Damage to non-structural elements and post-earthquake assessments
Following the post-earthquake usability inspections (Aedes sheets), it often happens that the building is declared unusable without significant damage to the load-bearing elements, but in the presence of potential risks deriving from internal rear-end collisions in the process of detachment, falling tiles, breaking of floors, etc.
The Technical Standards for Construction (NTC 2018) therefore underline the need not to neglect the risks linked to the so-called secondary elements such as tiles, chimney caps, cornices, infills (Fig. 1), false ceilings, pipes, systems, furniture.
Secondary structural and non-structural construction elements
More specifically, the 2018 NTCs distinguish between secondary structural elements and non-structural construction elements.
Regarding the first ones «(…) The stiffness and resistance to horizontal actions of these elements can be neglected. These elements are designed to resist only vertical loads and to follow the movements of the structure without losing load-bearing capacity (…)». They belong to this first category tiles, fireplaces, false ceilings and anything else does not have sufficient rigidity to influence the dynamic behavior of the structure.
Regarding the latter, the technical standards mean «(…) For non-structural construction elements (…) those with stiffness, resistance and mass such as to significantly influence the structural response and those which, although not influencing the structural response, are equally significant for the purposes of safety and/or safety ‘safety of people (…)». They definitely fall into this category external infills of reinforced concrete frameswhich in many cases can contribute, within their limits, to containing the movements of the frame during the seismic shock. Significant, for example, is the positive contribution of the infills closing the pilotais plane which represents a typical zone of seismic weakness.
Seismic safety of systems
The technical standards also require particular attention to seismic safety of systemsspecifying that «(…) of the anti-seismic design of the systems the manufacturer is responsibleof the anti-seismic design of the power supply and connection elements the installer is responsiblethe structural designer is responsible for the anti-seismic design of the horizontals, infills and partitions to which the systems are anchored. The capacity of the different functional elements making up the system, including the structural elements that support and connect them to each other and to the main structure, must be greater than the seismic demand corresponding to each of the limit states to be considered. It is the task of the designer of the structure to identify the demand, while it is the task of the supplier and/or installer to provide systems and connection systems of adequate capacity (…)»
Good practices for anti-seismic improvement
The common thread to guarantee the seismic safety of all non-structural elements are i connections with the load-bearing structures. They can be diversified depending on the type of element.
We can remember some good practices such as, for example, a greater stiffening of chimney caps by sheathing with flexible metal mesh and braces connected to the roof; bracing of false ceilings (whose load-bearing structure is made more integral with the floor joists), minimizing their horizontal movement in order to prevent the panels from falling and making the lighting fixtures connected to them self-supporting; consolidation and better connection of the cornices; anchoring of parapets and plasterboard walls with metal or fiberglass mesh (covered with subsequent plaster) appropriately connected to the load-bearing structures; cover the windows with protective films of large glass surfaces, so as to avoid shattering in the event of a fall; increase the number of roofing tile anchoring devices; fix large wardrobes and furniture to the walls (in many cases people’s safety was put at risk more by the fall or overturning of heavy furniture than by damage to the structures of the house); protect with metal elements with “parapet” function the goods on supermarket shelves.
Connection of systems to structures
Regarding the systems, they cannot be tied to the construction by relying on the effect of friction, but must be connected to it with rigid or flexible constraint devices.
As always happens in the seismic field, safety derives first of all from simple construction details that improve connections and limit movements: this applies both to the load-bearing elements of the building and to the non-structural ones.
