CUADRO CLÍNICO

In summary, the requirements for practical isolation system are defined by the performance objectives discussed above:

1. Flexibility. 2. Damping.

3. Resistance to service loads.

Additional requirements such as durability, cost, ease of installation and specific project requirements will influence device selection but all practical systems must contain these three essential elements. 2.9 TYPES OF ISOLATORS

Many types of isolation system have been proposed and have been developed to varying stages, with some remaining no more than concepts and others having a long list of installed projects. The following sections provide a discussion of generic types of system. Later chapters discuss devices that are commercially available.

2.9.1 SLIDING SYSTEMS

Sliding systems are simple in concept and have a theoretical appeal. A layer with a defined coefficient of friction will limit the accelerations to this value and the forces which can be transmitted will also be limited to the coefficient of friction times the weight.

Sliders provide the three requirements of a practical system if the coefficient of friction is high enough to resist movement under service loads. Sliding movement provides the flexibility and the force-displacement trace provides a rectangular shape that is the optimum for equivalent viscous damping.

A pure sliding system will have unbounded displacements, with an upper limit equal to the maximum ground displacement for a coefficient of friction close to zero. The system provides no restoring force and so the isolated structure will likely end up in a displaced position after an earthquake and may continue to displace with aftershocks.

The lack of a restoring force may be remedied by using sliding bearings in parallel with other types which do have a restoring force or by using a shaped rather than flat sliding surface, for example, a spherical sliding surface.

2.9.2 ELASTOMERIC (RUBBER) BEARINGS

Elastomeric bearings are formed of horizontal layers of natural or synthetic rubber in thin layers bonded between steel plates. The steel plates prevent the rubber layers from bulging and so the bearing is able to support higher vertical loads with only small deformations. Under a lateral load the bearing is flexible.

Plain elastomeric bearings provide flexibility but no significant damping and will move under service loads. Methods used to overcome these deficits include lead cores in the bearing, specially formulated elastomers with high damping and stiffness for small strains or other devices in parallel.

2.9.3 SPRINGS

There are some proprietary devices based on steel springs but they are not widely used and their most likely application is for machinery isolation. The main drawback with springs is that most are flexible in both the vertical and the lateral directions. The vertical flexibility will allow a pitching mode of response to occur. Springs alone have little damping and will move excessively under service loads.

2.9.4 ROLLERS AND BALL BEARINGS

Rolling devices include cylindrical rollers and ball races. As for springs, they are most commonly used for machinery applications. Depending on the material of the roller or ball bearing the resistance to movement may be sufficient to resist services loads and may generate damping.

2.9.5 SOFT STORY, INCLUDING SLEEVED PILES

The flexibility may be provided by pin ended structural members such as piles inside a sleeve that allows movement or a soft first story in a building. These elements provide flexibility but no damping or service load resistance and so are used in parallel with other devices to provide these functions.

2.9.6 ROCKING ISOLATION SYSTEMS

Rocking isolation systems are a special case of energy dissipation that does not fit the classic definition of isolation by permitting lateral translation. The rocking system is used for slender structures and is based on the principle that for a rocking body the period of response increases with increasing amplitude of rocking. This provides a period shift effect. Resistance to service loads is provided by the weight of the structure. Damping can be added by using devices such as yielding bolts or steel cantilevers.

2.10 SUPPLEMENTARY DAMPING

Some of the isolation types listed above provide flexibility but not significant damping or resistance to service loads. Supplementary devices that may be used include:

• Viscous dampers. These devices provide damping but not service load resistance. They have no elastic stiffness and so add less force to the system than other devices.

• Yielding steel devices, configured as either cantilevers yielding acting in flexure or beams yielding in torsion. These provide stiffness and damping.

• Lead yielding devices, acting in shear, provide stiffness and damping.

• Lead extrusion devices where lead is forced through an orifice. Added stiffness and damping. All devices apart from the viscous dampers are displacement dependent and so provide a maximum force at maximum displacement, which is additive to the force in the isolation device. Viscous dampers are velocity dependent and provide a maximum force at zero displacement. This out-of- phase response adds less total force to the system.