti-seismic frame structures were designed according to the current national seismic design code with fortification intensity of 7.5-degree,and viscous dampers were used based on minor earthquake in accordance with the requirements for increasing the fortification level to 8-degree.The damping design was carried out for the target with a 35% reduction of base shear force.Various damping designs with different speed index were used.Then,based on large-scale general finite element software,Msc.Marc,elasto-plastic analysis models of each structural example were established.(2)Seismic responses of single-story frame corresponding to different seismic intensity under different waves were calculated and analyzed using the equivalent high damping elastic response spectrum method.By comparing it with the time history analysis results,the effectiveness of this method for structural displacement response prediction was verified,and the possible error causes were analyzed.The calculation of equivalent damping ratio,including structural plasticity hysteretic energy and viscous dampers,were systematically discussed.On this basis,the reason for the declining trend with ground motion intensity increasing was deeply studied.At the same time,the difference in the damping effect between schemes using different parameters was compared and analyzed.(3)The spectrum method was promoted to calculate three-story and six-story RC frames.Compared with the time history analysis results,the validity of the spectrum method for the prediction of the displacement response of the multi-story frame structure(including the calculation of the top displacement and the story drift)was verified,and the possible errors were analyzed.It is shown that this set of method has better prediction for the displacement response of the three-story frame,but there is a larger error in the story drift calculation of sixstory frame.The possible causes of this error were analyzed in depth thus the scope of application of this method was limited.The damping effect of the multi-story frame was compared with the single-story frame to further confirm the characteristics of retrofitting scheme with viscous dampers.(4)Based on the above research results,a set of performance-enhancing design method for RC frame structures whose vibration shape is controlled by first-mode was proposed.This set of design method was based on a standardized response spectrum with large-earthquake displacement control.Then an example was designed by this method and compared with the time-history calculation to confirm the accuracy.At the same time,damping effects of different schemes with different parameters were compared.The results show that this method can well achieve the target performance,and also significantly improve the collapse resistance level under major earthquake.Finally,the cost-effectiveness of the damping structure was evaluated.It is suggested that dampers with a smaller speed index should be used in this method.