Keywords: seismic isolators, seismic hardware, steel hysteretic elements, sliding pendulum, friction pendulum.

The scope of the testing programme within the framework of the LESSLOSS Project was that of evaluating the benefits, as well as ascertaining the limitations, of two important types of isolators, namely: (a) a flat surface slider (sliding bearing) coupled with steel hysteretic elements (SHE), and (b) curved surface slider, usually referred to as a "Friction PendulumR" (FP). Sliding bearings with SHEs have been developed prevalently in Italy, but have found important applications elsewhere as well. After their success on the market, several concerns have been raised about repeatability, fatigue resistance and, above all, their re-centring capability. One of the purposes of this study is precisely that of allaying or confirming these doubts.

About the devices included in item (b) the specific objective of the testing programme was that of experimentally substantiating (or disproving) the characteristics and advantages claimed by FP systems. Actually, the testing campaign has investigated the effects of substantial changes in the supported mass and their claimed ability to minimize the adverse torsional motions that take place in asymmetrical structures. One objective common to the aforesaid sub-tasks was that of improving the knowledge of device behaviour in the presence of different seismic inputs, aiming to develop possible improvements thereof as well as the assessment and validation of numerical models in order to reliably and accurately predict the response of structures thus isolated.

Another objective common to both sub-tasks was that of experimentally validating the new energy based criterion for the evaluation of the re-centring capability of seismic isolation systems developed by the author [1].

The conclusions are the following:

1. The experience gained during the testing programme undertaken on the shake table for a mass seismically isolated by means of a flat surface sliders coupled with SHEs has proven that this type of isolator is: easy to design and manufacture, user-friendly, reliable and repeatable, durable (it survives least a dozen design-level earthquakes) and is cost effective. Nonetheless, its restoring capability needs to be verified on a case-by-case basis.
2. The results have substantiated the method for evaluating the re-centring capability of seismic isolation systems based on the energy approach.
3. The experience gained during the same testing programme on a 29.1 T mass seismically isolated by means of curved surface sliders has proven that with this type of isolator:

   a)

   no torsional effects occur in the presence of asymmetrical mass distribution in perfect accordance with the theoretical prediction

   b)

   the re-centring capability is higher than that indicated by the present norms.

1

R. Medeot, "Re-Centring Capability Evaluation of Seismic Isolation Systems Based on Energy Concepts", 13th World Conference on Earthquake Engineering, Vancouver, B.C., Canada, 2004.

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