Phd in STRUCTURAL AND GEOTECHNICAL ENGINEERING

Thesis title: Enhancing Seismic Safety and Energy Efficiency of Existing Buildings through External Low-Damage Exoskeletons

The European building stock, mostly built post-World War II with no regard to seismic design and energy efficiency principles, is unsurprisingly facing significant safety and energy efficiency challenges. The structural/seismic vulnerability of existing buildings has been further confirmed by recent earthquake disasters (e.g., L’Aquila 2009, Emilia 2012, Centre Italy 2016, Turkey & Syria 2023), whereas the energy inefficiency is underscored by high energy consumption rates. An unprecedented effort is therefore required to achieve energy savings and decarbonization targets by 2030 and 2050, respectively, to meet the ambitious goals of the European Green Deal. Although several technical solutions are available for improving energy efficiency, it is advantageous to pursue integrated renovation strategies (i.e., structural and energy efficient), especially when dealing with buildings located in zones with moderate-to-high seismicity, thus ensuring the transition towards a more resilient, energy efficient, and sustainable built environment. This work explores the application of exoskeleton-type solutions for the integrated renovation of existing reinforced concrete buildings. Specifically, external load-bearing systems consisting of low-damage structural members (i.e., implementing the PREcast Seismic Structural System technology, PRESSS; its extension to timber with the Prestressed Laminated timber technology, Pres-Lam; or hybrid solutions combining reinforced concrete and timber), that upgrade the seismic performance by working in parallel with the existing building, are considered. Such a solution is growing in interest, given the potential to execute the intervention entirely from outside the building, thus limiting occupant disruption and avoiding inhabitants’ relocation. This aspect is crucial in motivating owners to choose a combined renovation, rather than just focusing on the energy one, which could be easily impaired even in case of low-to-moderate earthquake shaking if implemented on structurally unsafe buildings. Concurrently, the exoskeleton operates as the support for a high-multi-performance “double-skin” facade system, contributing to enhanced energy efficiency and facilitating a holistic renovation. The first goal of this work is the proposal and validation of a fully analytical Displacement-Based Retrofit procedure for designing external exoskeletons in the form of frame systems. More specifically, considering several case-study buildings representative of constructions erected before the enforcement of modern seismic codes, the analytical procedure has been as first used for the design of exoskeletons. Consequently, several numerical simulations, including both non-linear static and dynamic analyses, have been used to validate the procedure itself. After the proposal and validation of the design procedure, this work aims to prove the effectiveness, thus supporting the attractiveness, of low-damage exoskeletons when compared to local interventions aiming at the inversion of the “hierarchy of strength” within beam-column joint subassemblies (e.g., the implementation of Fibre-Reinforced Polymers, or Concrete Jacketing). In this case, alternative retrofit strategies have been implemented on a pre-1970s reinforced concrete case-study building, and numerical analyses have been implemented to evaluate the seismic performance in terms of several indicators, including the Safety-Index, the Expected Annual Losses (EAL), and the probability of collapse. The results of this work confirm the improved performance of low-damage exoskeletons when compared to similar global strategies based on traditional reinforced concrete monolithic exoskeletons, rather than on low-damage technologies, and even further when compared with local strategies. Finally, this thesis explores the efficiency of the proposed integrated renovation strategy solution for the holistic renovation of the existing building stock. In this case, a direct comparison with traditional combined renovation approaches, widely adopted by practitioners, has been carried out. Such a comparison should involve several criteria, including seismic and energy performance indicators, as well as environmental considerations, rendering the selection of the best alternative far from straightforward. For this reason, a Multi-Criteria Decision-Making analysis, carried out in a Life-Cycle Thinking perspective, has been adopted. In this case, even dynamic energy simulations have been implemented to assess the energy performance of the considered alternatives. Moreover, Life-Cycle Assessments have been implemented in order to define all the data related to the considered environmental criteria. The results that will be presented in the following chapters support the efficiency and effectiveness of the proposed solution with respect to traditional strategies, especially if eco-friendly materials (i.e., timber) are implemented.