THE RELATIONSHIP BETWEEN THE PROJECT OF RESTORATION AND PROJECT STRUCTURAL STRENGTHENING
“A reasonable reassessment of empiricism”
Restoration and stability: a bit of history
The correct relationship between the conservation of a historic building and its stability is a very old problem, which has had various solutions at changing cultures and customs over time. (figure 1)
Already in the nineteenth century, the consolidation of the monuments was a current and controversial issue, so much so that we have witnessing of different attitudes also in the restoration of the same building.
For example is the consolidation of the Colosseum, in the nineteenth century it was precarious and it was the subject of two interventions of consolidation profoundly different a few years one after the other one.
Raffaele Stern in 1806 build a buttress to stabilize a precarious area but conserving its structural instabilities, without altering the uncertain position of the unstable blocks: an extremely actual conservation action. (figures 2a-c)
In 1823, instead, Giuseppe Valadier intervenes with a completely different philosophy: to stabilize the perimeter ring, he reconstructs a part of the arches collapsed with their original form. Valadier however uses a different material: bricks instead of travertine. The reconstructed part is, therefore, perfectly recognizable and, therefore, the intervention is still respectful of the authenticity of the monument. (figures 3a,b)
A completely different intervention was made by Emmanuel Viollet-Le-Duc, starting from 1849 he entirely reconstructs the crumbling towers and walls of the city of Carcassonne in medieval style. The new walls imitate those ancient, so they are a false without proper documentation: something similar to Disneyland's castles. (figures 4a-c)
These three interventions, very well known, can be considered examples of three different philosophies in the consolidation and restoration: the preservation of the historical document with the addition of different structural elements, the reconstruction differentiated with similar shapes and materials but such as to allow the recognition of intervention, and the reconstruction in style or an "ancient fake".
After nearly two centuries of discussion and debate on these strategies of interventions, we can say that now prevails certainly the desire to enhance the authenticity of the historical heritage preserving not only the visible form, but also the structural organization and the material reality, including the signs of trauma and the degradation of the past with additions also of modern materials, but recognizable. The desire is to not replace the original elements with new elements.
The stability of masonry buildings
Keep the original material and ensure the stability is not, however, always easy.
If there is no guarantee about the stability of a building, it is condemned to destruction, especially in countries with high seismicity hazard as Italy and Iran. It's about finding the right compromise, using most of the resources of ancient structures and integrating them in the respect of a fundamental principle of restoration: "the principle of minimum intervention."
The problem of stability and safety of historic buildings occurred in Italy, in modern times, since the 70s. In the short period of about ten years, in fact, Italy has suffered serious damage from a series of earthquakes, which produced the deaths of over three thousand people, destroyed entire town centers and damaged dozens of ancient monuments. (figures 5a-d)
Something very similar happened in recent decades in Iran, just remember the Manjil–Rudbar earthquake in 1990 and in Bam in 2003. For this earthquake I studied with the Polytechnic University of Milan a vegetable reinforcement for local masonry. (figures 6a,b)
Following the earthquakes of the 70s has started in Italy a process of research, whether in universities or within numerous industries in the construction field, to understand how it is possible to safeguard the architectural heritage but without altering it in an unacceptable way. This research process took about forty years before landing in a significant legislative result: the Italian law for the protection of the architectural heritage from earthquake is of January 2008! It is an innovative law that allows differentiated interventions for normal existing buildings, and for monumental constructions, in order to protect the great Italian architectural heritage.
Until the 70s in no Italian universities of engineering and architecture, but, I think, neither European or in other part of the world, has made research or deepened teaching on topics such as the stability of masonry buildings, the behaviour of arches and domes and dynamics of bell towers and minarets, or about the use of innovative materials (FRP) for consolidation.
Engineers and architects studied the structures only in steel or reinforced concrete, and the only solution that we knew to consolidate masonry structures was to insert in the ancient buildings some modern structural elements (steel frames or reinforced concrete panels) which took the external stresses and loads for entire, replacing the old structures.
The results of the first interventions were therefore uncertain and in many cases dangerous or unacceptable as opposed to the need for conservation of monuments.
Even traditional calculation methods soon proved totally inadequate to describe the complexity of the historical structures without tensile strength and built with traditional methods. Even the most modern methods on the market, with numerical finite element models, mostly elastic, have proven unreliable. (figures 7a,b)
By the time engineers and architects have understood that, to save the historic buildings without unnecessary and heavy interventions or excessive additions, you must make the maximum from the ancient structures stamina. To do this it's necessary to take the knowledge of past builders, based on their thousand-year experience on the behavior and criticality masonry construction (empirical knowledge).
If we study the ancient manuals, until the early nineteenth century, we find that the approach of the ancient builders to stability was deeply different from ours.
By building thick walls the problem for them was not the strength of materials, but the balance. The manuals contained the description of the main mechanisms of instability of individual structural components generated from equilibrium crisis.
The method of analysis based on the possible movements of the structures, divided into rigid bodies, is now called, with modern terminology, kinematic analysis, since it is based on virtual movements of rigid bodies (instability mechanisms) and studies the situations of equilibrium limit.
For centuries builders had constructed thick walls for which the issue was the balance and not the resistance of the materials and had gained enormous experience on the local crisis mechanisms (empirical knowledge). (figures 8a-e)
The beginning of the nineteenth century marks a major change in the architecture by the widespread use of iron.
At the time of introduction of iron elements in constructions, suddenly it was posed the question of shape and strength of the beams made with it. The resistance is linked to the internal tensions that break in the material. (figures 9a,b)
Not being able to use the experience and empirical knowledge, it was necessary to invent a new science (mechanics of materials) to assess internal stress and cracking of the beams, pillars and frameworks (stress analysis).
Since the beginning of the nineteenth century, for nearly two hundred years universities have studied only the predictive calculations of steel structures and reinforced concrete, forgetting the experience on masonry buildings, up to the modern numerical models.
Only facing earthquakes and the destruction of historic buildings, in the second half of the last century, particularly in Italy, we realized that the stress calculations were inadequate, even the most sophisticated with computers and numerical models, and that we have to return at the kinematics analysis based on the identification of local instability mechanisms.
The main reasons for which the numerical methods on the market find it difficult to describe (figure 10) the behaviour of masonry constructions are:
_the limited tensile strength of the masonry, which does not allow the diffusion of the stresses and the presence of a global behaviour,
_the lack of isotropy and homogeneity of masonry.
The analysis of local mechanisms, defined empirically on the basis of previous experience, has become the method of analysis recommended by the rules. (figure 11)
The Italian law of 2008
In 2008 it has been approved in Italy a law for the protection of historical and architectural heritage by the earthquake.
This law provides different rules for normal and for monumental buildings; also recognizes that in masonry buildings the collapse is achieved, in most cases, for lost of balance of limited portions of the building (the instability mechanisms), for which it suggests the use of the kinematic method for the detection of local criticality.
The law warns against an uncritical use of the global finite element calculation models, especially those made with elastic finite elements, as they provide for the most unreliable results.
The law also recognizes that it is not possible, in most cases, to make the historical buildings as safe as new ones without damaging the authenticity, then it allows for monumental buildings to have a security level lower than normal buildings.
Much importance is entrusted to historical analysis and the survey. Of utmost importance is the knowledge of the building response and similar buildings to earthquake (empirical knowledge).
The law contents:
"When the building does not show a clear behaviour of the whole, but rather tends to react to the earthquake as a set of subsets (local mechanisms), the verification with a comprehensive model hasn't compliance with respect to its actual seismic behaviour.
... The mechanical model, even if developed with the most accurate analysis tools, it is anyway inadequate ... the intervention can be fully satisfactory in the face of qualitative assessments of the structural behaviour based on observation of the construction and historical analysis.
The objective is to avoid superfluous works, thus favouring the principle of minimum intervention ... "
Even for non-protected historic buildings there are similar rules. A Ministerial Circular of 2009 contents:
"[In buildings without rigid floors] the global verify can be done through a comprehensive set of local checks, provided that the totality of the seismic forces is consistently distributed on the relevant local mechanisms and is properly takes into account the forces exchanged between the structural subsets considered."
In other words, the seismic analysis in masonry buildings, aimed at the enhancement of the ancient structures, consists in: to study the behaviour of individual building elements in masonry (walls, arches, vaults, steeples and minarets etc.); to know their historical behaviour; and to recognize, with careful inspections and surveys, any defects (badly performed walls, risk of tipping of the walls, rotten beams, lack of connections between the walls, and inadequate links between the wooden structures, lack of tie-rods etc.). It is a way of studying the buildings that is completely different from the design of a new building of reinforced concrete, for these we sit in the studio and make global virtual models. It is a method of study based on the experience, static sensitivity and accurate qualitative knowledge of empiricism type. If we look the drawings in the recent Italian rules we find that they are identical to those of the eighteenth-century manuals: they describe local collapse mechanisms.
Address an study in this way it will be understood that there are few different kind of buildings in masonry and that the instability mechanisms are always the same.
You will find that the first problem is the tipping of the walls out of their own plan; you will find that the arches push and rotate the walls, you will even find that the domes push; you will find that, in general, with simple tie-rods and rings you can hold together the buildings and prevent the frequent instabilities, saving the ancient structures.
Certainly remain the problems due to unreliability of some walls, such as those of Bam (which I analysed), but you will find that our colleagues in the past had often faced the problem of inconsistency of the walls (for example, those made by clay) and that they had solved the problem by placing the layers of plant fibres to hold the compact walls.
Understanding the mechanisms is also possible to perform calculations for the quantification of consolidating elements, but it is much easier and more reliable a calculation that starts from a local specific criticality well understood, than a calculation starting from the study for an entire building, not able to consider details of those problems that are the main cause of collapses.
During these meeting days various technical solutions will be presented mostly aimed to preserve the old structures while respecting the fundamental principle of minimum intervention.
As examples of restoration methodology beforehand described, I would like to show some works of consolidation and restoration made by Studio Comes on Islamic buildings.
> Consolidation of the Karajosbegova Mosque and its minaret damaged during the Bosnian war: the consolidation was made by old stone with hand finishing. (figures 12-13)
> Consolidation of Ayyubbid Hall in the Citadel of Damascus: the vaults are consolidated with tie-rods. The dome has a geometry that remembers the traditional architecture but with modern design. (figure 14)
> Consolidation of the Minaret Al-Hadba of Mosul: the minaret has a great declivity that will be reduced with a work on its foundations, like Pisa Tower, without damages on the ancients walls. The trunk will be consolidated with hooping of carbon ropes hidden inside the mortar joints. (figures 15a-c)
[by Carlo Blasi, University of Parma, Italy]