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Earthquake-induced poundings of seismically isolated structures

Final Activity Report Summary - EIPOSIS (Earthquake-induced poundings of seismically isolated structures)

Seismic isolation is an innovative earthquake resistant design approach, which can be used alternatively to conventional methods, in order to reduce the induced seismic loads in a structure and avoid structural and non-structural damage, as well as damage of the contents of the structure. A building is seismically isolated by inserting flexibility at its isolation level, usually at the base of the building, in order to shift its fundamental frequency outside the dangerous for resonance range with the predominant frequencies of common earthquakes and reduce the induced seismic loads. Both floor accelerations and inter-story deflections can be significantly reduced, while large deformations are confined at the isolators, which are purposely designed to accommodate cycles of inelastic deformations.

Nevertheless, a practical constraint for the utilisation of seismic isolation is the width of the clearance, known as seismic gap, that must be ensured around the building to facilitate the expected large relative displacements at the isolation level. Considering that there are often certain practical restrictions to the size of the available clearance around seismically isolated buildings, a reasonable concern is the possibility of poundings with adjacent structures during a stronger than expected earthquake.

Pounding incidences between fixed-supported buildings due to strong earthquakes motivated relevant research, which led to seismic-code reforms in order to mitigate the risks from poundings of adjacent fixed-supported structures. However, very limited research work has been carried out for poundings of seismically isolated buildings, which exhibit quite different dynamic characteristics from fixed-supported buildings. In particular, poundings of a seismically isolated building occur primarily as a result of the large relative displacements at the isolation level, while in the case of conventionally fixed-supported buildings poundings occur due to the deformations of the superstructure, usually at the building tops.

This research project has investigated the above described problem, aiming to understand the consequences of earthquake induced poundings on the effectiveness of seismic isolation and how the response of seismically isolated structures is affected by the various design parameters and the excitation characteristics. In order to be able to study this problem it was necessary to develop a specialised software application that could be used to efficiently and effectively perform series of simulations and parametric studies under various earthquake excitations. This software was designed and developed utilising recent advances in computing and modern software engineering, such as Java technologies and modern object-oriented oriented design and programming, in order to ensure the significant advantages that these technologies offer regarding the maintenance and extensibility of the developed software.

The numerical simulations demonstrate that poundings may substantially increase floor accelerations, especially at the floor where impacts occur. Higher modes of vibration are excided during poundings, increasing the inter-story deflections, instead of retaining an almost rigid-body motion of the superstructure, which is aimed with seismic isolation. Impact stiffness seems to affect significantly the acceleration response at the isolation level, while the displacement response is more insensitive to the variation of the impact stiffness. Finally, the results indicate that providing excessive flexibility at the isolation system to minimise the floor accelerations may lead to a building vulnerable to poundings, if the available seismic gap is limited. Furthermore, certain impact mitigation measures have been proposed in order to alleviate the detrimental consequences of earthquake induced poundings of seismically isolated structures.