Dynamic tests on an existing r.c. school building retrofitted with external steel "dissipative towers

This paper deals with the dynamic tests carried out on a school building (four story reinforced concrete frame) seismically retrofitted with an innovative system that uses external steel dissipative towers. Before the retrofitting, ambient vibration tests were carried out with the aim of evaluating the actual linear dynamic of the building including the contribution of non-structural components (e.g. external and internal walls). Modal parameters determined by means of experimental tests are crucial for the final design of the retrofitting system allowing the calibration of a predicting f.e. model. Three low noise servo-accelerometers per floor were opportunely positioned at each floor to monitor its rigid motion. The modal parameters are identified with the Enhanced Frequency Domain Decomposition technique obtaining the natural frequencies, mode shapes and damping ratios. After the building retrofitting, snap-back tests at different load levels were performed in order to evaluate the dynamic characteristics of the new structural system. The load was applied in a quasi-static manner by means of two Dywidag bars 47 anchored at the last floor and connected to a steel triangular truss pulled by two hydraulic jacks; the instantaneous release was obtained by cutting a dog-bone shaped steel plate with a blowtorch. In addition to the accelerometers, displacement transducers were positioned at the base of the steel towers, close to the viscous dampers, to measure the motion of the two towers. The natural frequencies and the damping ratios of the retrofitted building are estimated from the time histories of accelerations (free decay functions) by means of the crossing time and the logarithmic decrement techniques, respectively. The adopted experimental methodology adopted has revealed to be effective for the dynamic characterization, both at very low strain and at higher strain level, of a low-rise reinforced concrete frame building in service, which is characterized by a high overall stiffness due to the contribution of the non-structural components.