1.3. B ASE I SOLATION
1.3.2. Leading Base Isolation System
The basic features of base isolated systems consist of the horizontal flexibility and energy absorption capacity. The horizontal flexibility of the bearing can increase the
earthquake energy is dominant. However, due to the horizontal flexibility, the displacement of the isolated structure increases.
The energy absorption capacity and damping of the bearing counteract the excessive deformation of the structures. Owning to this characteristic of base isolated systems, they can attenuate the harmful horizontal acceleration transmitted to the super structure and reduce the member forces of the substructure. Various leading base isolated system, which have been used or are considered to have considerable potential for wide application, are briefly described.
(a) Pure-Friction (P-F) system
P-F system is classified as simplest system because it uses only stick-slip mechanism.
When there is a sliding in the friction plates, the P-F system limits the maximum acceleration to be transmitted to the superstructure to a certain value, which is proportional to the friction coefficient. However, there may an excessive deflection or a residual deformation in the friction surface after the seismic event, because the P-F system has no recovering force. A schematic diagram of pure friction based isolator is shown in Fig. 1.4.
Fig. 1.4 Schematic diagram of P-F base isolator.
(Lin et al., 1989)
(b) Laminated Rubber Bearing (RB) system
The RB system is widely studied and used all over the world. It consists of alternating layers of rubber and steel with the rubber being vulcanized to the steel plates. The rubber layers provide horizontal flexibility, while the interleaved steel layers are responsible to provide adequate vertical stiffness. The dominant feature of the system is parallel action of spring and dashpot as shown in the following Fig. 1.5(a). Since damping capacity of RB system is relatively small, it mainly shifts natural period of the isolated structure to avoid detrimental earthquake energy.
(a) Schematic of RB system.
(Lin et al., 1989)
(b) View of RB (Wang Y.P., 2002.) Fig. 1.5 Schematic and view of laminated rubber bearing.
Laminated rubber or elastomeric bearings can be classified into low-damping or high damping types. Schematic diagram of laminated rubber bearing is shown in Fig. 1.5(a) and view of laminated rubber bearing is shown in Fig. 1.5(b).
(c) Lead Rubber Bearing or New Zealand (NZ) system
The lead rubber bearing system or NZ system is also widely used like a RB unit. A central lead core improves performances of NZ system. It reduces relative deflection and provides an additional means of energy dissipation. Schematic diagram and pictorial view of NZ bearing system is shown in Fig. 1.6.
(a) Schematic diagram of NZ system base isolator (Lin et al., 1989)
(b) Pictorial view of NZ base isolation (Buckle I. G., 1985)
Fig. 1.6 Schematic diagram and pictorial view of NZ bearing.
(d) Resilient Friction Base Isolator (R-FBI) system
This isolator consists of concentric layers of plates with friction contact to each other and a central rubber core. The R-FBI system makes use of parallel action of resiliency of rubber and friction of teflon coated plates. This bearing does not slide below the frictional resistance similar to P-F system. However, the central rubber core provides an additional resistance to increase in deflection and a recovering force after sliding.
Schematic diagram of R-FBI bearings system is shown in Fig. 1.7.
Fig. 1.7 Schematic diagram of R-FBI bearing system.
(Lin et al., 1989)
TOP COVER PLATE
BOTTOM COVER PLATE
RUBBER LEAD
STEEL
(e) Electricite de France (EDF) system
The EDF system consists of the laminated neoprene pad topped by a lead bronze plate which is in frictional contact with steel plates anchored to the structures. During a low intensity earthquake, it behaves as a rubber bearing unit and return to its original position after the seismic event. When the frictional resistance is exceeded, slip will occur and the EDF system may have a residual deformation in the friction surface during a high intensity earthquake. This system behaves as a combination of elastomeric bearing and friction plates in series is schematically shown in Fig. 1.8.
Fig. 1.8 Schematic diagram of EDF bearing system.
(Lin et al., 1989)
(f) Friction Pendulum Bearing system
The friction pendulum bearings are made up of a dense chrome over steel concave surface in contact with a articulated friction slider and free to slide during lateral displacement. Bearings can be designed to accommodate different magnitude of displacements simply by adjusting the curvature and the diameter of the bearing surface. Cross sectional view of a friction pendulum bearing is shown in Fig. 1.9.
(g) Spring Type Base Isolation Systems
The spring type vibration isolation system is popularly used in many applications and
of spring type base isolation system. The GERB system for seismic isolation was developed originally for the vibration isolation of power plant rotating equipment like turbine generator, fans, mills etc. GERB vibration control systems typically consist of spring elements and viscodampers. It uses large helical steel springs that are flexible both horizontally and vertically. The vertical frequency is around 3-5 times the horizontal frequency. The steel springs are completely without damping and the system is always used in conjunction with the viscodamper to provide damping to the system.
Fig. 1.5(a) also represent the schematic of GERB base isolation system which consists of spring and viscodamper.
Fig. 1.9 Cross sectional view of FPB system (Zayas et al., 1989)