Diagrid solutions for a sustainable seismic, energy, and architectural upgrade of European RC buildings

The refurbishment of the existing building stock is nowadays becoming a priority in order to meet energy-saving and emission-control international targets and to foster safety and resilience of European communities. A new research recently introduced the concept of holistic seismic, energy, and architectural renovation of existing buildings targeting resilience, safety, and sustainability. Integrated retrofitting techniques have been proposed, and a new structural design procedure has been studied rethinking existing approaches by including sustainability principles. With reference to post-WWII RC buildings, which are often mid-rise isolated buildings located at the city outskirts, additional exoskeletons implementing the technologies and devices for an integrated upgrade have been proposed. Exoskeletons are entirely built from outside, thus avoiding the temporary dismissal of the buildings and the relocation of the inhabitants. Both ‘shear wall’ or ‘shell’ solutions, either dissipative or over-resistant, can be envisioned for structural retrofitting. In the first solution, shear walls can be integrated in the new exoskeleton, whereas energy efficiency upgrading is guaranteed by the envelope, thus the two structure-energy systems work in parallel. In the ‘shell’ solution, the building envelope has both energy and structural functions. In this paper, both over-resistant and adaptive diagrids are introduced for the holistic refurbishment of existing buildings. Over resistant diagrids are conceived for the seismic upgrade of those buildings having stiff masonry infill walls and staircase walls, for which dissipative solutions may be ineffective unless massive preliminary interventions are carried out to downgrade the existing building initial stiffness. Adaptive diagrids are conceived as over resistant ‘shell' structures to avoid any damage at the operational limit state, while dissipation is triggered through dissipative rigid-plastic supports to reduce shear at the grid foundations at the life safety limit state. Selection of materials and technologies, enabling maximum adaptability, reparability and maintenance, and total demountability-recyclability/reuse at end-of-life is also discussed.