Turkey
BASE ISOLATION
Base isolation offers important advantages over conventional protection methods because it reduces the earthquake forces transmitted into a structure.
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Base Isolation
Base isolation offers important advantages over conventional protection methods because it reduces the earthquake forces transmitted into a structure. Thus it protects not only the structure itself but also the contents and secondary structural features. Such a capability is particularly important for buildings such as hospitals and emergency facilities that need to maintain full serviceability after an earthquake.
Base isolation has now become an established and accepted technology all over the world. The technique is applicable to bridges and industrial structures, such as LNG tanks, in addition to buildings. It is suitable for upgrading existing buildings.
The capability to protect the contents is a major advantage for buildings such as hospitals and emergency centres where maintenance of functions during and after an earthquake is necessary, and in cases such as museums and advanced technology factories where the value of the contents is high.
In seismic isolation part or all of the superstructure is separated from the lower part of the structure by an interface that is soft and exible in the horizontal direction. Generally, the interface is placed between the foundation or basement and the ground oor and so the term base isolation can be accurately applied.
In a base isolated building, the structure is isolated from its foundations so as to minimize the effect of ground motion on the superstructure and, thereby, reduce its force and de ection response. The difference in how conventional structures and base isolated structures respond to earthquakes is illustrated in the gure below.
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RESPONSE OF CONVENTIONAL STRUCTURES AND BASE ISOLATED STRUCTURES TO EARTHQUAKES
Savings in the superstructure can be expected through a reduction of forces afforded by the isolation. The reduction in forces also allows for greater exibility in space planning and reduced footprint of structural elements. Additionally, foundations under lateral load resisting elements may be reduced with base isolation.
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The effect of the isolation system is that during an earthquake the structure moves virtually as a rigid body on the isolators. The deformation is concentrated at the isolation interface, but unlike the structure the isolation system can accomodate large deformations without signi cant damage. Because the technique is one of isolation it should be noted that high levels of damping (>15-18%) are not bene cial for structures (eg. multi-storey buildings) for which higher level oscillation modes may be excited.
THE PRINCIPLES OF BASE ISOLATION
Isolation is achieved by mounting the structure on a system of supports giving a low stiffness in the horizontal direction. The natural period of the structure on the isolation system is typically two seconds. This period is chosen to be long compared with both the dominant period of the earthquake ground shaking and the period of the superstructure in the xed-base conditions.
The gure below shows that the period lengthening achieved greatly reduces the acceleration response compared with that of a typical conventional structure. It also shows that the response to input excitations at the isolation period, and the amplitude of the horizontal movement of the structure.
It is important to realise that despite the need for some damping, the isolators are not principally acting to absorb the energy of the earthquake, but are providing an interface that re ects earthquake energy back into the ground so reducing its transmission into the structure.
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