Experimental investigation of dynamic behaviour of free-standing frames with friction

Many existing structures in seismic vulnerable areas require seismic upgrading to prevent the collapse in case of extreme earthquakes. This paper proposes an approach based on the separation of the columns base from the foundation, which allows the superstructure to slide in case of large magnitude ground motions. Such a system can cap the base shear transmitted to the superstructure by means of the friction force between the sliding surface and the column base. To achieve a proper value of friction coefficient graphite lubrication is applied on the sliding surface. In order to verify the efficiency of the proposed system, a series of shake table test was conducted. The specimen was designed to simulate the features of realistic structures and to take into account the possible influence of overturning moment and superstructure flexibility. Based on the experimental evidence, the lubricated system showed an efficient behaviour. The graphite lubrication reduced the friction coefficient to 0.16 and provided a stable behaviour, despite a large number of loading cycles. When increasing the magnitude of ground motion, the maximum base shear coefficient of the superstructure tended towards an upper limit close to 0.35. The overturning moment showed a negligible influence on the overall response.