Pre-stressed steel cables and bars in consolidation of monumental buildings

Steel, and stainless steel in particular, is taking on an increasing important role in the conservation of buildings of historical value. This is due to a series of obvious advantages: reduced bulk, limited cost, excellent strength, the possibility of immediate recognition and reversal of restoration work, and, lastly, durability comparable with the structure in which it is used. Ever since many centuries, chains, cramps, and connecting elements made of metal have been used to absorb traction stress produced by the horizontal component of forces in arches and domes or to improve faulty connections or replace missing ones. At present days reinforcing “passive” steel elements and “active” pre-stressing steel elements, in form of cables or bars, are more and more adopted and accepted. The paper shortly illustrates some examples in which steel cables have solved, efficiently and in some cases gracefully, difficult static problems involving damaged masonry structures. A tall medieval tower in Pavia, two castles of XV century in northern Italy, a pair of twin minarets in Azerbaijan and some damaged masonry vaults have been recently studied and fully restored. Technological, structural and execution aspects will be illustrated in relation to these examples in which a criterion of minimal intervention was chosen. INTRODUCTORY REMARKS Today there is no unanimous reply to the question “how a restoration has to be carried out?” It is therefore necessary to define what one means by the term “restoration” and the principles by which one operates in this field. The international documents – the various “restoration charters” – offer a useful though not exhaustive point of reference. The principles they set out demand reasoned adoption rather than a scholastic act of faith. Above all they demand that decisions be taken in the light of the specific situation of the monument to be restored. There is one element that may be considered common to all restoration projects: in-depth analysis of the constituent materials and the structural condition of the monument in question. In our opinion, the crucial value of reference is that the safeguarding of the memory constitutes the best foundation for the future. The monumental complex indeed offers unrepeatable testimonies to the culture of the people who produced it and who have looked after them ever since; the loss or alteration of a part is always irreversible and would seriously impair the cultural growth of a civilization. It follows that the conservation of these monuments must be planned and executed on the basis of a thorough analysis of the buildings, using materials and techniques that are compatible with those they embody already and so that they remain an expression of the place they occupy and of the continuity of their own history. The restoration must therefore strive to ensure the permanence of the monument with all the marks and layers of time and history that characterize its current state. With regard to any consolidation work of an historical construction it must be stated that it is part of a wider-ranging process that we may call “the practice of conservation”. The first principle to state is that we must accept the historical building in its current state (the building is the primary source of knowledge) as a significant testimony in its full complexity, and that we must maximise our total knowledge of the building, assigning equal value and importance to all components of the building and to all the materials contained in it. The work to be performed will, therefore, be determined through careful and specific observation. In other words, it is important to study the individual object as a unique, unrepeatable instance. The second principle is that strengthening, considered as a single, exceptional intervention or as a part of a larger project for work on a building, must be organised with a cognitive, scientific approach to phenomena of degradation and ruin to assess if and when there is an effective need for intervention. An object is irreplaceable in that it is unique, because of elements of its material culture, because of its historic nature, because of the relationship that links it to other events, because it can be studied through many distinct types of reading. The type of work and the technology that best achieve the aims listed above are then selected on the basis of these assessments. Ever since the Middle Ages, and even before, chains, reinforcement rings, cramps and connecting elements made of metal have been used to absorb traction stresses produced in the masonry by the horizontal component of forces in arches and domes or to improve faulty connections or replace missing ones. Important examples can be found in a number of monuments of the past, for example the metal components in Brunelleschi’s dome for Santa Maria del Fiore in Florence, or works involving use of chains in thrusting arches and vaults or encircling of cracked columns. Metal components frequently appear in historical buildings, either in restoration work or in the original construction. Not only is steel, generally speaking, compatible with the historical building in terms of both resistance and rigidity, but also in the case of stainless steel, any fear regarding lack of durability is really unfounded. Steel, especially stainless steel, can be used in work which will stand alongside the existing construction and permit it to be read. Nothing is replaced or removed, and a recognisable addition is made which can easily be removed and is therefore reversible. It is up to the designer to come up with a solution which is aesthetically satisfactory and harmonised with the whole. Steel is naturally predisposed to this compositional use of additions, providing considerable strength with minimum bulk. The projects described below are to be considered not only as examples of the use of steel, and more particularly stainless steel, in strengthening of historical buildings, but also as specific engineering choices which have tried to identify the best possible solution for the specific case in point in each project. EXAMPLES OF APPLICATION San Dalmazio Tower, Pavia (Italy) This 45 meters high, XI century, brick tower showed many local damages and lacks in the masonry and long vertical cracks along the four walls. The strengthening project involved construction of a new inner metal tower made partly of carbon steel and partly of stainless steel, which is fully exposed and completely renewable but entirely located inside the tower so that it could not be noticed from outside but for of a number of discrete signals declaring its presence; in other words, a “tower within a tower”. The dead load of the masonry is partially transferred to the inner tower by means of about 300 prestressed sub-vertical cables, laying in the interstice between the masonry and the steel tower. The new structure takes into account the culture of preservation, such as adoption of strengthening work which stands alongside the existing structure (it could be called a “crutch” if the term did not have such negative connotations), the reversibility of the work, and its easy recognition as belonging to a different historic period. The new construction collaborates structurally with the masonry tower. Not only does the new tower provide static assistance by removing part of the vertical load, increasing resistance to horizontal loads and improving ductility, but it helps make maintenance easier for itself and for the old masonry tower. In fact there is space inside the new metal tower for a rack elevator permitting easy access to the inside of the tower, allowing inspection and ongoing planned maintenance. Accessibility is an important factor in maintenance, for by encouraging use of all parts of a structure we promote work required for their use, at least in relation to frames, roofs, and finishes, which are where structural decay often starts. In the strengthening project on the masonry tower, additional stainless components were used to create radial tie bars that pass through regularly spaced horizontal holes just existing in the masonry. The aim is to “confine” laterally the walls adding horizontal loads, thus connecting the various vertical layers of the masonry. As seen, the more expensive stainless steel material is used in the areas which are most exposed to the air and most closely in contact with the old masonry. Visconteo Caste, Pavia (Italy) The Visconti’s XV century castle in Pavia has suffered the vicissitudes of time and now has only three remaining sides and two corner towers, which have been subjected to repeated restoration and modification. In the early XX century the Southwest Tower was in very poor conditions, with fissures and depressions in the large vault on the first floor, so that in 1925 the flooring and the filling material was completely removed. A new reinforced concrete floor was constructed above the vault, rigidified with rib beams, to provide support for live loads. For greater prudence (as it seemed at the time) the masonry vault was suspended from the reinforced concrete floor using twenty metal rods, 30 mm in diameter. In 1995 the concrete floor, which had been left exposed for many years, was paved. Only a few months later the pavement unexpectedly lifted and cracked visibly. The diagnosis was viscous yielding of the reinforced concrete floor subject to a considerable permanent load. The floor lowered and required the pavement itself to take on a structural role, subjecting it to compressive stress that leaded to a buckling phenomenon. This theory was confirmed by inspection of the hollow between the vault and the floor, which revealed that some of the 20 metal “safety” tie bars had buckled due to compressive stress, so that a part of the reinforced concrete floor was resting on the vault below it. The first action taken was a modification of the connection that permitted them to act as