Strengthening Techniques

In the following, a few anti-seismic strengthening techniques are illustrated. The presented techniques were adopted for the seismic upgrading of some churches after the 2004 earthquake.

3.1 Internal perimeter ties

The adoption of perimeter tie system is a traditional technique, which was effectively adopted to secure the

Figure 8. S.Pietro Church in Roe Vociano, Brescia, Italy: a) total and partial collapse of barrel vaults of the first and second bay of the main nave, and b) crack pattern induced by the differential rocking and deflection of the single leave groin vaults.

Figure 8. S.Pietro Church in Roe Vociano, Brescia, Italy: a) total and partial collapse of barrel vaults of the first and second bay of the main nave, and b) crack pattern induced by the differential rocking and deflection of the single leave groin vaults.

structure against wall overturning. Historic constructions often present perimeter ties embedded within the wall thickness.

When the tie confining effect is insufficient the tie system must be strengthened or replaced. This operation is usually done by placing a new external tie system, provided that the embedment of the ties within the wall width would require expensive and difficult drilling works (Figure 9a).

The static scheme of the external perimeter tie system is illustrated in Figure 9c. For the solution to be effective, the perimeter ties must confine the resisting ideal horizontal arch, developing within the masonry wall width, at the springing. This solution was applied in San Pietro and Paolo Church, Preseglie, Italy (Figure 10).

The same confining action can be effectively obtained by adopting an internal tie system, hidden on top of the nave mouldings (Figure 9d). This solution has the obvious advantage of being hidden from the sight, but it may also serve in case of irregular or preciously decorated external walls. The confinement of the resisting natural arch can still be obtained, provided that the inclined compressed strut forms in a)

Figure 11. Detail of the internal perimeter ties hidden over the nave moldings in San Pietro Church, Roe Volciano, Italy.

Figure 9. Introduction of internal or external perimeter ties.

Figure 10. San Lorenzo Church (Clibbio, Brescia, Italy): external perimeter ties adopted as safety emergency provisional solution following the 2004 earthquake.

Figure 12. Floor and roof box structure preventing perimeter walls overturning.

Figure 10. San Lorenzo Church (Clibbio, Brescia, Italy): external perimeter ties adopted as safety emergency provisional solution following the 2004 earthquake.

the masonry intersections (Figure 9d). This solution was applied in San Pietro Church, Roe Volciano, Italy (Figure 11).

Perimeter horizontal steel ties are inadequate in long-span buildings lacking strong transverse arches, as the wall span-to-thickness ratio is unfavourable and little constraint is provided to the toppling masonry walls (Figure 9b). In this case, regardless of the positioning of the perimeter ties, the resisting arch is excessively low-raised and its resistance is negligible. In case of long spanned churches, roof box structure, which are described in the following, were alternatively adopted.

Figure 12. Floor and roof box structure preventing perimeter walls overturning.

3.2 Roof box structures

As shown in the previous section, most of the analysed churches were found to be vulnerable with respect to overturning of the perimeter walls, as well as to excessive and to differential rocking of the diaphragm arch pillars (Table 1). In-plane shear resistant roof diaphragms, transforming the building into a box-structure, were adopted in these churches as a viable solution to avoid or limit these mechanisms (Giuriani and Marini 2006 and 2008).

Roof box structures behave like floor diaphragms, forming a sort of lit to the underlying masonries, thus preventing the perimeter wall overturning (Figure 12). The roof pitches are transformed into folded plates, which gather and transfer the horizontal seismic actions of the lateral walls, the diaphragm-arches and the roof to the shear resisting walls.

The roof box structure introduces an elastic constraint along the roof ridge, whose stiffness depends on the horizontal flexural deformability of the roof box structure (ye, Figure 13b). The box structure can c)

roof box structure ideal constraint roof box structure ideal constraint

Figure 13. Structure behavior a) prior to and b) following the introduction of the roof box structure preventing excessive and differential transverse arch rocking.

be proportioned in order to confine the maximum mid span displacement (ye) and the masonry wall drift (0), as well as to limit the shear distortion near the supports (ye', Figure 13b). This way the box structure can be an effective solution to the excessive and differential rocking of the diaphragm arches.

The main structural elements composing the roof box structure are: the pitch-panel (1), the eaves chords (c13), the head gables (2) (Figure 14).

In order to design the roof box structure of the analyzed churches, the same techniques usually adopted for the flexural strengthening of wooden floors, could have been addressed (Giuriani 2004). Among all possible techniques, however, lighter solutions were preferred in order to prevent the increase of the dead loads, and thus of the seismic action.

The wooden lightweight box structure proposed in Giuriani and Marini (2008) were adopted. With the proposed technique roof diaphragms were obtained by placing overlaying plywood panels on the existing wooden roof pitches (Figure 14). Plywood panels were connected to each other by means of nailed steel flanges. The whole pitch diaphragms w nailed to the perimeter steel eaves chords, and to both roof rafters and masonry walls by means of steel studs and vertical anchored bars.

The adopted lightweight wooden box structure technique is mainly reversible and minimally impairing of the building integrity, and thus respectful of the modern restoration principles.

As for the structure analysis and proportioning, reference to Giuriani and Marini (2008) was made.

plywood panels
Figure 14. Wooden roof box structure by means of overlaying nailed plywood panels. Principal structural component and detail of the plywood panel assembly.

By reference to the load distribution shown in Figure 15, in order to analyze the roof box structure behavior, a two step approach was adopted. In the first step vertical and horizontal idealized additional constraints, having r1z and r1y reaction respectively, were introduced along the roof ridge line (Figure 16). In the second step these provisional constraints were removed and their reaction forces (r1z and r1y) were backed out.

In the first step, the unit-width stripe (a) with additional vertical and horizontal constraints, behaves like a frame undergoing the seismic actions (case A Figure 16). In this phase, significant uplifting vertical force per unit length nA may arise along the top

h BEHAVIOR STRUCTURE

h BEHAVIOR STRUCTURE

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