Prefabricated responsive diagrids for holistic renovation of existing MID-RISE RC buildings

The holistic renovation of existing buildings is nowadays acknowledged as an essential and urgent action to reduce the environmental impact and increase the resilience of the existing building stock. Such holistic interventions envision the adoption of an exoskeleton, entirely built from outside. In this paper, the exoskeleton is designed as an external diagrid, consisting in a lattice structure. Among possible structural typologies, diagrids are particularly interesting for their remarkable architectural potential and for the possible standardization of the components, which highly increases replicability of the retrofit solution. Furthermore, prefabrication of components speeds up the assembly process and reduces the construction time. In the paper, focus is made on the dynamic behaviour of a retrofitted building featuring a responsive diagrid. The responsive behaviour is attained by changing boundary conditions at the diagrid’s base supports as a function of the earthquake intensity, while the diagrid lattice structure remains elastic. At the Operational Limit State, the diagrid is designed as hinged at the base, whereas at Life Safety Limit State, beyond a target base shear, hinges are conceived to downgrade into non-linear supports allowing for the controlled sliding of the diagrid’s base. Non-linear restraints limit the maximum base shear force, while avoiding excessive horizontal displacements and second order effects. Non-linear time history analyses are carried out to study the responsive behaviour of a reference 3 storey building strengthened with an external responsive diagrid. Results show that responsive diagrids require preliminary interventions at the existing building ground floor to reduce the damage following the onset of the diagrid sliding. Such interventions are for instance the disengagement of the infills from the RC frame and the local increase of the column ends ductility.