Influence of higher vibration modes on the seismic performance of seismically isolated structures: Chilean hospital as a case study

The demographic growth of population and the contemporary environmental disasters are two of the main challenges faced by civil engineers. As buildings are getting taller and their seismic isolation systems are getting more sophisticated, understanding the structural behavior of these isolated structures is a complex task. Assessing the behavior of base isolated structures in seismically active countries is a main topic of the research work conducted by the Prof. Juan Carlos de la Llera at the Pontificia Universidad Católica (PUC) de Chile. Indeed, Chile is one of the most active seismic regions in the world and is the home to the tallest skyscraper in Latin America. Furthermore, the city of Santiago is a vast city, with a dense population and a complex healthcare network system that is also growing with new healthcare facilities. This thesis focuses on the seismic performance of base isolated buildings such as hospitals and more specifically on the effect of higher modes on the response of the structure (in displacement and accelera-tion). The higher mode effect relates to the influence of high vibrational frequencies on the displacement or acceleration. These high modes are that more important when considering non structural elements such as acceleration sensitive equipment present in hospitals. Through the implementation of linear and non linear models of base isolated buildings that share the same geometry of typical Chilean hospitals on the open-source software OpenSees, the effect of higher modes is evaluated and quantified by conducting dynamic history analyses using various sets of natural ground motion records. The base case for the comparison and extraction of the higher modes is analysed and a methodology using a physical filter and a numerical filter is elaborated. The objective being to identify the different modes and to extract their contribution to the response. The results obtained indicate that the first mode governs the displacement regardless of the isolator, however, the second mode and third modes have an important contribution to the acceleration response of the upper floors of the tall buildings. A result that is even more apparent for the non linear isolator. The key takeaway would be to incorporate the higher mode effects in the design codes and to be able to use the findings in practice when studying tall isolated buildings. Thus, this research work aims to fill the gap in knowledge regarding the impact of high frequency modes on the response of base isolated complex structures such as hospitals.