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Brussels, 06 September 1999
EU research results protect Assisi's basilica from future damage
Keywords: cultural heritage, shape memory alloy, earthquake damage, seismic protection
On 26-27 September 1997, a series of five earthquakes caused severe damage to the Basilica of St Francis in Assisi. This was not the first time the basilica had suffered from seismic activity - nor, probably, will it be the last. The question was how to limit future damage to this historic building. The answer has come through an innovative technique developed by a project funded under the EU's Fourth Framework Programme for RTD: custom-made shape memory alloy (SMA) devices will be used to connect the tympanum wall to the roof. Even though their super-elasticity makes them ideal for the seismic protection of monuments, this is the first time that SMAs have ever been used in the field of cultural-heritage structural engineering. Project tests show that structures protected with SMA devices can withstand an earthquake at least 50% stronger than one which would destroy structures reinforced with traditional steel bars.
Restoring ancient monuments is a tricky business at the best of times - even more so when the buildings concerned are in an earthquake zone. The steel bars normally used to reinforce historic buildings will protect up to a point, but can be too rigid to survive a large earthquake. What is needed are devices which are both strong and flexible - able to hold the building together and absorb the shocks.
The Istech project, supported by the European Union's Cultural Heritage research activity (part of the Fourth Framework Programme's Environment and Climate programme) was set up to develop innovative techniques for improving the stability of historic European buildings, concentrating in particular on seismic protection. The Istech team came up with the idea of incorporating new nickel-titanium Shape Memory Alloys into devices to increase the strength and energy dissipation capacity of a building, and help it resist the effects of earthquakes. The project has shown that, amongst the very unusual properties of SMAs (see below), their super-elastic behaviour - the ability to recover from large deformations in loading-unloading cycles - is the most valuable feature for the seismic protection of monuments. The European Commission contributed to the project not only by funding it but also through research carried out by the Joint Research Centre's European Laboratory for Structural Assessment (ELSA), which undertook the material characterisation tests and proved the effectiveness of the system with full-scale tests on masonry walls protected against earthquakes by SMA devices.
Such devices are exactly what were needed to help reinforce and protect the Basilica of St Francis in Assisi after the 1997 earthquakes. Following the positive results of stringent tests, the Italian committee responsible for the basilica's restoration has decided to use the devices developed by the Istech team to restore the basilica's tympanum.
This is not the EU's only involvement in the restoration of the basilica in Assisi, where, in September last year, the Italian Ministry of Culture and the European Commission jointly organised a research conference to discuss the damage to, and restoration of, the basilica's frescoes, as well as measures to protect such cultural heritage from the effects of future earthquakes and environmental degradation, and mass tourism, a particular problem in popular destinations such as Assisi.
Research into the protection of Europe's cultural heritage will be substantially increased through the EU's recently launched Fifth Framework Programme for RTD, where it forms part of the key action on The City of Tomorrow and Cultural Heritage.
For further information, please contact:
Ms Julia Acevedo
Scientific Officer, DG XII D.I.4
Mr Stephen Gosden
Press and Information Officer, DG XII
Shape Memory Alloys and their use in the Istech Project
Gabriella Castellano, Istech Project Scientific Coordinator
The first recorded observation of the shape-memory transformation was in 1932. However, research into both the metallurgy and potential practical uses actually started in 1962, when the shape-memory effect was discovered in equiatomic Ni-Ti by a US military research centre that patented the material under the name of NITINOL. Study of Shape Memory Alloys has continued at an increasing pace since then and more products using these materials are coming to the market each year. In 1990, it was estimated that the world-wide business in Shape Memory Alloys exceeded 30 million US dollars and was growing at over 25% a year. The main products are fine medical wires (for orthodontic and orthopaedic applications and self-expanding micro-structures used in the treatment of hollow-organ or duct-system occlusions), electrical switches, spectacle frames, pipe couplings (one of the oldest applications) and antennae for cellular telephones.
The potential use of Shape Memory Alloys in the civil engineering field has been studied, both theoretically and through some experiments, but only during recent years and mainly in the USA. However, a survey of the literature does not yield any information on applications of Shape Memory Alloys in the field of cultural-heritage structural engineering.
The idea of using Shape Memory Alloys for protecting cultural-heritage structures against earthquakes was at the base of the Istech Project as a possible solution to fulfil the need for new techniques that are more effective and/or less invasive than traditional practices.
The Istech Project has shown that, amongst the very unusual thermo-mechanical properties of Shape Memory Alloys, their super-elastic behaviour, that is, the ability to recover large deformations in loading-unloading cycles, is the most useful feature for the seismic protection of monuments.
Since the beginning of the project, the following promising characteristics of super-elastic Shape Memory Alloys - in terms of possible structural applications in monuments - have been identified:
1. ability to reach - and recover from - high deformations (super-elasticity): Shape Memory Alloys have notably high values for maximum recoverable deformation, more than 10 times that recovered by a conventional metal;
2. force limitation: the force-displacement curve is characterised by a "plateau": i.e., the force remains nearly constant with increasing displacement;
3. low stiffness: Shape Memory Alloys, in their elastic and even more super-elastic range, have a Young's modulus lower than that of conventional steel;
4. energy dissipation: a Shape Memory Alloy subjected to cyclic deformations (loading-unloading), of the kind that take place during an earthquake, dissipates energy;
5. excellent fatigue and corrosion resistance.
The studies carried out within the framework of the Istech Project have shown that Shape Memory Alloys (SMAs) are particularly suitable for use as wires and/or strands as part of devices capable of increasing both the in-plane and out-of-plane seismic resistance of masonry structures. In effect, two different types of application of SMA-devices can take advantage of the properties described above: 1) in series with conventional steel tendons for masonry post-tensioning (to increase the in-plane flexural and shear capacity of masonry structural elements); and 2) in series with horizontal conventional steel ties (to improve the out-of-plane behaviour of masonry elements, in particular to prevent the collapse of outside walls poorly connected at floor level). It is worth noting that the application to the Basilica of St Francis in Assisi is of the second type; in this case the Shape Memory Alloy devices will connect the tympanum wall to the roof.
In both types of application, the main additional advantage derived from the use of SMAs is their ability to act as force-limiting devices owing to the super-elastic plateau that guarantees a fairly constant force, even in the face of substantial deformation. With traditional masonry reinforcement using steel bars, the forces transmitted to the masonry structures are substantially higher, because of the high stiffness of steel bars. Furthermore, SMAs can dissipate a certain amount of energy when subjected to large cyclic deformations. Thus, during unloading, SMA-based devices do not return all the energy accumulated upon loading, as conventional elastic bars do. This effect could be crucial in avoiding localised damage. SMA devices are also able to increase the energy dissipation capability of the masonry itself (by friction in masonry joints and fractures), thanks to a better distribution of the damage in the structure, when damage can not be avoided.
The numerical studies and tests carried out by the Istech partners have shown that the Shape Memory Alloy devices are able to substantially increase the stability of masonry structures. For example, out-of-plane tests on masonry mock-ups have shown that a structure protected with Shape Memory Alloy devices can withstand, with no damage, an earthquake of intensity at least 50% higher than one which would cause the collapse of a structure reinforced using traditional techniques (steel bars).
As far as the application of SMA devices in the Basilica of St Francis in Assisi is concerned, the expected benefit, compared with the traditional technique of connecting the tympanum to the roof with steel bars, will be to prevent the collapse of the tympanum at the design-value earthquake and reduce damage to it in the case of a stronger earthquake. This benefit is due to the higher flexibility provided to the wall by the SMA devices. Such flexibility results in a reduction of the accelerations, and consequently the forces, transmitted to the tympanum. In addition, the forces are limited by the super-elastic plateau of the SMA devices.
PRESS RELEASES | PRESS RELEASES OF 1999 | 09.02.2000