01-10-2012, 01:34 PM
Technique of Earthquake Resistant Engineering.
Technique of Earthquake.pptx (Size: 2.94 MB / Downloads: 154)
INTRODUCTION
The purpose of this seminar is to provide a brief overview of many new technologies that are rapidly becoming more prevalent in the seismic design of building structures.
These all technologies involve the use of special details or specific devices to alter or control the dynamic behavior of buildings.
These technologies are categorized as Seismic Base Isolation, Energy Dissipation Devices, Active Control Devices.
Base isolation is a well known and widely used technique in seismic design.
The isolating system absorbs part of the earthquake energy before it is transferred to the structure.
large displacement appear at the base isolation level. In order to decrease these displacements and to yield additional improvement in structural seismic response, dampers are added to base isolators.
Addition of viscous dampers to base isolation systems (BIS) reduces the base displacement at the expanse of increasing floor accelerations and inter-story drifts to overcome it, active and semi-active dampers were used.
Seismic Base Isolation Technique for Earthquake Resistance Building
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. A base isolated structure is supported by a series of bearing pads which are placed between the building and the building’s foundation.
A variety of different types of base isolation bearing pads have now been developed. For our example, Plain and Laminated elastomeric bridge bearings, ptfe- Teflon sliding bearings, and lead-rubber bearing.we’ll discuss lead–rubber bearings.
Lead-rubber 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.
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.
How it Works:
To get a basic idea of how base isolation works, first examine the above diagram.
As a result of an earthquake, the ground beneath each building begins to move.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 deforming.
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.
.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.
.Energy Dissipation Devices for Earthquake Resistant Building Design:
The second of the major new techniques for improving the earthquake resistance of buildings also relies upon damping and energy dissipation, but it greatly extends the damping and energy dissipation provided by lead–rubber bearings.
As we’ve said, a certain amount of vibration energy is transferred to the building by earthquake ground motion. Buildings themselves do possess an inherent ability to dissipate, or damp, this energy.
However, the capacity of buildings to dissipate energy before they begin to suffer deformation and damage is quite limited.
The building will dissipate energy either by undergoing large scale movement or sustaining increased internal strains in elements such as the building’s columns and beams.
Commonly used Seismic Dampers:
Viscous Dampers (energy is absorbed by silicone-based fluid passing between piston cylinder arrangement)
Friction Dampers (energy is absorbed by surfaces with friction between them rubbing against each other),
Yielding Dampers (energy is absorbed by metallic components that yield).
Viscoelastic Dampers (energy is absorbed by utilizing the controlled shearing of solids).