Steel exoskeletons for low-impact and Integrated seismic retrofit of existing buildings

A significant portion of Europe's building stock was constructed following World War II. The absence of updates and enhancements has left this constructed environment exceedingly vulnerable to seismic events and energy inefficient. Due to PNRR tax incentives, there is a distinct necessity to delineate and supply qualified professionals with integrated, efficient, prompt, and cost-effective retrofit solutions. These treatments ought to be formulated with a comprehensive approach and grounded in the principle of Life Cycle Thinking. Traditional seismic retrofitting is often hindered by high costs, long execution times requiring building halts, and environmental concerns stemming from excessive material usage. This thesis investigates modern, non-disruptive, and environmentally conscious alternatives, focusing on external steel frame structures, or exoskeletons, which facilitate an integrated approach to resilience and energy improvements. Exoskeletons leverage the simplicity of steel assembly to reduce costs, minimize operational disruptions, and improve structural resilience. They are also compatible with dissipative or self-centering devices for improved seismic performance. The integration of energy-dissipating devices allows for the absorption and dissipation of seismic energy via inelastic deformations, thereby decreasing stress on the existing reinforced concrete structure. Such a retrofit provides a precise strategy, preventing excessive strengthening and concentrating on seismic demands. This leads to lower loads for new foundations and minimal impact on the original building, potentially diminishing overall intervention costs. In this scenario, innovative devices have been selected that can dissipate energy resulting from seismic activity, as well as seek to improve the resilience of the structure through devices that allow the building to return to its plumb position. To achieve this objective, the devices analyzed within this thesis work are rectangular and hourglass-shaped Buckling-Restrained Aluminum Shear-Yielding Plates; Dumbbell-shaped Steel Slit Dampers; Shape-Memory-Alloy-based dampers. This thesis applies and sizes several exoskeleton systems for strategic RC structures. The investigated retrofit strategies, all characterized as steel exoskeletons, include Dissipative Eccentric Braced Frames, Self-Centering Concentrically Braced Frames, Self-Centering Dual-Rocking Frames, and Base-Rocking Dual-Core Frames. The design of these interventions utilizes the Displacement-based Design approach. This method features constraints that may emerge when employed to build a retrofit for an actual case study. This thesis addresses several issues arising from the retrofit system or the as-built structure. The considerations encompass the energy dissipation capacity of the installed device and the connection system between the structure and the exoskeleton, which must guarantee the complete transmission of stresses from seismic forces while preserving rigid behavior, including the validation of the initial rigid diaphragm assumption. The retrofit strategies investigated in the matter of this work were thought and designed for real-world strategic RC buildings case studies. Their capacity to handle seismic forces was assessed starting from the linear (eigenvalue) modal and nonlinear (pushover) static analyses by Eurocode 8. Based on collapse mechanisms of each structure, the target displacement was defined, which is useful for designing the various interventions. The effectiveness of the retrofit strategy and the design procedure have been assessed by nonlinear Time-History analyses under a set of earthquake-strong ground motions selected and scaled for the Collapse Prevention Limit State based on the spectrum-compatibility rules of the Italian Code (2018).