The seismic regulations of the 70s and 80s
In the era of reconstruction by the greats seismic events of the 70s and 80s (earthquakes in Friuli and Irpinia), the technical-regulatory corpus was linked to one predominantly prescriptive approach aimed atadaptation of existing buildings to the seismic safety requirements of newly designed buildings.
Among the requirements to be implemented included the execution of reinforced concrete (RC) top curbs, the diffusion of which was almost total not only in post-seismic reconstruction sites but also in the culture of building maintenance until the end of the 1990s.
The problems that emerged from the 1997 Umbria-Marche earthquake
It was the 1997 Umbria-Marche earthquake to highlight how the type of concrete curb has not always workedfor a series of errors in the construction details and in the evaluation of the correct interaction with the wall structurewhich in fact did not guarantee the correct closure of the wall box, but actually favored the partial collapse of portions of the underlying masonry. A series of seismically adequate buildings following the traditional guidelines of the 70s and 80s on the seismic recovery of existing buildings, which also included the replacement of wooden roofs and floors with heavy reinforced concrete slabs, resulted in the subsequent earthquake many partial collapsesa large part of which is due to the presence of very rigid reinforced concrete curbs, both at the top and between the floors.
THE’excessive stiffness of the reinforced concrete has effectively transformed the curb into a beam on two supports, transmitting most of the weight of the roof bearing on it only to the terminal wall supports, and consequently leaving the central part of the underlying wall unloaded. Instead of uniformly distributing the vertical load over the entire length of the underlying wall, in fact the latter has actually been weakened at a seismic level since, made even more unloaded by the vertical weights above which should have stabilized it, it becomes more vulnerable to seismic actions at it orthogonal (fig. 1). In fact, the top concrete curb was often cast simply adhering to the top of the wall, without providing any type of vertical connection that could better bind the curb to the wall below.
The revision of the technical standards after 1997
After the 1997 Umbria-Marche earthquake, the technical standards aimed at repairs took into account specific vertical groutingwhich however, 20 years later (2016 Central Italy earthquake) in some situations showed the same collapses, due both to the insufficient anchoring length of the vertical bars and to the poor quality of the walls. In particular for this last situation it has been verified that even in correspondence with very deep and well-designed grouting, the underlying walls are still “collapsed” due to the poor quality of their weavingoften multilayered, leaving the grouted bars hanging from the curb (fig. 2). Especially this last case teaches how the quality of the wall texture on which you are going to grout must first be analyzed in depthand possibly preliminarily provide for an improvement in its monolithic and structural behavior.
However, they were not lacking positive cases in which the presence of vertical bars of adequate length is one good quality of the underlying masonry have allowed the summit curb in reinforced concrete effectively carry out its role as an anti-seismic protection limiting damage and improving the box-like behavior of the building.
Alternative solutions to reinforced concrete curbs
Over time, alongside the traditional ca technique, they have matured new and alternative construction solutions to overcome the excessive rigidity of the traditional solution. Today the progress of technologies also allows for the execution of summit curbs with lighter materials and techniques that are compatible with existing buildings, for example in lamellar masonry by reinforcement with composite materials of at least three mortar joints of the top wall perimeter (fig. 3), or in steel by means of perimeter dowelling of metal profiles or plates (fig. 4).
These new techniques have the advantage of do not stiffen further the top ensuring an effective connection with the underlying walls which are correctly stabilized by the vertical loads coming from the roof. In all cases, great attention must be paid to the quality of the wall texture on which the curb is made and the vertical grouting bars are inserted.
