Seismic retrofit of non-ductile RC frames using friction dampers

Recent earthquakes around the world have shown the disastrous effects of earthquakes on infrastructural facilities. The risks are particularly high in the eastern and central parts of United States, where most of the existing stock of buildings were designed without any consideration for seismic loads, and there is a potential for earthquakes to occur. In light of this, the aim of the research presented in the thesis was to propose and evaluate a practical seismic retrofit scheme for such structures. Earlier research had shown that the existing nonductile RC frames would undergo large drifts even under moderate earthquake loading. The proposed method aims to limit the drift levels without increasing the loading on the foundations. The proposed retrofit scheme involves the use of slotted bolt type friction dampers incorporated along with masonry infill walls. The proposed scheme has the advantage of being easy to install in an existing building and being economically realistic. The evaluation procedure for the scheme can be categorized into three parts: experiments, time history analyses and proposal of a design methodology. The scheme was experimentally evaluated using shake table tests on a 1/3rd scale one story one bay specimen. Under two contrasting ground motions, the wall-damper scheme exhibited different effects on the frame response, indicating the importance of ground motion characteristics on the effectiveness of the retrofit. An analytical model was used to simulate the test results. To further understand the effects of using friction dampers in multistory nondurable RC frames, nonlinear time history analyses were performed for a three bay three story nonductile RC frame. Nine different ground motions with two levels of loading were used. For each level of loading, the expected performance criteria in terms of maximum interstory drifts and maximum foundation loads were defined. The analyses showed that introducing the friction dampers improved the performance of the frame for the wide range of ground motions applied. The success of the retrofit scheme however depends on the slip load setting in the friction dampers. From the analysis process it was determined that a large number of time history analysis runs would be required to obtain the optimum slip load setting. To make the design of friction damper retrofit schemes more acceptable for practicing engineers, a simplified design method, called the Inelastic Demand Spectrum (IDS) method, was proposed. For two different frames, a three story and a ten story frame, it was shown that the IDS method predicted the optimum slip load setting accurately. The method also provided interesting insight into the behavior of nonlinear structures under seismic loading.; To summarize, a practical and economical seismic retrofit scheme was proposed; the scheme was experimentally evaluated using shake table tests; several time history analyses were performed to study its effectiveness and a simplified design methodology was developed for obtaining the optimum slip load.