When box-like behavior ceases
Unfortunately, in the majority of existing buildings the construction quality or the continuous building transformations that have taken place over time do not guarantee a complete connection between the resistant elements, the perfectly box-like behavior disappears. It is sufficient that even just one wall or floor is not effectively connected to make those elements very vulnerable to tipping out of plane in correspondence with seismic actions orthogonal to them.
These are the more frequent damage which have been encountered in the various seismic crises of recent years, and unfortunately also the most dangerous ones whose magnitude seriously affects the usability of the building as well as the safety of people. Out-of-plane kinematic mechanisms fall within seismic damage classified as mode I (Fig. 2a), whose activation does not allow the development of more dissipative seismic damage or in the plane of the walls, classified as mode II (Fig. 2b).
For this reason it is important, even before carrying out complex three-dimensional modeling, verify visually or by inspection aim sand the connections between the structural elements are satisfactory and such as to guarantee the hypothesis of box-like behavior and seismic dissipation in the building plan. Otherwise the structural model remains only theoretical.
Since seismic safety is based first and foremost on details, It's uselessfor example, have very resistant masonry walls but not connected to each other and the floorssince the earthquake will immediately highlight the lack of connections, favoring the onset of out-of-plane overturning, rather than the activation of mechanical resistance in the plane.
The frequent cases and interventions to counter them
Also there construction quality plays an important role, because where the wall texture is multilayered or bag-like, with inconsistent internal fillings made up of pebbles or variegated rubble, chaotic damage mechanisms are found (Fig. 3) more similar to landslides and disintegration than to kinematic mechanisms of monolithic block. Even more so in these cases, the distance from the ideal box-like behavior that the wall structure should guarantee is evident.
A covering corner pushing onto the walls (Fig. 4), an old and flexible wooden floor not attached to the entire perimeter wall, a wall added or rebuilt over time but not attached to the orthogonal ones (Fig. 2a). These are just some of the frequent cases which in conjunction with a major seismic event develop into partial collapses.
The interventions to counter them they are very simple and of a punctual nature: dry stitching, tie rods, reinforcement of the floors represent rapid anti-seismic protection works with a lower economic impact, but are already such as to restore sufficient overall behavior to prevent partial collapses and bring the damage back to the plane of walls with less serious damage than out-of-plane kinematics. In the box-shaped system, floors and roofs also play an important role, therefore it is essential to provide a balanced stiffening of the floor and punctual connections, such as for example top covering ridges tied to the underlying walls, and punctual stitching of the extrados of the floors to the entire perimeter masonry.
There reduction of seismic vulnerability it can be pursued first of all with local interventions to restore the box-like behaviour, as an indispensable basis for planning subsequent seismic improvement works which also involve the in-plane increase in the mechanical resistance of the walls.
