Consequently, the possible occurrence of seismic events has been bringing to spend an increasing interest in the conservation of vulnerable historical buildings located in seismic areas. These pieces of cultural heritage, thus, need to be seismically evaluated and strengthened by appropriate methods. Upgrading historical buildings in order to survive strong ground motions is a challenging task: their historical value must be protected and this means that the originality of the structure must remain intact, together with providing improved seismic performance. The more difficult question with respect to the maintenance problem and the restoring technique is to have a correct measure of the stability degree, by means of a model able to simulate the mechanical behaviour as close as possible. Hence, the seismic response of the historic construction is a subject in continual elaboration.

Besides the Egyptian one, the architecture nowadays most known by means of its objects d’art is that of Greece, from whose sacred monuments – namely the temples – is deduced all the knowledge of its principles. Grecian Architecture is very much common not only in Greece, but also in its colonies, like those of Magna Graecia formed in the southern Italian peninsula by Greek colonization starting from eighth century B.C.. One of these is Paestum, where there is a still existing proof of Greek settlement, constituted by three temples, whose oldest one is dated at 540 B.C. Despite the fact that these temples do still exist, it has to be pointed out that they are exposed to a high seismic risk, in fact Paestum arises in Campania, a region characterized by such a risk. For instance, the recent Irpinia earthquake, happened in 1980 and gifted of a moment magnitude 6.89, hit the region itself and the surrounding areas, causing almost three thousands death.

As earlier highlighted, seismic updating of historical buildings should be obtained without affecting their original specific characters. Advanced structural engineering technologies are capable of achieving this purpose by means of structural control concepts, one of which is the Tendon System. This system introduces a rigid body mechanism into the structure which is controlled through steel or different material (FRP, SMA, etc.) tendons with very little pre-stressing. In an effective Tendon System, the forces in the structure remain limited and are controlled at low levels by the tendons, not the earthquake. The very simple and effective idea is to tie loose building blocks with tendons that run through their centres allowing  the reducing of displacements and dissipating energy at the joints. Practical applications of this strategy are already available, nevertheless to fully understand its potentiality and to provide general design rules, an analytical model for the controlled system, with emphasize on non-linear joint dynamic behaviour should be validated by means of experimental tests.

According to this purpose, the present work is aimed to prove the efficiency of Tendon System, by performing an experimental campaign focusing on the dynamic response of small scale specimens representing Greek columns of Paestum, retrofitted by Tendon System. The experimental campaign has been executed at the laboratories of Steel & Composite Structures Department of Kassel University, Germany, under the supervision of Prof. Uwe Dorka. The testing facilities, among which the shaking table for the application of input excitations and the basic components of the specimens are provided by the same University.

In regard to Seismic Engineering experimentation, the field which manages the performance of experimental activities, it follows a synthetic digression about. Its paramount objective is to contribute in deep for the policies of prevention, formation or development leading to the mitigation of the seismic risk concerning the structural damage or collapse. The safety verification of a seismic action involves several concepts: necessary conditions, performance criteria, probabilistic reliability, hazard scenarios and quality guarantee. In spite of several scientific areas being under constant development, it is still nowadays very common to get important knowledge following the occurrence of each new catastrophic earthquake in a developed zone of the globe. In fact, there is no better seismic testing “laboratory” than the real place where an earthquake takes place, because it is absolutely impossible to reproduce artificially, and simultaneously, all the concerned phenomena. Anyway, and despite the wide numerical methods and computer capability developments, the tests will be always indispensable because they are the only way to reproduce seismic actions allowing its systematic repetition and the necessary scientific observation of their effects. Consequently, the main objective of the Seismic Engineering experimentation is, basically, to allow the observation and analysis of the damage and behaviour of different types of structures, under different conditions, while submitted to a seismic action. Seismic Engineering experimentation is also indispensable for the validation of analytical models and for the verification of new design methodologies or new strengthening or reinforcement structural techniques. The study of new materials behaviour and the analysis of the new methodologies still in a pre-normative phase need also the use of experimental tests. It should be noticed too that, apart the multiple aspects already referred, experimentation with earthquake simulators provides the certification of several types of anti-seismic equipments, such as structural control devices, in order to ensure their proper way of functioning during the occurrence of a major earthquake. In fact, such mechanical, electrical and electronic equipment of various types are main-stations and sub-stations material, nuclear power plants control units or hospital vital devices, just to quote a few of the more relevant examples.

Testing of complete structures is normally used to enable the understanding of the global behaviour of a construction and to capture the interplay of the response of its different components. It corresponds to quite expensive tests but, in principle, provides quite realistic information on the expected response of the specific structure under testing. On the contrary, tests on single structural elements or sub-assemblages are much less expensive but can only provide information of what could be called local nature. They are mostly suitable for the calibration and validation of analytical models of the elements that can be incorporated into a computer code with which the simulation of the response of a complete structure may be achieved. Hence, tests on elements or sub-assemblages are normally conducted on a large series of specimens so that the effect of different variables (for instance, geometrical proportions, mechanical properties or sequence of loading) can be checked and considered in the above referred validation of analytical models.

A rigid-bodies system, like a Greek column, responds to a ground acceleration showing one of the following five possible motions: rest, slide, rock, slide-rock or free-flight; they, depending on magnitude and duration of excitation, may or not turn into overturning. What is investigated in this research work, is the benefit produced in such a rigid block system by the installation of the above mentioned seismic retrofit.

In conclusion, by analyzing model response to the performed tests, it is stated that the desired improvement to be provided by the installation of a retrofit system – such as tendon passive control system – in a rigid-bodies system is obtained. By introducing a pre-stressing cable the eventual collapsing is avoided and by increasing the pre-stressing force in the cable, the total displacements of the system are reduced, which means that the relative rocking between the several bodies is also reduced, provided that sliding is however prevented by the installation of Tendon System.

L'articolo Seismic upgrade of historical monuments proviene da Evolveeng.

The following lines are a summary of the paper by E. Tortorella, I. Marino, L. Petti (University of Salerno, Italy), M. N. Khanlou, U. Dorka (University of Kassel, Germany), presented at the Conference “Seismic Protection of Cultural Heritage” held in Antalya, Turkey in 2011.

Testimony of past classical stone architecture is present in Greek, Roman, Byzantine, Romanesque and Gothic monuments. The buildings, which remain nowadays, are only a few examples of the wide activity of this ancient culture, since many of them, characterized by high seismic vulnerability, have been located in areas of severe seismicity.

The ever present seismic threat to those monuments that have survived or been rebuilt has triggered an increasing interest of the scientific community in the conservation of these important historical structures. The difficulty to improve the seismic strength by preserving the cultural heritage is well known. Non-invasive strengthening methods are requested, since the uniqueness of the building must remain intact and any structural intervention must be essentially reversible to allow for a return to the original historic state, before intervention, at any time.

The paper focuses on monuments of classical Greek architecture: the temples, which are not only common in Greece, but also in its colonies, like those of Magna Graecia in the southern Italian peninsula. Starting around the 8^th century B.C., these monuments were generally made of large structural stone elements that lie on top of each other without mortar, like drums in the case of columns.

This type of structures behaves as rigid-labile systems with unilateral constraints. In the case of the columns excited by seismic events, for example, the drums usually slide and/or rock, independently or in groups, with consequent permanent dislocations or even probable failure due to the overturning of the whole structure or one of its parts also for low seismic actions.

Recently some new concepts are being developed, among others, structural control concepts, which can be active, semi-active or passive. One of these possibilities is the Tendon System. The basic idea is to tie the loose stone blocks of a historical monument with tendons that run through their centres, but without continuous connections between tendons and stones (like grouting). Ideally, no pre-stressing is applied. Only during motions under earthquakes, forces develop in the tendons that limit these motions and prevent collapse. These forces can be controlled through special devices, like active hydraulic actuators or passive shape-memory alloy devices (SMA). Under regular loading conditions (dead load, wind, etc.), the historical state of stress is untouched. This system works particularly well in the case of a rigid body mechanism, like the one, which characterizes the Greek columns.

The tendon system can be removed any time easily, thereby returning the structure to its state before the intervention.

The paper describes the tendon control strategy in the case of Greek columns. In particular, the drums of the columns have been considered as held together through single cables running through the vertical axis, from the bottom to the top. On the top, the cables are connected to devices, which could control the axial tensile force and hence, provide a compressive pre-stress to the column to counteract seismic actions. The dynamic behavior of a rigid block with vertical load has been considered as a basic model to describe the behavior of such systems analytically. This analysis has been confirmed in preliminary shaking table tests on scale models at the University of Kassel representing the columns of the Temple of Neptune at Paestum, Italy.

Whereas the columns without Tendon System will collapse even under moderate ground motions, no collapse was observed with the Tendon System in place, even without initial pre-stressing. The obtained results clearly show the effectiveness of the control strategy presented here encouraging further in-depth studies for innovative applications of this system to monumental historic buildings.

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