ELSA, or how to construct earthquake-proof building       The construction of bridges, viaducts, buildings or tunnels, which can withstand earthquakes, involves using particularly sophisticated simulation laboratories. The most prestigious of these is ELSA, which is a technological flagship for the Europe an Commission's Joint Research Centre. The art of building structures which are capable of withstanding earthquakes progresses only through simulation. The leader in this field is European. ELSA, the European Laboratory for Structural Assessment within the Institute for Systems, Informatics and Safety at the European Commission's Joint Research Centre (JRC) in Ispra benefits from a technology that is envied by the United States and Japan. ELSA makes it possible to analyse the ability of a building to withstand extreme stresses placed upon it by a substantial earthquake, such stresses varying according to the intensity of the energy released, the nature and duration of the shocks, and so on. Above all, the purpose of such tests is to validate the harmonised standards - Eurocodes - on construction in the countries of the European Union. Preventing the unforeseeable "This simulation laboratory was set up using all the accumulated expertise of bodies specialised in seismic phenomena in the Community countries," explains Jean Donea, one of ELSA's initiators. "The powerful computer systems that we use to reproduce the stresses on structures under test and to record their reaction incorporate the latest developments in digital technology. This makes it possible for us to carry out tests according to the so-called "pseudo-dynamic" method, one of the advantages of which is that it makes it possible for us to carry out tests "in slow motion", by spreading over an hour, for instance, the shocks which would normally be engendered in 10 seconds by a real earthquake. Such controlled simulation affords us the opportunity of using complex models reproducing precisely the conditions of a particular earthquake and making it possible to observe the deformations experienced by the structure, and particularly the appearance of cracks. The monitoring technique enables us to measure exactly displacements of even 1 micron, i.e. a thousandth of a millimetre." Used to dealing with complex calculations, such as those which make it possible to predict the resistance of steel vessels in nuclear reactors, the ELSA team comprises about 20 specialists (engineers specialising in mechanics, electronics engineers, computer specialists, and civil engineers). While some define the forces to be applied to the building from an analysis of earthquakes, others design the thrust programme, install the monitoring apparatus, analyse the building's responses, follow the smallest crack and prepare the technical data making it possible to validate the European standards. Life-size tests ELSA looks like a "wall" 16 metres high and 20 metres wide and two concrete slabs 25 metres long. This rigid structure makes it possible to test the resistance of different types of buildings. If it is given, for example, the concrete framework of a five-storey building - including support beams and floors - this is fixed to the "wall" by hydraulic servo-jacks driven by pressurised oil (200 times atmospheric pressure). Since they are connected to pumps of 800 kilowatts, these jacks are capable of delivering forces of 100 tonnes which can push, pull, twist and slowly deform this framework and even destroy it. On the basis of previously calculated earthquake models, the computer system dictates the intensity of the various thrusts and automatically modifies them according to the resistance of the structure and the materials. A technology of the future ELSA, a technological wonder valued at ECU 7 million, has been raised to the rank of "major scientific installation" within the context of the programme for the Training and Mobility of Researchers, which aims to promote mobility among European researchers. Its potential is exceptional. The simulator is currently being used to evaluate the vulnerability of and to test reinforcement methods for historical monuments in Portugal and Sicily. ELSA can also simulate damage and cracks done to different types of structures: buildings, engineering structures, silos, tanks, pylons, bridges and oil rigs. Its performance can only improve as advances are made in other research fields, such as knowledge of materials, computer modelling, scientific calculations and civil engineering.     The European Laboratory for Structural Assessment