Seismic retrofitting of academic buildings using roof isolation as a tuned mass damper

A large percentage of buildings in Colombia do not have proper earthquake resistance features, as in the case of the city of Cali, where 70% of structures are vulnerable to seismic events. An example of this is the Faculty of Engineering of the del Valle Del Valle University, where the buildings, which were built more than 40 years ago, need to be evaluated and retrofitted according to Regulation NSR-10. However, conventional reinforcement techniques for buildings are highly invasive, disrupt building operations and can be very costly. As an alternative, this article analyzes a retrofitting technique that uses the roof isolation system. In this system, the roof slab is disconnected from the structure and is supported by elastomeric seismic isolators, functioning as a tuned mass damper (TMD), which is an easy configuration due to the type of structure of the Faculty's buildings. To verify the effectiveness of the proposed system, the buildings were classified into four types according to their geometric characteristics and floor layout, and both the conventional reinforcement and the TMD were designed. A dynamic characterization was obtained through ambient vibration tests, with four seismic accelerometers placed on top of the buildings to identify the predominant frequencies and damping ratios of each vibration mode. The parameters were determined using three identification techniques, and these data were used to fit finite element models of the buildings. The roof isolation system reduces the dynamic response and allows the structural elements to withstand the stress of earthquakes, which shows that the design of the TMD is acceptable. The cost of the roof isolation alternative is between 68 and 86% less than the conventional proposal. This form of seismic retrofitting represents a convenient option for existing buildings of all types of use.