Essay: SEISMIC DAMPERS

3.3.1 Introduction:
Another approach for controlling seismic damage in buildings and improving their seismic performance is by installing seismic dampers in place of structural elements, such as diagonal braces. These dampers act like the hydraulic shock absorbers in cars — much of the sudden jerks are absorbed in the hydraulic fluids and only little is transmitted above to the chassis of the car. When seismic energy is transmitted through them, dampers absorb part of it, and thus damp the motion of the building. Dampers were used since 1960s to protect tall buildings against wind effects. However, it was only since 1990s, that they were used to protect buildings against earthquake effects.
Energy dissipation by dampers help in overall reduction in displacements of the structure. This technique is most effective in structures that are relatively flexible and have some inelastic deformation capacity. In recent years, efforts have been undertaken to develop the concept of energy dissipation or supplemental damping into a workable technology and a number of these devices have been installed in structures throughout the world. In general, they are characterized by the capability to enhance energy dissipation in the structural systems in which they are installed. This may be achieved either by conversion of kinetic energy to heat, or by transferring of energy among vibrating modes.
Fig. 3.8 One of the first dampers used in structural engineering (Taylor, 2003)
This energy is applied in two types of kinetic and potential (strain) to structure and it is absorbed or amortized. If structure is free of damping, its vibration will be continuously, but due to the material damping, vibration is reduced. Input energy caused by earthquake to structure is presented in the following equation:
In this equation, E is earthquake input energy, Ek is kinetic energy, Es is reversible strain energy in the elastic range and Eh is the amount of wasted energy due to inelastic deformation and Ed is the amount of amortized energy by additional damper.
The energy dissipating devices are dampers which transform earthquake energy into heat which is dissipated into the structure. A wide range of energy dissipation devices have been developed
and are now being installed in real buildings. Energy dissipation devices are also often called damping devices. The various types of dampers are described below.
3.3.2 Types of dampers:
The commonly used types of seismic dampers are as follows:
1. Metallic yield damper:
In this type of damper the seismic energy is absorbed by yielding of mild steel.
2. Friction damper:
In this type of damper the seismic energy is absorbed by the surfaces with friction between them which are rubbing against each other.
3. Viscous fluid damper:
In this type of damper the energy is absorbed by silicon-based fluid which is passed between piston cylinder arrangements.
4. Viscoelasticdamper:
In this type of damper the energy is absorbed by viscoelastic action in polymeric materials-controlled shearing of solids.
Fig. 3.9 Seismic Energy Dissipation Devices
1. Metallic Yield Damper :
It is one of the effective mechanisms available for the dissipation of energy, input to a structure from an earthquake is through in elastic deformation of metals.
The idea of using metallic energy dissipaters within a structure to absorb a large portion of the seismic energy began with the conceptual and experimental work of Kelly et al. (1972) and Skinner et al. (1975). Several of the devices considered include torsional beams, flexural beams, and V-strip energy dissipaters. Many of these devices use mild steel plates with triangular or hourglass shapes so that yielding is spread almost uniformly throughout the material. A typical X-shaped plate damper or added damping and stiffness (ADAS) device is shown in Fig. 3.10.
Fig. 3.10 X-shaped ADAS Device
In this damper, transferred energy to the structure is spent to submission and non-linear behavior in used element in damper. In these dampers, metal inelastic deformation is used such as for formability metals such as steel and lead for energy dissipation. In all conventional structures, energy dissipation is based on deformation of steel members after the submission.
X-shaped metal dampers have a significant performance. Massive submission on steel volume, providing Hysteretic damping and extraordinary energy dissipation are unique features of this type of damper. These dampers have a high lateral stiffness, in addition to providing damping. So, they were entitled as Added Damping and Stiffness (ADAS).
These dampers are installed between head chevron tracings and floor beams. And by good connections, these dampers can be installed in concrete frames.
2. Friction Damper :
Fig. 3.11 Pall Friction Damper
Friction provides another excellent mechanism for energy dissipation, and has been used for many years in automotive brakes to dissipate kinetic energy of motion. In the development of friction dampers, it is important to minimize stick-slip phenomena to avoid introducing high frequency excitation. Furthermore, compatible materials must be employed to maintain a consistent coefficient of friction over the intended life of the device.
In this type of damper, seismic energy is spent in overcoming friction in the contact surfaces. In such systems the friction surfaces are clamped with prestressing bolts. The characteristic feature of this system is that almost perfect rectangular hysteretic behavior is exhibited. In this system the amount of energy dissipated is proportional to displacement. These dampers are installed in parallel to bracing.
Another type of friction damper is Pall friction damper. This damper includes a bracing and some steel plate with friction screws. And they should be installed in the middle of bracing. Steel sheets are connected to each other by high strength bolts and they have a slip by a certain force, to each other.
3. Viscous Fluid Damper :
In this damper, by using viscous fluid inside a cylinder, energy is dissipated. Viscous fluid dampers are used with the objective of permitting slowly developing displacements due to thermal movements, but limiting the response under dynamic actions.
Fig. 3.12 Taylor device fluid damper
These systems dissipate energy by forcing a fluid through an orifice similar to the shock absorbers of an automobile. They may be constituted of a piston moving in every direction in very viscous elastomers like silicon or bitumen. Viscous fluid dampers, are widely used in aerospace and military applications, and have recently been adapted for structural applications. Due to ease of installation, adaptability and coordination with other members also diversity in their sizes, viscous dampers have many applications in designing and retrofitting.
This type of dampers are connected to the structure in three ways:
• damper installation in the floor or foundation
• connecting dampers in stern pericardial braces
• damper installation in diagonal braces.
In connecting dampers on the floor or foundation of structures, we can use a combination of dampers with isolators.
4. Viscoelastic Damper:
The metallic and frictional devices described are primarily intended for seismic application. But, viscoelastic dampers find application in both wind and seismic application. A typical viscoelastic damper, developed by the ‘3M Company’, formerly known as the ‘Minnesota Mining and Manufacturing Company’, is an American multinational conglomerate corporation, is shown in Fig. 3.13. It consists of viscoelastic layers bonded with steel plates.
Fig. 3.13 Viscoelastic Damper
Viscoelastic dampers were first used structurally in the World Trade Centre buildings in New York in 1973. Ten thousand viscoelastic dampers were installed so as to limit the sway in the top floors to about 3 feet. The viscoelastic material absorbed the energy by being slightly displaced and then returning to its original position. These dampers have since been used in a number of tall structures as an energy dissipating system, and are often used for structural retrofits due to their ability to easily be incorporated into an existing structure.
Further studies on the dynamic response of viscoelastic dampers have been carried out, and the results show that they can also be effectively used in reducing structural response due to large range of intensity levels of earthquake. Viscoelastic materials used in civil engineering structure are typical copolymers or glassy substances. The materials are usually bonded to steel and dissipate energy when sheared. Stiffness properties of some visco-elastic materials are temperature and frequency dependent.
3.3.3 Tuned Mass Damper (TMD):
A tuned mass damper, also known as a harmonic absorber, is a device mounted in structures to reduce the amplitude of mechanical vibrations. Their application can prevent discomfort, damage, or outright structural failure.
Fig. 3.14 Tuned Mass Damper
Tuned mass dampers stabilize against violent motion caused by harmonic vibration. A tuned damper reduces the vibration of a system with a comparatively lightweight component so that the worst-case vibrations are less intense.TMD is attached to a structure in order to reduce the dynamic response of the structure. The frequency of the damper is tuned to a particular structural frequency so that when that frequency is excited, the damper will resonate out of phase with the structural motion.
Tuned mass dampers (TMD) have been widely used for vibration control in mechanical engineering systems. In recent years, TMD theory has been adopted to reduce vibrations of tall buildings and other civil engineering structures. Dynamic absorbers and tuned mass dampers are the realizations of tuned absorbers and tuned dampers for structural vibration control applications. The inertial, resilient, and dissipative elements in such devices are: mass, spring and dashpot (or material damping) for linear applications and their rotary counterparts in rotational applications. Depending on the application, these devices are sized from a few ounces (grams) to many tons.
Working principle of TMD:
TMD works on by combining of two main principles it works on pendulum principle and counter acting movement (energy). Lightly damped structures may develop large amplitude vibrations for loads acting near the resonance frequency. These vibrations may be reduced by attaching a secondary mass through a suitably selected spring and damper .The ‘tuning’ of the spring and damper to produce optimal reduction is an important feature, and the device is therefore called a tuned mass damper.
The figure below shows a TMD attached to the structure to be damped. The terms m1, K1, C1 and X1 represent the mass, stiffness, damping and displacement of the structure respectively. The terms m2, K2, C2, X2 represent the mass, stiffness, damping and displacement of the TMD respectively. F(t) represents the excitation force acting on the structure and f(t) represents the force on the TMD.
Fig. 3.15 A schematic representation of a TMD system
A TMD consists of a mass mounted on a structure via a spring system. The spring and mass are “tuned” so as to have a natural frequency close to that of the primary structure. When properly tuned, the TMD mass oscillates in the opposite direction from the primary structure.
The motion of the mass relative to the main structure can be very large when the system is properly tuned and this provide opportunity to dissipate a large amount of energy in the damper linking the mass to the main structure.
3.3.4 Examples:
Following are some examples of buildings in which dampers are used.
1.Torre Mayor (Mexico City, Mexico):
Fig. 3.16 Torre Mayor Building Mexico
The new 57-story Torre Mayor building is the dominant structure in the Mexico City skyline. It is also the first tall building to utilize large Fluid Viscous Dampers as a primary means of seismic energy dissipation. Total of 98 dampers are used, including 24 large dampers, each rated at 570 tonnes of output force, located in the long walls of the building. The short walls utilize 74 smaller dampers, each rated at 280 tonnes of output force. Dampers are installed in mega-brace elements, up to 20m in length, where a single damper spans up to six floors. The damping technology successfully implemented for Torre Mayor is now being used on five other tall buildings, including three in the USA, and two in Japan. A total of one hundred and thirty structures throughout the world utilize Fluid Viscous Dampers for earthquake, hurricane, and typhoon protection.
2. The Arrowhead Regional Medical Center at Colton, California:
Fig. 3.17 Arrowhead Regional Medical Center, California
This project was the first application for fluid viscous dampers in the seismic protection field. The five buildings of this complex use a total of 186 dampers, each being rated at 145 tonnes force. The dampers are used to dissipate seismic energy, and are installed in systems parallel with rubber base isolation bearings. The 79.000 square meter medical center is located in San Bernardino County, between the cities of Ontario and San Bernardino. The location is within 8 km of the San Andreas Fault, and 10 km of an intersecting fault. The location of the medical center was determined by available Federal Government funding, provided with a requirement that the hospital complex be located between the two cities noted, and with a very easy access.
3. Taipei 101 (Taipei, Taiwan):
(a)Taipei Tower 101 (b) Tuned mass damper (TMD) situated in the Taipei
Fig. 3.18 Taipei 101 building, Taiwan
The Taipei 101, Figure 3.18, contains the world’s most famous example of a TMD. The building sits just 200m from a major fault line in the city of Taipei, making it susceptible to seismic events as well as high winds and typhoons. For this reason, it was decided that a TMD, in the form of a 730 ton, giant circular pendulum was required. The sphere, which is made up of steel plates welded together, sits between the 87th and 92nd floors and sways to reduce movements in the building by 30 to 40 percent. The structure is designed so as to withstand gale force winds of 60m/s and an earthquake with a 2500 year return period.
FUTURE WORK
 Use another different earthquake resistant techniques to resist seismic force and apply any one technique on structure.
 Prepare prototype of a model as per the design at required time.
 Prepare the model in required time to achieve the highest accuracy and performance.
 Complete the model in budget.
CONCLUSION
There is an optimum scope for Earthquake Resistant Techniques to protect each and every structure. Efforts are required to find the solutions for the situations like near fault regions where wide variety of earthquake motions may occur. Further research work can lead to more safe and economical structures which will protect our mankind as well as valuable property.
REFERENCES
1) Murty, C.V.R., 2004, IITK-BMTPC, Earthquake Tips, New Delhi.
2) Duggal, S.K., 2007, Earthquake Resistant Design of Structures, Oxford university press, New Delhi.
3) Varyani, U.H., 2002, Structural Design of Multi-storied buildings, South Asian publishers, New Delhi, second edition.
4) “A Study on Earthquake Resistant Construction Techniques”, AJER, e-ISSN: 2320-0847 & p-ISSN: 2320-0936, Volume-02, Issue-12, pp-258-264, (2013).
5) Alex Y. Tuan and G. Q. Shang, “Vibration Control in a 101-StoreyBuilding Using a Tuned Mass Damper, Journal of Applied Science and Engineering, Vol. 17, No. 2, pp. 141-156 (May 2014).
6) Douglas P. Taylor, “Fluid dampers for applications of seismic energy dissipation and seismic isolation”, Paper No. 798, Eleventh World Conference on Earthquake Engineering, (June 1996)
7) Alireza Heysami, “Types of dampers and their seismicperformance during an earthquake”, Current World Environment, Vol. 10(Special Issue 1), 1002-1015, (April 2015).