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Base Isolation.

Recent years, base isolation designs for isolation and protection of buildings from damages due to earthquake has become an increasingly applied structural design technique in highly seismic prone areas. Many types of structures have been designed and built with base isolation, incorporating different concept of building base isolation, and many others are in design phase or under construction. Most of these completed buildings and those under construction use rubber base isolation bearings, which are found to be most efficient in base isolation designs. We at www.pretread.com, your reliable source for neoprene bridge bearing, bearing pads, laminated elastomeric bearing, elastomeric bearing have been involved in many of these projects for base isolation and our home page and connected links provide information on a wide range of rubber products for construction industry.

It is seen that conventionally constructed buildings with seismic protection in mind have been crumbled under the tremendous destructive strength of earthquake in many parts of the world. Conventional structure design approach to earthquake resistant of buildings have found to be followed all over world, by mostly depends upon providing building with strength, stiffness and inelastic deformation capacity which are great enough to withstand a given level of earthquake-generated force. This is generally accomplished through selection of an appropriate structural configuration and careful detailing of structural members, such as beams and columns, and connections between them. Practically they all proved to be failing under seismic activates, damaging human lives and properties all over the world.

Fixed base and base isolated building.

First let us explain a fixed base building and base isolated building and then explain concept of base isolation. Buildings with foundation base fixed to the super structure are known as fixed base, and buildings with rubber or similar isolation between to base foundation and structure are known as base isolated buildings.

There are two basic types of base isolation systems. The system that has been adopted most widely in recent years is typified by use of elastomeric bearings, of different sizes and shapes. In this approach, building or structure is decoupled from horizontal components of earthquake .. ground motion.. by interposing a layer with low horizontal stiffness between structure and foundation.

In base isolation with rubber bearings, large rubber bearings are used to connect structure and base of building isolating structure and its movements from foundation. A variety of different types of base isolation bearing pads have now been developed, and a base isolated structure will be supported by a series of bearing pads, which are placed between building and building's foundation, providing isolation to building base.

These base isolation bearings are manufactured by vulcanization, elastomer used will be either natural rubber or neoprene, bonding of sheets of thick rubber to thin steel reinforcing plates. These bearings are very stiff in vertical direction and very flexible in horizontal direction, and under seismic loading, bearing layers isolates building from the horizontal components of ground movements, while vertical components are transmitted to structure, relatively remains unchanged. Although vertical accelerations do not affect most buildings, the bearings also isolate building from unwanted high-frequency vertical vibrations produced by underground railways and local traffic. Rubber bearings are suitable for stiff buildings up to seven stories in height. For this type of building, uplift on the bearings will not occur and wind load will be unimportant.

Rubber base isolation gives structure a fundamental frequency that is much lower than its fixed-base frequency and also much lower than predominant frequencies of ground motion. The first dynamic mode of isolated structure involves deformation only in isolation system, structure above being to all intents and purposes rigid. The higher modes that will produce deformation in structure are orthogonal to first mode and consequently also to ground motion. These higher modes do not participate in motion, so that if there is high energy due to ground motion at these higher frequencies, this energy will not be transmitted into structure. The isolation system does not absorb the earthquake energy, but rather deflects it through the dynamics of the system. This type of isolation works when the system is linear and even when un-damped however some damping is beneficial to suppress any possible resonance at the isolation frequency.

Deformation and Damages to fixed base building.

Let us see what happens to fixed base building during an earthquake. Conventionally constructed buildings with seismic protection in mind with fixed base will displace to right when ground moves to left and to left when ground moves to right. This displacing changes shape of the building from a rectangle to a parallelogram. We say that building is deforming. The primary cause of earthquake damage to buildings is this deformation which building undergoes as a result of the inertial forces acting upon it. Acceleration is increased, shortening building's period of vibration, causes damages to structure and probably the magnitude of earthquake brings the structure down, with permanent damages including human life losses.

The different types of damage, which buildings can suffer, are quite varied and depend upon a large number of complicated factors. But to take one simple example, one can easily imagine what happens to two pieces of wood joined at a right angle by a few nails suddenly starts to move very quickly--the nails pull out and connection fails.

Response of Base Isolated Building to earthquake.

To get a basic idea of how base isolation works, examine how earthquake acting on a base isolated building. As a result of an earthquake, ground beneath each building begins to move, imagine first to left, building responds with movement, which tends toward the right. We say that the building undergoes displacement towards right. The building's displacement in direction opposite ground motion is actually due to inertia. The inertial forces acting on a building are the most important of all those generated during an earthquake.

By contrast, even though base isolated buildings too displaces, base isolated building retains its original, rectangular shape where as a conventional fixed base building changes to parallelogram. During displacement, it will be the base isolated rubber bearings, supporting the building will be deformed. The base-isolated building itself escapes deformation and damage--which implies that inertial forces acting on base isolated building have been reduced. Experiments and observations of base-isolated buildings in earthquakes have been shown to reduce accelerations to as little as 1/4 of acceleration, comparable to fixed-base buildings, which each building undergoes as a percentage of gravity. As noted above, inertial forces increase, and decrease, proportionally as acceleration increases or decreases.

Acceleration is decreased because base isolation system lengthens a building's period of vibration, the time it takes for 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. It is important to know that, inertial forces which the building undergoes are proportional to the building's acceleration during ground motion. It is also important to realize that buildings don't actually shift in only one direction. Because of the complex nature of earthquake ground motion, the building actually tends to vibrate back and forth in varying directions.

The basic approach underlying more advanced techniques for earthquake resistance is not to strengthen building, but to reduce the earthquake-generated forces acting upon it by choosing the right and appropriate base isolation bearing system.

There has been many experiments and designs followed with concept of base isolation for the past 30 odd years and at many places they seem to be out-performing conventional fixed base with conventional structure strengthen approaches.

Here we notice that the building with base isolation is safer than the conventional base structure.

These base isolation bearings are very stiff and strong in vertical direction, but flexible in the horizontal direction. Finally, since they are highly elastic, the rubber isolation bearings don't suffer much damages, but the lead plug in the middle bearing experiences same deformation as the rubber. However, it also generates heat as it does so. In other words, the lead plug reduces, or dissipates, the energy of motion--i.e., kinetic energy--by converting that energy into heat. By reducing the energy entering structure building, it helps to slow and eventually stop building's vibrations sooner than would otherwise be the case--in other words, it damps the building's vibrations. (Damping is the fundamental property of all vibrating bodies which tends to absorb the body's energy of motion, and thus reduce the amplitude of vibrations until the body's motion eventually ceases.)

There is a second basic type of base isolation system typified by the a sliding system. This works by limiting the transfer of shear across the isolation interface. Many sliding systems have been proposed and some have been used. Another type of isolation containing a lead-bronze plate sliding on stainless steel with an elastomeric bearing has been used. The friction-pendulum system is a sliding system using a special interfacial material sliding on stainless steel and has been used for several projects in the United States, both new and retrofit construction.

Types of Bearings manufactured by us for base isolation.

High Damping Rubber Bearing (HDRB)

High damping rubber bearing (HDRB) are large laminated elastomeric bearings which is ideal for seismic isolation with one device - supporting the structure, for base isolation, providing elastic restoring force and required amount of damping up to a maximum of 10-15% of critical. Moderate damping is achieved with this type of the bearing.

HDRB isolation bearings are vertically stiff, capable of supporting vertical gravity loads, while being laterally flexible, capable of allowing large horizontal displacements. In effect, the ground is allowed to move back and forth under a base isolated during an earthquake, while leaving the building to remain "stationary." By means of its flexibility and energy absorption capability, with HDRB rubber bearings base isolation system partially reflects and partially absorbs some of the earthquake input energy before this energy can be transmitted to the structure. The net effect is a reduction of energy dissipation demand on the structural system, resulting in an increase in its survivability.

Significantly reductions of structural and non-structural damage may be achieved through use of HDRB rubber bearings seismic isolation. Reduction in elastic-force reductions by factors of 5- to 10- are possible. Expressed in simple terms with regard to building performance, this is roughly equivalent to a reduction of a Richter-magnitude-8 event to an event in the 5-to-6 magnitude range. Clearly, this is a very significant reduction. These potential benefits are greatest for stiff structures fixed rigidly to the ground, such as low- and medium-rise buildings, nuclear power plants, bridges and many types of equipment. Some of the heady duty bearings supplied by us and installed in structures in Pakistan has helped to minimize damages to the structure as will as human life.

Recent disasters caused in many parts of the world have due to earthquake has made many structural designer to start using with HDRB rubber bearings base isolation systems to building and structures. We have few Projects in hand are very keen to work on such Projects and you are requested to contact us.

Lead Rubber Bearings LRB.

A lead rubber bearing is a bigger laminated bearing manufactured from layers of rubber, sandwiches together with layers of steel, except for that in middle of bearing there will be a solid lead "plug." Top and bottom of the bearing is fitted with steel plates, which are used to attach bearing to building through its foundation for base isolation.

These lead rubber bearings are designed in such a way that bearing is very stiff and strong in vertical direction, but flexible in horizontal direction. Lead, inserted as center core of bearing dissipates the energy of earthquake while the rubber, reinforced with steel plates, provides stability, supports structure and isolates vibrations. Lead plug in the middle bearing experiences same deformation as rubber. However, it also generates heat as it does so. In other words, the lead plug reduces, or dissipates, the energy of motion--i.e., kinetic energy--by converting that energy into heat and reduces energy entering the structure. Such produced heat energy, melts and weakens structure in the case of fixed base building and increases the damages occurred.

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