Design of diagrid exoskeletons for the retrofit of existing RC buildings

The pursuit of a sustainable society requires an extensive intervention on the existing buildings, which are responsible for the major share of greenhouse gas (GHG) emissions. In addition, such constructions have exhausted their nominal structural service life and are vulnerable to seismic hazard. In such a scenario, new integrated retrofit techniques have been proposed to foster the holistic and sustainable renovation of the European obsolete building stock, thereby boosting the current renovation rate. In this paper, diagrids are proposed as structural exoskeletons for the renovation of existing reinforced concrete (RC) buildings. The diagrid system is an inclined structural grid withstanding both vertical and horizontal loads to which a building is subjected. Such a system was initially proposed and is usually adopted in tall new buildings with the aim of creating structures with strong architectural identity, without vertical columns. Diagrids are suitable solutions for the integrated renovation (energy, architecture and structure) of existing buildings, and they may be applied from outside to avoid the occupants' relocation. They may be assembled in different steps over an extended period of time by adopting an incremental rehabilitation strategy, thereby increasing the economic sustainability of the interventions; finally, they may be designed in full compliance with the principles of Life Cycle Thinking. In this paper, two methods for the design of elastic diagrids as retrofit intervention are proposed. The first method is an analytical design method which can be regarded as the extension of previous studies on diagrid systems for tall new buildings. The second method entails the definition of design spectra from which both stiffness and strength of the diagrid exoskeleton can be obtained. The latter is obtained from sensitivity analyses carried out on a simplified SDOF system, and it stems as the extension of existing procedures for the design of bracing systems. Both methods are then applied for the design of the structural retrofit of a RC building typical of the post-WWII European building stock. Theoretical results have been compared with results obtained with nonlinear time history analyses, showing the effectiveness of the proposed design methods.