Figure : Classification of Seismic Protection Systems.
  1. ELASTOMERIC BEARINGS
An elastomeric isolation bearing consists of a number of rubber layers and steel shims, bonded in alternating layers, to produce a vertically stiff but horizontally flexible isolator. This flexibility lengthens the fundamental period of the isolated building and reduces the seismic forces in the superstructure. But this reduction may be accompanied by large horizontal displacements in the isolators, which, together with their lateral flexibility, may lead to significant reduction in their critical axial load Buckle and Kelly 1986; Koh and Kelly 1986; Buckle and Liu 1994; Nagarajaiah and Ferrell 1999. Elastomeric isolation bearings are required to be stable at high shear strains, which occur during strong earthquakes. Hence, rigorous determination of the critical axial load during design is important [13].
  1. LOW DAMPING NATURAL OR SYNTHETIC RUBBER BEARING
Elastomeric bearings have been used widely in bridges as bearing pads between the girder and the supporting structure for many years. Elastomeric bearings have multiple layers of steel shims and rubber laminated together under high pressure and heat in a mold. Steel shims prevent lateral bulging of the rubber when axially loaded. They do not resist shear forces and do not prevent the horizontal deformation of the layered rubbers. Therefore, steel shims increase the vertical stiffness of isolators but do not increase the lateral stiffness of elastomeric bearings.
Figure 2.5 shows elastomeric bearings subjected to dynamic shear test. The uneven surface of the elastomeric bearings shown is due to its steel shims. Generally, elastomeric bearings have low critical damping resistance, approximately 2% to 3% of critical viscous damping; and have minimal resistance under service loads. Therefore, elastomeric bearings need to be improved. The result is a high damping elastomeric bearings and the lead rubber bearings.
  1. HIGH DAMPING RUBBER BEARING
As an alternative to elastomeric bearings, high damping rubber bearings provide critical damping from 10% to 20% at 100% shear strains. The construction methodology is the same with elastomeric bearings; however, the damping is increased by adding carbon block and other fillers. In addition, it has an adequate resistance to service loads. The damping characteristic is in between hysteretic and viscous. The energy dissipation is linear and quadratic for hysteretic and viscous, respectively. The energy absorption capacity help reduced the earthquake energy transmitted to the superstructure. The load capacity of HDRB can be computed using the same method as elastomeric bearings. The damping value can be computed using the equivalent damping ratios for specific elastomeric compounds.
  1. LEAD RUBBER BEARING
Lead rubber bearings are elastomeric bearings that contain one or more lead plugs inserted into their preformed holes. The lead provides significant stiffness under service loads and low lateral loads as compare to elastomeric bearings. Figure 2.6 shows the alternating sheet of steel shims and rubbers circumscribing a lead core. In addition, the lead serves as energy dissipation mechanism under severe lateral loads.
During high lateral loads, the lead yields and the lateral stiffness of the LRB is significantly reduced. This increases the duration of the period of the structure and thereby serves the purpose of base isolation system. The bearing is cycled into a hysteretic damping as it absorbs the energy. LRB has a range of damping from 15% to 30% which is a function of displacement. The LRB are the most common base isolator used for isolating midrise buildings. It is usually designed and optimized according to a specific performance based target design. It combines the stiffness needed for service loads and low lateral loads while providing the flexibility and damping needed for high lateral loads. A wide array of damping and stiffness is possible through the use of LRB. As for the case of HDRB design, the formulas of elastomeric bearings are suitable for the design of LRB.
  1. SLIDING BEARING
One of the most popular and effective techniques for seismic isolation is through the use of sliding isolation devices. The sliding systems perform very well under a variety of severe earthquake loading and are very effective in reducing the large levels of the superstructure’s acceleration. These isolators are characterized by insensitivity to the frequency content of earthquake excitation. This is due to tendency of sliding system to reduce and spread the earthquake energy over a wide range of frequencies. The sliding isolation systems have found application in both buildings and bridges. The advantages of sliding isolation systems as compared to conventional rubber bearings are (i) frictional base isolation system is effective for a wide range of frequency input, (ii) since the frictional force is developed at the base, it is proportional to the mass of the structure and the center of mass and center of resistance of the sliding support coincides. Consequently, the torsional effects produced by the asymmetric building are diminished.
  1. FLAT SLIDING BEARING
Flat sliding bearings consist of PTFE (Teflon) disc that slides on a stainless-steel plate. They provide a perfectly plastic hysteresis shape, and adequate stiffness under service loads with high damping properties. In addition, the coefficient of friction is a function of both pressure and velocity of sliding. It provides the resistance under service loads. However, it must be combined with other bearings (i.e. HDRBs, LRBs) because it has no capability to return to its initial position. Figure 2.7 shows an assembly of flat sliding bearing, which has the stainless-steel plate supporting the circular disc. There is no other part in the assembly that shows that it has the capability to return to its initial position. A modified version of flat sliding bearing that has self-restoring force is the friction pendulum bearings.
  1. SPHERICAL SLIDING BEARING
The friction pendulum bearings have the same properties as the flat sliding bearings. However, the sliding surface is concave in shape rather than flat as shown in Figure 2.8. The hemisphere at the center of the concave surface is the pendulum slider. The spherical concave surface provides a restoring force to the pendulum slider to return to its initial position. Varying the radius of the concave surface varies the stiffness of the friction pendulum bearings. In addition, once the coefficient of friction is overcome the lateral movement of the mass is accompanied by a vertical movement of the mass because of the curved shape of the slider. As for the case of LRB, the friction pendulum bearings have a wide array of damping and stiffness design capabilities.
  1. PASSIVE ENERGY DISSIPATION SYSTEM
Passive energy dissipation systems encompass a range of materials and devices for enhancing damping, stiffness and strength, and can be used both for natural hazard mitigation and for rehabilitation of aging or deficient structures. Some structures have very low damping on the order of 1% of critical damping and consequently experience large amplitudes of vibration even for moderately strong earthquakes [14]. Methods of increasing the energy dissipation capacity are very effective in reducing the amplitudes of vibration In recent years, serious efforts have been undertaken to develop the concept of energy dissipation or supplemental damping into a workable technology and a number of these devices have been installed in structures throughout the world [7, 15]. In general, they are all characterized by a capability to enhance energy dissipation in the structural systems to which they are installed. This may be achieved either by conversion of kinetic energy to heat, or by transferring of energy among vibrating modes. The first method includes devices that operate on principles such as frictional sliding, yielding of metals, phase transformation in metals, deformation of viscoelastic solids or fluids, and fluid orificing. The latter method includes supplemental oscillators, which act as dynamic vibration absorbers.