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 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 (fig 1b), then some effect of the ground shaking will be transferred to the building above. 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 (fig 1c). 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. Also, 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.
Lead-rubber bearings 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.
Seismic Isolation absorbs the shock and reduces the stiffness as well as damping in the structure It is most effective for low to medium rise structure. Instructional Material Complementing FEMA 451, Design Examples Seismic Isolation 15 -7-19 Types of Seismic Isolation Bearings Elastomeric Bearings - Low-Damping Natural or Synthetic Rubber Bearing - High-Damping Natural Rubber Bearing - Lead-Rubber Bearing (Low damping natural rubber with lead core) Sliding Bearings - Flat Sliding Bearing.
How it Works
To get a basic idea of how base isolation works, first examine the above diagram. This shows an earthquake acting on base isolated building and a conventional, fixed-base, 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.
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.
Spherical Sliding Base Isolation
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
NEW SEISMIC ISOLATION TECHNOLOGY
The preservation of ancient buildings (churches, palaces and so on) having a cultural or historic value is a very complex problem. The difficulties do not depend on the design of the technical solutions, but on the specific requirements that govern this specific subject in Italy. In accordance with the philological criterion, very restricted requirements must be fulfilled to design the rehabilitation works of historic buildings. This trend derives from a strong popular willingness to wish to preserve our historical traditions. Mainly, it is strongly recommended that the rehabilitation works be 'non-invasive'. The former limitation does not allow, for example, that the building could be strengthened by the use of additional structural elements. Both the requirements limit the extensive use of new materials to strengthen the existing structural elements. In any case, the existing architectural morphology, the structural configuration and even the original materials should be preserved and they cannot undergo significant modifications.
Seismic Isolation Devices For Sale
For instance, often the floors at ground level are irregularly connected over a large area and sometimes they rest directly on the soil. Therefore, the application of the current base isolation system requires the construction of a new floor and a new foundation.
More than the technical problems, the use of the current seismic isolation devices is severely limited by suspicion and mistrust due to the novelty of the system even in the field of new constructions.
This attitude is obviously reflected in the sphere of cultural heritage [Parducci A., 1999].
ADVANCE UNDERGROUND INSTALLATION OF THE JETMINE SEISMIC ISOLATORS
Seismic Base Isolation
The JETMINE seismic isolation is a simple way to reduce both the seismic stresses on the structural elements and withstand the horizontal accelerations transmitted to the buildings.
During the installation of the seismic isolator slabs, no structural elements of the building are altered, no any of the walls are modified and the elaborate stone floors are remaining intact. The installation is performed well below the surface with limited equipment visible above the ground. As a result of this installation, the historic building will preserve its original features and will remain like that for centuries and the future generations can enjoy its beauty.
Unique abilities of the cutter head during the installation of the seismic isolator slabs is to remove the ground ahead of the slab in the horizontal direction. The cutter head is pushed in a horizontal direction under the force of the hydraulic devices, that assembled on the one side, with following subsections of the isolator slabs. In this direction, the cutter head will slice through the underground formations and all loose material is sucked out of the way and make space for slabs that following immediately behind. This installation process will have to be carried out in stages until the whole ground underside of the building is separated.
(TECHNICAL SPECIFICATIONS OF THE SLABS CAN BE SUPPLIED ON REQUEST) Ultraedit old versions.
Seismic Isolation Bearing
This performance would pave the way toward the use of the seismic isolation to protect the buildings containing high-cost equipment (for example, computer centres). Likewise, seismic isolation can be applied to reduce the seismic accelerations of the buildings containing artistic or historical works.