On the occasion of medium-high seismic tremors intensity is common to encounter the breaking and overturning of the external wall infills at the closure of the buildings with reinforced concrete frames, unfortunately one of the damages more dangerous for public and private safety. In their original design, in accordance with technical standards of the time less sensitive to seismic risk, most of the existing concrete buildings it did not contain anti-seismic deviceswhich they are mandatory today for new buildings and for seismic improvement interventions on existing buildings.
Therefore, in the varied panorama of civil construction, built between the old and new centuries, infill panels are frequently created without effective connections with the frame in approx (Fig. 1) or with poor quality of materials and mortars (Fig. 2).
How does the external infill break down?
Sometimes the failure of the external infill happens without be accompanied by significant damage to the structural framehowever the overturning even only partial of the outer lining can result serious risks for the safety of people, as well as the unusability of the home. The infill panels can in fact be affected and stressed, beyond the limits of their lower resistance and sometimes high slenderness, by the displacements induced by the reinforced concrete frame during seismic action.
In these cases we witness a crushing failure at the corners. Although they are not structural elements, if designed with adequate rigidity and effective connections they can contribute, within their limits, to containing the displacements of the RC 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.
Him too external edgeswhen without suitable constraints, represent critical points tending to detach under dynamic stress (Fig. 3).
How to intervene? The techniques
How can you intervene? make safe the external partitions in case of earthquake?
The techniques they are simple, easy to execute and they do not cause any particular inconvenience to the users of the buildings as they work mainly from the outside. Therefore, their safety can be associated with other external restructuring interventionssuch as the application of a thermal coat or a traditional renovation of the facade plaster.
It is useful to remember that the same Guidelines for the Classification of Seismic Risk, closely linked to the recent Sismabonus incentives, provide that, for buildings framed in both main directions, it is possible to guarantee the improvement of a seismic class if you intervene to insert anti-tipping constraints of the external infillsas well as carrying out the node confinement and the restoration of any critical issues due to degradation or crack patterns.
The intervention includes a preliminary stage removal of part of the plaster external between the upper RC beam and the panel, possibly also to be extended to the border area with the pillar or, in the case of more extensive reinforcement, along the entire perimeter or even to the entire surface of the wall panel (Fig. 4).
Subsequently yes they will prepare holes at regular intervals on the reinforced concrete elements that will serve as grouting of anti-tipping mesh strips in fiberglass or PBOlaid using two thin layers of structural mortar (one for support and the other for closure, for a total thickness of no more than 6-10 mm as this is an FRCM intervention).
The extension of anti-tipping net it depends on the raised plaster area and must be placed between the reinforced concrete frame and the infill (Fig. 5). Finally, the net is anchored using flakes of the same material as the net, grouted with specific fluid mortar and loosened onto the second layer of structural mortar used to lay the net itself (Fig.6).
At the end you can apply the finishing plaster.
The type of processing results non-invasive and does not stiffen the existing structure, on the contrary making a significant contribution to countering tipping and ejection of the external wall linings.
the improvement of safety also comes through timely interventions like this one.
