Among the various techniques of active reinforcement, this work focuses on the CAM^® system (active confinement of masonry: CAM is the Italian acronym for active stitching of masonry) [16,17,18,19,20]. This system is an evolution of the strengthening method with post-tensioned horizontal and vertical tie rods [21,22,23,24,25,26,27]. The evolution consists of the strengthening elements of the CAM^® system [28], which are stainless steel straps instead of metal bars. The stainless steel straps pass through the thickness of the masonry thanks to openings made in the masonry by core drilling.

When four stainless steel straps share the perforations and the loops have horizontal and vertical directions, the CAM^® system replicates the reinforcement scheme with horizontal and vertical ties. In this case, known as a rectangular arrangement, the perforations divide the masonry wall into volume units in the shape of right parallelepipeds (Figure 1). Some theoretical analyses [11,29] revealed that the CAM^® system adds confinement forces only in the transverse direction (Figure 1), except for volume units located near the free ends of the masonry wall. Recently, the numerical results of a FEM simulation [15] also confirmed that the confinement provided by the CAM^® system to the wall is not isotropic.

Seismic events impose two types of dominant loads on structures—the in-plane shear load (Figure 2a) and the out-of-plane bending load (Figure 2b)—which causes two types of dominant failure modes in URM structures [33,34]: the in-plane shear mechanisms and the out-of-plane bending mechanisms. The CAM^® system with a rectangular arrangement (Figure 1) is suitable for increasing the out-of-plane strength of masonry walls when used in conjunction with other strengthening systems [29,35,36]. In the plane of the wall, however, the rectangular arrangement does not bring any increase in strength or stiffness. Both in-depth experimental tests [37] and recent numerical analyses [38], in fact, have shown that the rectangular arrangement of the stainless steel straps leads to an almost negligible increase in the strength of the shear-loaded masonry panels while providing a significant increase in ductility. The use of steel grids on both faces of the masonry wall, conversely, increases both the shear strength and the ductility, but the ultimate displacement achieved with the steel grids is lower than that achieved with the CAM^® system [38].

The lack of increase in shear strength with the CAM^® system depends on the arrangement of the perforations, which makes the strengthening system labile to horizontal loads [29]. On the two wall facings, in fact, the straps of the rectangular arrangement form unbraced rectangular frame structures with hinged nodes. Under the action of horizontal forces, these nodes sway laterally exactly like the nodes of the simplified mechanical model in Figure 3a. Therefore, the perforations for the common passage of the straps are cylindrical hinges (Figure 3b), around which the loops connecting the two wall facings can rotate freely.

The in-plane lability of the strengthening system with a rectangular arrangement makes the CAM^® system useless under horizontal loads. In order to allow the CAM^® system with a rectangular arrangement to also improve the in-plane behavior of masonry walls subjected to seismic loads, it is therefore mandatory to eliminate its in-plane lability.

In rectangular frame structures, it is customary to cancel the horizontal displacement of the hinged nodes by bracing the structures along one or both diagonals, as shown in the simplified mechanical models of Figure 4a and Figure 4b, respectively. However, bracing is not the only possible solution to remedy the lability of the rectangular arrangement in the CAM^® system: a 45° rotation of the straps makes the rectangular arrangement effective even in the plane of masonry walls.