The conventional approach to design earthquake resistant building is based upon equipping the building with stiffness, strength and deformation due to elasticity which can afford a given level of earthquake-generated forces. This is mostly done selecting an appropriate structural configuration and careful detailing of the parts of the structure, such as beam and columns and the connections between them.
On the contrary, the basic approach to look for advanced techniques for earthquake resistant should not be to strengthen the building itself, but to reduce the magnitude of forces generated by the earthquakes. The most important advanced techniques of designing and constructing earthquake resistant buildings are:
(i) Base isolation
(ii) Energy dissipation devices
(iii) Active control systems
184.108.40.206 Base Isolation
This method is used most extensively to resist earthquake damages. A base isolated structure is supported by a series of bearing pads placed between the building and the foundation of the building. Nowadays, a verity of base isolation pads is available, including ones which are known as lead-rubber bearings.
A lead-rubber bearing is made from sandwiching of layers of rubbers with layers of steel. There is a solid lead "plug" in the middle of the bearing. On the top and bottom of the bearing steel plates are fitted which are used to attach the bearing to the building and its foundation. This bearing is very stiff and strong in the vertical direction but equally flexible in the horizontal direction.
As a result of an earthquake, the ground beneath each building begins to move (as shown in the figure to the left direction). Resultantly, each building undergoes displacement towards the right direction due to inertia. The unisolated building also changes its shape into more of a parallelogram from a rectangular shape together with the displacement to the right. This is known as the process of deforming and in fact the primary cause to damages due to earthquakes to the building is this deformation which the building undergoes as a result of the force of inertia acting on it.
The building with an isolated base is also experiencing displacement but it is able to retain its original rectangular shape. Only the lead-rubber bearings supporting the building are deformed.
The building with an isolated base avoided the deformity and damages showing that the forces of inertia acting on the basic-isolated building have shown remarkable reduction in building acceleration to as low as one-fourth of the fixed-base buildings.
Being highly elastic, the rubber isolation does not suffer any damage. The lead plug on the other hand experiences the same deformation as the rubber, but it also generates heat. Therefore, the lead plug dissipates the energy of motion (kinetic energy) by converting it into the heat energy. Thus, by reducing the energy that enters the building, it helps in slowing and finally stopping the vibrations experienced by the building.
There are other types of basic isolation known as spherical sliding isolation systems. In this type of base isolation, the building is supported by bearing pads having curved surface and low friction.
When an earthquake strikes, the building is free to slide on the bearings. As the bearings have a curved surface, the building slides horizontally as well as vertically. The force needed to move the building up words restricts horizontal or lateral forces which would otherwise cause deformities to the building. At the same time, by adjusting the radius of the bearing's curved surface, this property can be used to design bearings that have the capacity to lengthen the time period of the vibration of the building.
220.127.116.11 Energy dissipation devices
The second major new technique to improve resistance against earthquakes of building is also dependent upon damping and energy dissipation, but it exceeds the amount of the damping and energy dissipation provided by lead-rubber bearings.
As already mentioned, a certain amount of energy due to vibrations is transferred to the building by the ground motion due to an earthquake, buildings themselves have an inherent capability to dissipate or damp this energy. But this capacity of the building to dissipate or damp the energy is limited before suffering any deformations or damages.
The building can dissipate energy either by experiencing great movement or sustaining increased internal strains in the columns and beams. Both of these reactions result in varying levels of damages. Therefore, the seismic energy entering the building could be largely decreased by providing the building with additional devices that have high capacity of damping the energy, thereby decreasing the probability of damages to the building. For this purpose a variety of energy dissipation devices have been developed and are now being installed in the buildings. These devices are also known as damping devices. These devices can be classified into following categories:
(i) Friction dampers: these utilize frictional forces to dissipate energy.
(ii) Metallic dampers: utilize the deformation of metal elements within the damper.
(iii) Viscoelastic damper: utilize the controlled shearing of solids.
(iv) Viscous dampers: utilize the forced movement of fluids within the damper.
In this type of dampers, the mechanism of frication between solids developed when two solid bodies slide relative to each other is utilized to achieve the desired levels of energy dissipation. During development of these devices, it is necessary to minimize the stick and slip phenomenon to avoid occurrence of high frequency excitations. Materials used in the production of these devices must be compatible to maintain a constant coefficient of friction over the presumed life of the devices. An example of these devices which can be installed in a structure in an X-braced frame is the pall devices as shown in the figure (5.22 (a) and (b)).
In the most of the friction dampers, sliding interfaces of steel on steel, brass on steel, or graphite impregnated bronze on stainless steel are utilized. These dampers are designed in such a manner that they do not slip during the windstorms or earthquake of moderate intensity. But when a severe earthquake strikes, the devices slip at a predetermined optimum load before any yielding takes place in the primary structural members. During recent year, structural application of friction dampers to provide increased protection against seismic activities has been experimented with in the new and retrofitted buildings. When compared with the unbraced and conventionally braced frames, it has been found that friction dampers reduce displacement effectively while maintaining comparable acceleration levels.
It is a very effective and reliable mechanism for the dissipation of the energy generated by an earthquake before it enters the building through inelastic deformation of metals. Mild steel plates with triangular or x-shapes are used in a number of these types of devices so that yielding of metal can spread throughout the material. The schematic diagram of a typical x-shaped added damping stiffeners (ADAS) device is shown in the figure below.
These devices have the quality of stable hysteresis behavior, low cycle fatigue, long-term reliability and relative insensitivity to environmental temperature. In spite fo different geometrical configurations of these devices, dissipation of energy takes place through inelastic deformation of the metal.
Before utilization of metallic dampers within a structure, design guidelines and knowledge acquired on the basis of theoretical and experimental studies are very important.
While the friction and metallic dampers are intended for seismic isolation, the viscoelastic dampers can be utilized for seismic as well as wind protection of structures. The dynamic response of viscoelastic dampers and the seismic response of viscoelastically damped structures have demonstrated the efficiency of these dampers in reducing seismic structural responses over varying intensity levels of earthquake ground motions.
The materials used in these dampers for structural applications are generally copolymers or glossy substances which dissipate energy through shear deformation. A particular viscoelastic damper consisting of viscoelastic layer bonded with steel plates is shown in the figure 5.24 (a) and (b).
While installed in a structure, shear deformation and consequently dissipation of energy occurs when the structural vibrations produce relative motion between the outer steel flanges and the central plates. The role of viscoelastic devices under dynamic loading depends upon the frequency of vibrations, strain and ambient temperature.
Experiments have shown that the viscoelastic dampers are effective in reducing the inelastic ductility demand of the experimental structural during strong earthquake ground motions.
Recent experiments and analyses have demonstrated the efficiency of viscoelastic dampers when applied to both steel and reinforced concrete structures during earthquake of varying intensities. In comparison to steel structures, seismic responses of reinforced concrete structures under very strong excitations are inelastic and are often accompanied by permanent deformations and damages. If viscoelastic dampers are used in the reinforced concrete structures, dissipation of energy can be achieved at the early stages of cracking and can reduce the further damages significantly.
In these dampers viscous fluids are utilized to dissipate energy generated during an earthquake. A large number of such devices have been developed and used in the structures, for example viscous wall, taylor viscous fluid dampers etc. the characteristic equality of these devices that are of utility in structural engineering are the linear regions response over a large range of frequencies, insensitivity to temperature and compactness. The viscous nature of these devices is obtained by the use of specially designed orifices, which are responsible for generating damper forces that are out of phase with displacement.
A viscous damper consists of a piston in the damper housing filled with a compound of silicone or oil. Dissipation of energy is achieved through the movement of the piston in the highly viscous fluid. The fluid being ideally viscous, the output damper force is directly proportional to the velocity of the piston.
18.104.22.168 Active control systems
During the most recent years, considerable attention has been paid to the field of research and development where motion of a structure is controlled or modified by using control devices by supplying some external energy, thus, reducing the impact of forces acting on the structures and all structural responses like floor acceleration, velocities and displacements etc. Excitations to the structure might be internal or external, such as machinery or traffic noises, wind, earthquake etc. where safety and comfort levels of the occupants are affected. The active control systems have the power to react to the changes in structures and to control the vibrations of the structural system. Different devices, such as active mass dampers (AMD), hybrid mass dampers (HMD), active variable stiffness system, tendon control etc. are utilized for this purpose.
The basic feature of all the active control devices is that an external power is utilized to trigger the controlling action. Presently, the active control systems are in the developmental phases as is the testing for their applicability in the structures on account of a number of factors including the following:
(i) With the development of innovative materials and new construction methods, structures are growing taller, longer and more flexible. The application of the active control systems is among one of the measures to safeguard such structures against earthquakes as well as any wind induced vibrations.
(ii) These devices can be of great help in retrofitting or strengthening the existing structures against threats due to earthquakes. The passive measures of using interior shear walls or base isolation systems are invasive to the structures. Active control systems, on the other hand, can be more effective and can be accommodated into the existing structures with less alteration.
(iii) In the eventuality of reverse loading, active control systems may be the only measures to avoid collapse of structures.
(iv) If any valuable and sensitive equipment are in a structure, active control system can be applied at the sub structural level to ensure proper operating conditions for such equipments.
(v) Though the passive devices like isolating systems, viscoelastic dampers etc. have been utilized in some existing structures with good results in the structural performance, however the passive devices have their inherent limitations.
(vi) The advent of active control systems itself is not only attractive but potentially revolutionary, as it takes ahead the structural concepts from a static and passive level to one of dynamism and adaptability.
Active control systems have the potential to adapt to different loading conditions and to control vibration modes of the structures.
It comprises of sensors to measure external excitations, devices to process the measured information and to compute necessary control forces needed based on a given control algorithm, and actuators powered by external sources of energy, to produce the required control forces. In these systems, the signals sent to control actuators are a function of responses of the system measured with physical sensors. The basic objective for an active control system designer is to determine a control strategy that the measured structural responses to calculate appropriate control signals to be sent to actuators. Different control strategies have been proposed and investigated, for example robust control sliding mode control, Adaptive control, neural network control nonlinear control, mode predictive control etc.
It is important to remember that, since each control method has its own advantages and disadvantages, the design approach for each control method is different. Applicability of a given method depends on the characteristics of the controlled target and desired performance adjectives.
Some types of active control systems are discussed below:
(i) Tendon control :
In this type of active control system a set of prestressed tendons is connected to a structure whose tensions are controlled by electrohydraulic servo-mechanisms. Many existing structures already have tendons, so active tendon control may utilize these tendos. This system is especially attractive for retrofitting or strengthening of existing structures.
(ii) Active mass dampers :
In this device a mass spring damper system is used. Additionally, it has an actuator which is used to position the mass at each instant to increase the amount of damping achieved and the operational frequency range of the device.
(iii) Hybrid control devices :
These devices were innovated to overcome the weaknesses of passive of passive system, that is, its inability to respond to load applied suddenly like earthquakes and wind actions. These devices are the combination of a passive tuned mass damper (TMD) an active control actuator, such that the energy required to operate a typical hybrid mass damper (HMD) is much less than those for a fully active mass damper (AMD) system of comparable performance. The force from the control actuator are used to increase the efficiency of the HMD and to increase it robustness to change in the dynamic features of the structure during the strong winds or an earthquake of moderate intensity, the structure's first mode of vibration can be considered as dominant, the control will act as a device, but in the event of a stronge earthquake, the actuator is activated.