Base Isolation (Seismic Rubber Bearing Pads)
In recent years base isolation has become an increasingly applied structural design technique for buildings and bridges in highly seismic prone areas. Many types of structures have been built using this approach, incorporating base isolation and many others are in design phase or under construction. Most of the completed buildings and those under construction use rubber base isolation bearings in some way in the isolation system. We at http://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 and our home page and connected links provide information on a wide range of rubber products on base isolation too.
Recently we have seen that conventionally constructed buildings have been seen to be crumbled under tremedus destructive strength of earthquake which runs as high as 7.0 on Richter scale in many parts of the world. The conventional approach to earthquake resistant design of buildings have seen to be followed all over the world, 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 the selection of an appropriate structural configuration and the careful detailing of structural members, such as beams and columns, and the connections between them. They are all proved to be failing and seismic activities have been damaging human lives and properties all over the world.
There has been many experiments and designs been followed with the concept of base isolation for the past 30 odd years and at many places they seem to be out performing the conventional fixed base isolation with conventional structure strengthens approaches.
The concept of base isolation are quite simple.
First let us explain a fixed base building and base isolated building which is clear from the following figures.
Here we notice that the building with base isolation is safer than the conventional base structure.
We notice that large rubber bearings are used to connect the structure and base of the building isolating the structure and its movemetns from foundation. A variety of different types of base isolation bearing pads have now been developed, and a base isolated structure is supported by a series of bearing pads which are placed between the building and the building’s foundation, where as fixed base buildings are everyday conventional foundations .
There are two basic types of isolation systems. The system that has been adopted most widely in recent years is typified by the use of elastomeric bearings, the elastomer made of either natural rubber or neoprene. In this approach, the building or structure is decoupled from the horizontal components of the earthquake ground motion by interposing a layer with low horizontal stiffness between the structure and the foundation.
The bearings are made by vulcanization bonding of sheets of rubber to thin steel reinforcing plates. Because the bearings are very stiff in the vertical direction and very flexible in the horizontal direction, under seismic loading the bearing layer isolates the building from the horizontal components of the ground movement while the vertical components are transmitted to the structure relatively unchanged. Although vertical accelerations do not affect most buildings, the bearings also isolate the 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.
This layer gives the structure a fundamental frequency that is much lower than its fixed-base frequency and also much lower than the predominant frequencies of the ground motion. The first dynamic mode of the isolated structure involves deformation only in the isolation system, the structure above being to all intents and purposes rigid. The higher modes that will produce deformation in the structure are orthogonal to the first mode and consequently also to the ground motion. These higher modes do not participate in the motion, so that if there is high energy in the ground motion at these higher frequencies, this energy cannot be transmitted into the 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 undamped; however, some damping is beneficial to suppress any possible resonance at the isolation frequency.
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.
Among most important advanced techniques of earthquake resistant design and construction are:
Energy Dissipation Devices Active Control Systems
It is easiest to see this principle at work by referring directly to the most widely used of these advanced techniques, which is known as base isolation.
Lead-rubber bearings and High density Rubber Bearings.
These are among 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 bearing is a 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
To get a basic idea of how base isolation works, first examine Figure 2 This shows an earthquake acting on both a base isolated building and a conventional, fixed-base, building. As a result of an earthquake, the ground beneath each building begins to move. In Figure 3, it is shown moving to the left. Each building responds with movement which tends toward the right. We say that the building undergoes displacement towards the right. The building’s displacement in direction opposite the 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.
It is important to know that the 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. So, Figure 3 is really a kind of “snapshot” of the building at only one particular point of its earthquake response.
Deformation and Damages
In addition to displacing toward the 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 acting upon it.
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, when the very heavy building containing them suddenly starts to move very quickly–the nails pull out and the connection fails.
Response of Base Isolated Building.
By contrast, even though it too is displacing, the base-isolated building retains its original, rectangular shape. It is the lead-rubber bearings supporting building that are deformed. The base-isolated building itself escapes deformation and damage–which implies that inertial forces acting on the base-isolated building have been reduced. Experiments and observations of base-isolated buildings in earthquakes have been shown to reduce building accelerations to as little as 1/4 of the acceleration of comparable fixed-base buildings, which each building undergoes as a percentage of gravity. As we noted above, inertial forces increase, and decrease, proportionally as acceleration increases or decreases.
Acceleration is decreased because the base isolation system lengthens a building’s period of vibration, the time it takes for the 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.
Finally, since they are highly elastic, the rubber isolation bearings don’t suffer any 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–IEEE, kinetic energy–by converting that energy into heat. And by reducing the energy entering the building, it helps to slow and eventually stop the 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.
Buildings that can expect to benefit from the effects of base isolation.