The adjectives "necessary, focused, minimum, optimum, lightweight, compatible, durable, inexpensive and reversible" are all criteria very often required in a structural consolidation project concerning historical heritages.
On this last term, "reversible", I believe that in close etymological sense no intervention can be defined “completely reversible”, since, even after its removal, the building will preserve memory and traces of the past. However, it should be appropriate to accept and include the concept of “reversibility” into the design process.
Furthermore, I suppose that the consolidation intervention must manifest itself only to those who want to see it, but should remain subdued for those who prefer the original image of the building.
Lastly, but of course not less important, the intervention must be structurally effective in response of vertical and horizontal loads, especially seismic ones.
Thus, the "way" to perform the intervention becomes the fundamental question to ensure the unrepeatable originality of the historic monument.
Geometry, materials and loads are the mechanical parameters that characterize the building, involving the resistance and the stiffness of the whole structure after a consolidation intervention.
The most common trend among designers is to propose changes to the geometry or the materials, but very often changes to the loads would be easier and more effective to be implemented.
For example, it’s well known that placing an archive on the ground floor instead on the first floor, or adding new loads on a vault, to be symmetric or opposed to existing ones, or simply better distributing the new loads on a floor, could be the optimal solution and the most affordable.
In the last decades, the experiences of the author in the field of consolidation of ancient buildings have demonstrated that exist some alternative ways of intervention, that act on loads for strengthening the structure. Some of them are here described, sending to the website www.jurina.it for detailed information.
First, it must be highlighted the importance of temporary consolidation of historic buildings, mainly if not in use or even in ruins. These are frequently subjected to structural instability that causes serious and irrecoverable losses of precious antique materials and sometimes causes risks for people nearby the building. There can be also situations of post-earthquake damage that require interventions for structural safety, where efficacy, easy operability as well as cheapness are needed.
The most usual safety systems make use of props or scaffoldings which, although providing versatility and good resistance, at the same time can be bulky and do not allow free access to the building.
As an alternative to scaffolding, the use of steel stay cables can ensure an adequate level of safety and a reduction of lost space. Some example are represented by the masonry wall of Trezzo sull’Adda (Milan- Italy) [figure 1], the temporary encirclement of the Torrione di Cassina de’Pecchi (Milan- Italy) [figure 2] and a proposal for Pompei Regiones (Italy).
As a further possible solution, in recent years, polyester straps offer excellent confinement possibility, incrementing resistance to traction, with the possibility of pre-tensioning the system by means of "ratchets", so that an immediate and active intervention can be obtained.
Some applications of this technique were realized for the dome of Villa Arconati in Bollate (Milan- Italy), for the tower of Masserano Palace in Biella (Italy) [figure 3], for Gorani tower in Milan and proposed for the ancient tomb of Imedghassen in Batna (Algery) [figure 4].
Regarding the definitive consolidation of arches, vaults and domes it has been developed by the author the so called “RAM – Reinforced Arch Method” [figure 5]. The solution consist in modifying the distribution of loads acting on the arch, vault or dome so that the combination of the old loads plus the new loads could become the “right one” for the given and known geometry of the masonry structure.
Steel tensioned cables, placed at the intrados or extrados, give rise to a distribution of radial forces applied to the surface of the arch, whose value depends on the local curvature. This new load distribution induces a pure axial compression between the blocks so that the thrust line is re-centered, such avoiding or retarding the formation of plastic hinges.
Large scale experimental tests were conducted to measure the improvement of safety obtained with the adoption of RAM Method. More than 500 tests were undertaken by the author, where the arches were subjected either to vertical loads or to horizontal-seismic loads.
Tests showed that the RAM technique induces a significant increase of the collapse load and of the ductility.
Recently, the RAM solution was adopted for the consolidation of the dome of Santa Caterina church in Lucca (Italy) [figure 6], in the consolidation of Cremona Cathedral [figure 7], of Colorno Cathedral (Parma - Italy), for the depressed vault of Galleria dei Poeti in Masino Castle (Turin-Italy) [figure 8] and proposed in many other situations.
Another solution for the consolidation of masonry arches, vaults and domes is represented by the so called “Graffetta Method”. The clamp is composed by an horizontal element (a steel rigid beam, stiff enough to support bending moment) and two inclined ties, tangent to the arch, inserted in the abutments. Ties are able to contrast the horizontal thrusts of the arch, preventing the formation of plastic hinges. “Graffetta Method” was applied to Cremona Cathedral, Manta Castle and Masino Castle (carriages room).
Even high isolates walls, or slender structures, typical of many historical buildings, are particularly vulnerable when subjected to horizontal seismic loads.
This is another interesting challenge that the designer is called to deal with. Thus, the use of diagonals stays and "light diaphragms " made by steel cables can be a solution able to brace the cantilever and to give it back the necessary structural safety. Examples include the Forte Fuentes in Colico (Italy) and the proposals for the bell tower of the Abbey of Chiaravalle Milanese (Italy) [figure 9] and Echmiadzin (Armenia) [figure 10].
Finally, a recent proposal refers to the consolidation of masonry columns through the insertion of a circumferential thin stainless wire, tensioned, stretched inside the joint between the bricks. This addition, very simple and minimally invasive, significantly increases the ductility and strength, without modifying the stiffness of the masonry columns and, from a formal point of view, it is particularly respectful of the existing material.
[by Lorenzo Jurina, Politecnico of Milan - ABC dept. - Italy]
figure 1. Trezzo Castle
Styes bracing solution for an isolated tall masonry wall of the Castle of Trezzo (MI).
figure 2. Torrione of Cassina de' Pecchi
Temporary encirclement of Cassina de'Pecchi Torrione (MI) with contrast plates to "pyramid" shaped and steel cables
figure 3. Masserano Tower
The tower of the Masserano (Biella). For the provisional encirclement we have been used straps in polyester and wooden boards.
figure 4. Imedghassen Batna
The Tomb of Imedghassen in Batna (Algeria) – A consolidation proposal with parallel encirclement.
figure 5. Experimental model of RAM
The effectiveness of the ''Reinforced Arch Method" was confirmed by a series of experimental campaigns performed on wooden small scale models and on real scale masonry ones, with arches of different geometric configurations (round-headed, polycentric, lowered, with slight central depression and with accentuated central depression).
The test campaigns have highlighted significant increases of the collapse load of the arches respect to the not consolidated ones, both in presence of vertical loads and horizontal loads, for any geometry of arch (including those strongly depressed) and for any position of application of the external load.
These results were comparable to those experimentally obtained by the use of reinforced concrete layer, which, on the other hand, are much more impactful, invasive and provided with additional masses not acceptable in earthquake zones.
Experimental and theoretical studies demonstrate a linear relationship between the collapse load and the tensile force imposed on the cable. These results were obtained with post-tensioned cables applied either on the external and on the internal side of the arch.
figure 6. Dome of Santa Caterina church in Lucca
A particular and innovative system of confinement was designed to consolidate the elliptical dome of the church of Santa Caterina in Lucca.
The applied solution combines two systems of cables: one is represented by hoops in steel cables, located as rings along the main parallels, and the other is the use of radial "reinforced arches" along the ribs.
Numerical experiments showed that, besides the traditional confinement along the parallels, the added cables along the meridians (the reinforced arches) play an important role, especially in terms of decrease of seismic displacements, reducing them by 65 %.
A further light and reversible intervention was performed on one of the two wooden trusses, which showed a clear inflection of the chain, caused by point loads transferred by the two monks. The solution implemented consisted in the insertion of two new wooden inclined props, section 20x20 cm, that rise towards the center of the chain from the perimeter walls, reducing the span of the wooden element. The inclination allows not to reduce the open space of the attic. It was provided a system of "reinforcement" of the truss, made with steel cables, in order to create two additional intermediate support points for the chain, in correspondence of the monks.
figure 7. Galleria dei Poeti at Masino Castle
The illustrated consolidation involved the barrel vault of the polycentric "Gallery of Poets" of Masino Castle in Caravino (TO) (XVI cent.), a room almost 35 meters long and 5 meters wide. An evident longitudinal crack in correspondence of the key of the vault is present, together with numerous lateral cracks, accompanied by a depression of the central portion of the vault (more than 15 centimeters). The most obvious reason can be found in insufficient contrast offered by the outside wall.
A passive system was first applied on the extrados, by means of strips of carbon fiber, embedded in a matrix of hydraulic mortar, to which a series of active interventions was added , i.e. 9 reinforced arches, orthogonal to the main axis of the vault.
The geometry of the cable follows exactly the one of the masonry arch, also in the central depressed area, where, thanks to the upward forces exerted by the tie rods, also the depressed portion on the vault works properly. A series of inclined cables, parallel to the roof, depart from some "A shaped" props located along the main central wall, and prevent the opening of the abutments of the vault.
figure 8. Cremona Cathedral
The structural consolidation of the Cathedral of Cremona consists in a system of various reinforcing "V-shaped" cables, which allow to replace the traditional method of chains of contrast (that pull the abutments towards each other) through a system of cables located externally to the nave (which push the abutments towards each other).
Seven metallic twin trusses (called " boomerang beam " for their arched shape) were placed in the attic, side by side with the arches of the nave, to offer a partial contrast to the lateral forces induced by the vaults. The boomerangs were then connected together by robust longitudinal bars, post-tensioned and crossed, with the function of bracing in the direction of the axis of the nave, so that to resist the horizontal seismic loads.
To provide a lateral contrast to the pushing vaults of the central nave, each "boomerang" was linked to a contrast system, placed in the two matronei. They consist in two “V-shaped” steel bars and a "telescoping strut". The latter is able to apply a local contrast in correspondence of the abutment of the vaults, by exerting, from the outside inwards, an horizontal thrust on the masonry, equal to 30t, in correspondence of each span.
At the level of the extrados of the vaults of the matronei, two large steel truss-beams work as diaphragms and ensure an adequate contrast to the thrusts of the lateral vaults by means of the “graffette” method.
figure 9. Tower of the Abbey of Chiaravalle Milanese
Solution for the consolidation of the central tower of Chiaravalle Milanese Abbey (near Milan), through the insertion of post tensioned steel cables, arranged in the shape of hyperbolic paraboloids, connected to the wall, offering a confinement action. In this way, the "masonry + cables" system will function as a truss, in case of an earthquake (horizontal load).
figure 10. Echmiadin Armenia
The Consolidation Proposal for Etchimiadzin Cathedral (Armenia) with steel cables organized to "star".