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Base Isolation


Published on Jan 10, 2017

Abstract

Base isolation is one of the most popular means of protecting a structure against earthquake forces. It is one of most powerful tools of earthquake engineering pertaining to the passive structural vibration control technologies. It is easiest to see the principle at work by referring directly to the most widely used of these advanced techniques, known as base isolation. A base isolated structure is supported by a series of bearing pads, which are placed between the buildings and building foundation.

The concept of base isolation is explained through an example of building resting on frictionless rollers. When the ground shakes, the rollers freely roll, but the building above does not move. Thus, no force is transferred to the building due to the shaking of the ground; simply, the building does not experience the earthquake.

Now, if the same building is rested on the flexible pads that offer resistance against lateral movements, then some effect of the ground shaking will be transferred to the building. If the flexible pads are properly chosen, the forces induced by ground shaking can be a few times smaller than that experienced by the building built directly on ground, namely a fixed base building. The flexible pads are called base-isolators, whereas the structures protected by means of these devices are called base-isolated buildings.

The main feature of the base isolation technology is that it introduces flexibility in the structure. As a result, a robust medium-rise masonry or reinforced concrete building becomes extremely flexible. The isolators are often designed, to absorb energy and thus add damping to the system. This helps in further reducing the seismic response of the building. Many of the base isolators look like large rubber pads, although there are other types that are based on sliding of one part of the building relative to other.

Base Isolation

Base isolation is not suitable for all buildings. Mostly low to medium rise buildings rested on hard soil underneath; high-rise buildings or buildings rested on soft soil are not suitable for base isolation.

Response of Base Isolated Buildings

The base-isolated building retains its original, rectangular shape. The base isolated building itself escapes the deformation and damage-which implies that the inertial forces acting on the base isolated building have been reduced. Experiments and observations of base-isolated buildings in earthquakes to as little as ¼ of the acceleration of comparable fixed-base buildings. Acceleration is decreased because the base isolation system lengthens a buildings period of vibration, the time it takes for a building to rock back and forth and then back again. And in general, structures with longer periods of vibration tend to reduce acceleration, while those with shorter periods tend to increase or amplify acceleration.

Base Isolation

Spherical sliding isolation system:

Spherical sliding isolation systems are another type of base isolation. The building is supported by bearing pads that have a curved surface and low friction. During an earthquake the building is free to slide on the bearings. Since the bearings have a curved surface, the building slides both horizontally and vertically. The forces needed to move the building upwards limits the horizontal or lateral forces which would otherwise cause building deformations. Also by adjusting the radius of the bearings curved surface, this property can be used to design bearings that also lengthen the buildings period of vibration.

Types of bearings:

Lead-rubber bearings

These are the frequently-used types of base isolation bearings. A lead rubber bearing is made from layers of rubber sandwiched together with layers of steel. In the middle of the solid lead “plug”. On top and bottom, the bearing is fitted with steel plates which are used to attach the bearing to the building and foundation. The bearing is very stiff and strong in the vertical direction, but flexible in the horizontal direction. Lead is a crystalline material which changes its structure temporarily, under deformations beyond its yield point, and regains its original structure and elastic properties as soon as the deformation is removed by the restoring force in the rubber. Note that lead has good fatigue properties for subsequent cycles of loading beyond its yield point.

How it works?

To get a basic idea of how base isolation works, first examine the above diagram. This shows earthquake acting on base isolated building and a conventional, fixed-base and building. As a result of an earthquake, the ground beneath each building begins to move. Each building responds with movement which tends towards the right. The buildings displacement in the direction opposite the ground motion is actually due to inertia. The inertia forces acting on a building are the most important of all those generated during an earthquake. In addition to displacing towards right, the un-isolated building is also shown to be changing its shape from a rectangle to a parallelogram. We say that the building is deforming. The primary cause of earthquake damage to buildings is the deformation which the building undergoes as a result of the inertial forces upon it.

Elastomeric isolation system

The most popular seismic isolation systems use elastomeric bearings which consist of thin rubber sheets bonded onto thin steel plates and combine with an energy dissipation mechanism. The rubber sheets are vulcanized and bonded to the thin steel plates under pressure and heat.

The inner thin steel plates provide the vertical load capacity and Stiffness, and prevent lateral bulging of the rubber.In particular the steel plates laterally constrain the rubber sheets as vertical load is applied to the elastomeric bearing, providing the vertical stiffness. Horizontal flexibility is provided by the shearing deformability of the rubber sheets which are not restrained from deform in that direction by the steel plates. Thick mounting steel plates are bonded to the bottom and top surfaces allowing the isolator to be firmly connected to the foundation below and the superstructure above.

The energy dissipation mechanism is based either on the plastic deformation of a metal or on the inherent damping properties of the rubber. In the first case either lead plugs are inserted in the elastomeric bearings or auxiliary dampers based on deformations of lead or steel are used.

Lead rubber bearings (LRBs) and high-damping rubber bearings (HDRBs) are most useful in seismic isolation since they provide the following in a single unit:

 Vertical support due to the high vertical stiffness, which is usually several hundred times the horizontal stiffness .Sufficient vertical stiffness is necessary to avoid rocking of the structure.

 Horizontal flexibility which shifts the fundamental frequency of the structure out of the dangerous for resonance frequency range.

 An energy dissipation mechanism, either via the plastic deformation of the lead plug or through the inherent damping properties of high damping rubber.

Finally, there are also some systems that use natural rubber bearings (NRBs) with additional steel or lead damper; in this case energy dissipation results from the plastic deformations of the damper.

High-damping rubber bearings (HDRBs)

This type of bearing consists of thin layers of high damping rubber sandwiched between steel plates. The same manufacturing methods for vulcanization and bonding that are used for LRBs are also used to construct HDRBs. The only difference is the composition of the rubber compound, which provides increased damping.

High-damping rubber is actually a filled rubber compound with inherent damping properties due to the addition of special fillers, such as carbon and resins. The addition of fillers increases the inherent damping properties of rubber without affecting its mechanical properties. When shear stresses are applied to high-damping rubber, a sliding of molecules generates frictional heat which is a mechanism of energy dissipation. In unfilled natural rubber, used for LRBs, frictional heat is negligible because the molecular attraction in physical cross links is very weak. The energy dissipation mechanism of an HDRB is available for both small and large strains, is constant and is characterized by smooth elliptical hysteresis loops. Experimental studies of high damping rubber bearings verified the anticipated energy dissipation capacity, which is, typically, equivalent to about 15% damping ratio of equivalent linear elastic models.

However, HDRBs may not provide the necessary initial rigidity under service loads and minor lateral loads, although some initial rigidity is provided by high-damping rubber compounds which exhibit higher stiffness under small strains. A structure isolated with HDRBs essentially has a constant, large fundamental period due to the flexibility of the isolation system, which makes the structure vulnerable to wind action with dominant frequencies close to the fundamental frequency. In addition, the damping and mechanical properties of the HDRB appear to be temperature dependent while the hysteretic energy dissipation mechanism of the LRB is not. HDRBs are not as widely used in seismic isolation as LRBs.

Hybrid type: lead high-damping rubber bearing (LHDRBs)

A hybrid type of lead high-damping rubber bearing (LHDRBs) may consist of layers of high-damping rubber sandwiched between steel plates and a smaller diameter lead cylinder plug firmly in a hole at its center. This hybrid bearing may have the advantages of both isolation systems discussed above. The LHDRBs has both an initial rigidity, due to the presence of the lead plug, and a continuous energy dissipation mechanism, due to the damping properties of the high-damping rubber. Therefore, the isolation system is expected to perform well in both weak and extreme earthquakes as well as under minor lateral loads. In addition, the properties of this bearing will be less dependent on temperature changes and shear strains than those of HDRB.

Also, the use of LHDRBs allows the reduction of the initial stiffness of the isolation system which is responsible for higher frequency effects. A final advantage is that the effectiveness of such a device and its compact size make it suitable for cases where installation space is limited. LHDRBs offer a practical and cost-effective trade-off between the advantages and limitations of LRBs and HDRBs.

When practically possible, it may be more effective to place the high-damping rubber bearings with lead plugs at the perimeter of the building, preferably under the columns situated as far away as possible from the center of stiffness and mass of the isolated structure. High-damping rubber bearings with no lead plugs may be used under the internal columns. This configuration will allow lower prior-to-yielding stiffness of the lead plugs and consequently, a smoother change of the stiffness during yielding and reverse loading. The high initial stiffness and its sudden changes are responsible for higher mode effects and acceleration increases, which may be avoided by reducing the initial stiffness.

In addition, lower initial stiffness will provide a higher degree of isolation at the prior-to-yielding stage. Placing the bearings with the lead plugs under the external columns away from the center of stiffness will provide higher resistance against torsion, due to the larger diameter between the points of application of forces and the center of stiffness of the isolation system. Note that this configuration may be used only when there is a rigid diaphragm at the isolation level to redistribute the inertia forces to the LHDRBs.

Maintenance and Management of the Isolation System:

The isolation system must remain operational for the world expected lifetime of the structure under all possible environmental effects. These may corrode the metallic parts of the isolation system and deteriorate the elastomer; such effects may be reduced by using a protective rubber cover. The maintenance of the isolation system, and especially that of the seismic gap, must be frequent to ensure the vertical loads which they must sustain. When the construction of a diaphragm is not possible, the bearings must be designed in proportion to the magnitude of the lateral forces carried by the members above them. It is difficult to take into account such design issues due to the uncertainties involved; therefore, the construction of a diaphragm above the isolation level must be anticipated. Note that here, the existence of a rigid diaphragm is assumed. Finally, the selected location of the bearings must be such that access to them is enabled for inspection and possible replacement purposes.

Factors which enable the use of base isolation

Several factors favor the development and practical application of seismic isolation. First of all there are increased requirements for the performance of structure in severe earthquakes which are not met by current conventional design philosophy. These are crucial to structures containing sensitive and expensive equipment vulnerable even to micro tremors.

Second, the advance of computer technology and modern structural analysis method enables the development of reliable software to stimulate the response of structures. The development of seismic engineering and earthquake engineering to level where reliable prediction can be made for expected earthquakes, is another important factor.

In addition, the construction of shaking tables which can stimulate actual earthquake excitations makes possible the experiment validation of the behavior of seismic isolation system. The study of other minor loads such as wind load and the reliable quantifications of their expected intensities and frequency of occurrence, also enables the use of seismic isolation.

Finally, development, manufacture and extensive research in the area of structural material enable the reliable use of modern material for seismic isolation device. The development of device which decipate energy and provide the restoring force to avoid permanent displacement also allow the practical implementation of the seismic isolation concept.

CONCLUSION

We can use base isolation technique to construct the earthquake resistant building. Proper materials and design should be selected to get the best result. The safety of people should be the main aim.

REFERNCES

P. Komodromos- Seismic isolation for earthquake resistance structure.- Henry J. Lagorio

Earthquake – An architect’s guide to nonstructural seismic hazards.- Charles K. Erdey

Earthquake Engineering.-Yousef Bozorgnia & Vitelmo V. Bertero

Earthquake Engineering-Wai – Fan Chen & Charles Scawthorn














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