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SEISMIC ISOLATION AND STRUCTURAL CONFIGURATIONS

Seismic isolation is coming out from its pioneer period and now it is on the way to become a popular system for the seismic protection of ordinary structures. The criteria concerning the architectural morphology, the selection of the structural configurations and other design options of seismic isolated buildings shall be discussed to popularise its use, improve the effectiveness of ordinary applications, cut down the construction costs and open the way to new applications. Moreover, the higher reliability of the usual design procedures when they are applied to isolated structures rather than to fixed-base structures must be emphasised. The target is to avoid that new applications are discouraged by unnecessary and complex analyses required by specific codes. Taking the opportunity of some demonstrative initiatives promoted by the local Government of the Umbria Region after the seismic swarm that struck central Italy in 1997 and 1998, this papers deals with practical subjects concerning the application of seismic isolation.

SEISMIC ISOLATION AND STRUCTURAL CONFIGURATIONS. THE ITALIAN EXPERIENCE

Alberto Parducci

Introduction
We know that the structures of civil engineering, built with the usual construction materials, cannot resist without any damage the intensities of the major earthquakes. On the other hand, too often the seismic damages are out of the real proportion that can be achieved with the up-to-date seismic approaches. If our engagement is to reduce the seismic hazard of the populations we must pay a growing attention to popularise the most effective construction systems set out by the applied research, that can really improve the seismic protection of the buildings. Since this engagement requires the use of new construction techniques, some customary design habits must be revised, including those related to the selection of the appropriate architectural options and structural configurations. 
At present, Seismic Isolation (SI), often applied as Base Isolation (BI), is the most promising technique to reduce the effects of the major earthquakes. SI is now out of the pioneer period and is ready to become a popular construction technique that can be used by many designers in all the seismic zones of the world. Besides the significant results of the theoretical and experimental research, its real effectiveness was clearly shown by the optimum performances of the isolated buildings struck by the severe earthquakes of Los Angeles in California in 1994 [7] and Kobe in Japan in 1995 [11,12,13].
In spite of these unquestionable successes, many architects and structural designers still consider SI with a suspicious attitude, due to some persistent prejudices and false opinions (a certain time can be necessary to remove the previous habits and apply the new results of the scientific research). Often, the negative attitude depends on the poor knowledge of the new design philosophy, and can be favoured by lack or inadequacy of code's requirements.  Therefore, important targets are the elimination of the wrong beliefs, the definition of the practical ways to achieve the best seismic performances and the research of appropriate architectural approaches and simple design procedures. Then, the questions are these: SI is really a winning option in seismic design that can be currently and suitably applied by many architects and engineers? To reach this target, which design criteria should be revised?

 
Typical performances of SI structures
The first point is the dependence of the effectiveness of SI on the selection of appropriate architectural morphologies and structural configurations. Everybody knows that the seismic performance of a structure is strongly related to its architectural configuration [1,2,15]. All the reports emphasise this aspect when they illustrate the damages of the real earthquakes [7,10,11,12]. On the other hand, this basic problem is not yet taken into the right consideration at the time of the applications. This is a critical point in order to achieve high seismic performances and reduce the construction costs, particularly in the ordinary applications of SI.
Besides the reduction of the structural stresses, other performances make BI a profitable technique in seismic design. For instance, it is a simple way to reduce the horizontal accelerations transmitted to the floors of multi-storey buildings. This performance, that cannot be achieved if the seismic resistance is obtained by the strengthening of the structural elements, is generally disregarded in the design practice and is not considered by the codes. On the contrary, the reduction of floor accelerations must be regarded as a primary result in seismic design, because it avoids damages of non-structural elements and contents of the buildings. This is an important performance of the strategic buildings (like hospitals) and buildings containing high values or objects that cannot be reproduced (like museums).
Finally, unlike some false opinions, there is a further significant aspect in favour of BI, because the ordinary numerical procedures used in the anti-seismic design are more reliable when they are applied to base isolated rather than to fixed-base structures [15].

 

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