Extending the life-span of cultural heritage structures
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Monuments and historic buildings represent our historic heritage, the witnesses of the history that have arrived up to our age. We have inherited them from the previous generations, and it is our duty to preserve and to transfer them to the future generations. Therefore, we must preserve our cultural heritage from the natural ageing and the natural catastrophes, such as earthquakes. This can be done by means of a continuous monitoring and periodic controls, using non-destructive techniques. The recent applications of structural health monitoring to cultural heritage structures are introduced in this paper. They present some interesting cases, which are examples of good practise for the future.
KeywordsStructural health monitoring Experimental dynamic analysis Cultural heritage
Structures are usually designed to perform their function for a certain life time. This is usually assumed to be 50 years, which is the usual value for ordinary buildings. It is assumed equal to 100 years or more for special structures, such as long span bridges and plants.
What is the life time of a cultural heritage structure?
We do not know the nominal life time at the time of construction, when this concept likely did not exist, but we certainly would like it to be very long, tending to infinite. Actually, monuments and historic buildings represent our historic heritage, the witnesses of the history arrived up to our age. We have inherited them from the previous generations, and it is our duty to preserve and to transfer them to the future generations.
What do we have to protect the monuments from?
It is well known that the erosion related to wind and rain, as well as the daily and seasonal thermal cycles contributed to the natural ageing of the materials of exposed surfaces of any structures. This phenomenon is usually accelerated by the presence of pollution, especially in urban areas. As a result, the strength of the materials can be immensely degraded and consequentially, a significant reduction of the static and seismic capacity of the structures. The deterioration process can be facilitated by vibration effects, such as those induced by the traffic . Therefore, any structure becomes more and more vulnerable to natural catastrophes, especially to earthquakes. Seismic events in the past caused the definitive destruction of several monuments and historic buildings and, in some cases, of the disappearance of entire civilizations.
The described deterioration process is unfortunately quite common for monumental structures. As a matter of fact, most of them were built when there was no traffic issue and, in most cases, without accounting for horizontal actions, such as those due to earthquakes. On the other hand, also in buildings designed with seismic retrofit concept, the mentioned effects can be very dangerous.
In several cases, the existing structures are damaged by the works related to new adjacent buildings or roads around them. The case of the settlements due to the excavation for the foundations and/or the underground floors of an adjacent building is quite common.
The importance of the historical and archaeological sites imposes a big effort in their preservation, but this is not the only reason. They are good attractions for tourists and therefore are also important from an economic point of view. Furthermore, the presence of hundreds or thousands of people imposes severe controls and maintenance efforts to guarantee the conservation of these structures in conjunction with an acceptable level of safety.
How can we preserve and/or improve the level of safety of monuments?
Historical buildings are often characterized by an irregular form, both in plan and in elevation, the lack of vertical joints and transversal braces, in-plane flexibility or absence of floor slabs, and shallow foundations. These characteristics make them very vulnerable even to moderate seismic events. Furthermore, the structural rehabilitation of historical buildings is quite delicate, and should aim at the protection of human life, but also of the historical and artistic testimony. The interventions should be non-invasive and reversible, that make use of materials compatible with the original ones and not to determine changes in the original structural concept .
It is worth noting that also the usual concept of smart structure, which dissipates the seismic energy by means of the damage accumulated during a seismic event, is not acceptable for monumental structures. Furthermore, inadequate interventions were often the cause of the collapses of cultural heritage buildings during earthquakes, but the absence of a suitable maintenance and/or improving interventions could be even worse, as demonstrated during the last earthquakes in Italy. The health conditions of several monuments do not allow avoiding or postponing significant structural strengthening interventions.
What should we do to extend the life time of cultural heritage structures? Two important factors should be accounted for: to know the history of the construction and its architectural and structural characteristics and managing the maintenance of the structure.
Cultural heritage structures were built in different ages, using different materials, whose mechanical characteristics are not known and are difficult to model, and using technologies, which are not used any more. Their geometry is often not known and inspections are difficult due to the complexity of reaching all the structural components, especially the foundations. Besides, secondary structural and non-structural elements could contribute significantly to support loads, but this contribution is very hard to evaluate. Obviously, to define a retrofitting intervention and/or a suitable maintenance programme, it is compulsory to know the “objects of interest” in detail. This is also an essential issue for the mathematical modelling .
For the second aspect, the usual maintenance procedures are the classic maintenance on request, which implies a retrofitting intervention only if a damage is already occurred, and the cyclic preventive maintenance, which is aimed to prevent any damage and is based on an evaluation of the optimum maintenance period. The worst disadvantage of the first approach is that the damage is already occurred and the maintenance works, if still possible, require the interruption of the use of the building or at least an important limitation to it, with obvious negative economic effects. The disadvantage of the second approach is the difficulty in the definition of the optimum maintenance period.
The continuous monitoring, which is to be preferred, whenever possible;
The periodic controls by means of non-destructive techniques, which should be done in any case for those tests that cannot be substituted by the online monitoring .
The first approach allows checking the health status of the structure and its components continuously. A continuous dynamic monitoring, for example, allows to record the effects of any vibration source, especially those due to earthquakes . The final goal is the reduction of the damage and so of the maintenance works and costs, and the increase of the safety check level.
The basics and the most interesting applications of structural health monitoring in the preservation of cultural heritage structures have already been presented in previous papers and books [6, 7, 8]. The importance of the multidisciplinary approach is to be emphasized, as demonstrated especially by the studies carried out on impressive archaeological sites [9, 10, 11, 12, 13, 14] and monumental structures [15, 16].
Interesting specific studies have been conducted on the conservation of ancient stones , the health status of ancient timber structures [18, 19, 20] and historical bridges [21, 22]. The structural health monitoring is important also in the retrofitting process of heritage constructions  especially when using new technologies .
In March 2017, this Journal launched a call for papers on Structural Health Monitoring of Cultural Heritage Structures with the author of this paper as guest editor. Several proposals were received between July and September 2017. They were subjected to the usual review process. Only ten of them were accepted and published in the last three issues of the Journal. In the following the accepted and published papers are introduced. They present some interesting case studies, which are examples of good practise for the future.
2 Examples of structural health monitoring of cultural heritage structures
Preservation of cultural heritage structures requires monetary investments that will immediately be paid off by tourism and, more importantly, transfer of cultural heritage to future generations. To achieve this, the public as well as the authorities in charge of such structures need to be convinced of the economic and cultural benefits associated with the preservation of historic structures.
In some cases, important interventions are necessary. These should be non-invasive and reversible and should aim at a balance between the needs of safety and the needs of preservation of the cultural testimony. But it is important to stress that “do nothing” is not a suitable solution. The collapse during the earthquake of October 30th, 2016, of the San Benedetto of Norcia Basilica, which had been damaged by the earthquake of August 24th, 2016, is a typical bad example.
Structural health monitoring appears to be the only way to guarantee this equilibrium, reducing the damage and planning the suitable interventions on time; in other words, to extend the life time of cultural heritage structures. The papers presented in this article and published in this Journal show some interesting cases, which are examples of good practise for the future.
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