CHAPTER 1 INTRODUCTION 1.1 PASSIVE ENERGY DISSIPATION SYSTEM
Passive energy dissipation systems encompass a range of materials and devices for enhancing damping, stiffness and strength, and can be used both for seismic hazard mitigation and for rehabilitation of aging or deficient structures. In general, such systems are characterized by their capability to enhance energy dissipation in the structural systems in which they are installed. 1.1.1 Principles of Operation
These devices generally operate on principles such as frictional sliding, yielding of metals, phase transformation in metals, deformation of viscoelastic (VE) solids or fluids, fluid orificing and sloshing. 1.1.2 Basic Function
The basic function of passive energy dissipation devices when incorporated into the superstructure of a building is to absorb or consume a portion of the input energy, thereby reducing energy dissipation demand on primary structural members and minimizing possible structural damage.
Excitation
Structure
Response
Figure 1.1 Conventional Structure
PED
Excitation
Structure
Response
Figure 1.2 Structure with Passive Energy Dissipation (PED)
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1.2 CLASSIFICATION OF PASSIVE ENERGY DISSIPATION SYSTEMS
A large number of passive control systems or PED devices have been developed and installed in structures for performance enhancement under earthquake loads. A variety of passive energy dissipation devices are available and have been implemented worldwide for seismic protection of structures. Passive energy dissipation systems are classified herein in three categories as follows. 1. Rate-dependent system 2. Rate-independent system 3. Others 1.2.1 Rate-Dependent System
Rate-dependent system consists of dampers whose force output is dependent on the rate of change of displacement across the damper. The behaviour of such dampers is commonly described using various models of linear viscoelasticity. This system is also called as velocity-dependent or viscoelastic system. It may or may not impart additional stiffness to the structure. This system works on the principle of fluid orificing or deformation of viscoelastic solids. Examples: Viscoelastic Fluid dampers and Viscoelastic Solid dampers. 1.2.2 Rate-Independent System
Rate-independent systems consist of dampers whose force output is not dependent on the rate of change of displacement across the damper but rather upon the magnitude of the displacement and possibly the sign of the velocity i.e., the direction of motion. The behaviour of such dampers is commonly described using various nonlinear hysteretic models. This system is also called as displacement-dependent or hysteretic system. It always adds stiffness to the structure. This system works on the principle of yielding of metals or sliding friction. Examples: Metallic dampers and Friction dampers.
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Energy dissipation systems which cannot be classified by one of the above basic systems depicted are classified as other systems. These systems work on the various principle of operation and can be further classified as follows. 1. Re-centering System 2. Dynamic Vibration Absorbers 1.2.3 Re-centering System
This system utilizes either a preload generated by fluid pressurization or internal springs, or a phase transformation to produce a modified force-displacement response that includes a natural re-centering component. Examples: Pressurized fluid dampers, Preloaded spring-friction dampers, and Phase
transformation dampers. 1.2.4 Dynamic Vibration Absorbers
In these systems, supplemental oscillators involving mass, stiffness and damping are introduced in order to significantly enhance performance, the dynamic characteristics of the supplemental oscillators must be tuned to those of the primary structure. The objective of incorporating a dynamic vibration absorber into a structure is basically to reduce energy dissipation demand on the primary structural members under the action of external forces. The reduction, in this case, is accomplished by transferring some of the structural\vibrational energy to the absorber Examples: Tuned mass dampers and Tuned liquid dampers
Explanations on these various dampers are given in the following chapters.
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CHAPTER 2 METALLIC DAMPERS AND FRICTION DAMPERS 2.1 METALLIC DAMPERS
Metallic dampers are hysteretic systems that dissipate energy with no significant rate dependence and utilize the yielding of metals as the dissipative mechanism. The mechanism involved in energy dissipation in metallic dampers can be categorized as one form of internal friction. One of the effective mechanisms available for the dissipation of energy input to a structure from an earthquake is through inelastic deformation of metals. Many of these devices use mild steel plates with triangular or X shapes so that yielding is spread almost uniformly throughout the material. Single round hole metallic damper and double X shaped metallic damper are commonly used.
Figure 2.1 X-shaped Plate Damper
Figure 2.2 Triangular Plate Damper
The idea of utilizing supplemental metallic hysteretic dampers within the superstructure is to absorb a large portion of the seismic energy during earthquakes. The performance objectives of using metallic dampers within the superstructure are energy dissipation and strength enhancement. Other configurations of steel yielding devices include bending type of honeycomb and slit dampers and shear panel type. Two major types of metallic dampers are
Buckling-Restrained Brace (BRB) dampers
Added Damping and Stiffness (ADAS) dampers.
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2.1.1 Buckling-Restrained Brace Dampers
A BRB damper consists of a steel brace usually having low-yield strength with a cruciform cross section that is surrounded by a stiff steel tube. The region between the tube and brace is filled with a concrete-like material and a special coating is applied to the brace to prevent it from bonding to the concrete. Thus, the brace can slide with respect to the concrete-filled tube. The confinement provided by the concrete-filled tube allows the brace to be subjected to compressive loads without buckling i.e., the damper can yield in tension or compression with the tensile and compressive loads being carried entirely by the steel brace. Under compressive loads, the damper behaviour is essentially identical to its behaviour in tension. Since buckling is prevented, significant energy dissipation can occur over a cycle of motion. In many cases, BRB dampers are installed within a chevron bracing arrangement
Figure 2.3 Typical Arrangement of BRB Damper
Figure 2.4 Sectional View of BRB Damper 5
2.1.2 Added Damping and Stiffness Dampers
An ADAS damper consists of a series of steel plates wherein the bottom of the plates are attached to the top of a chevron bracing arrangement and the top of the plates are attached to the floor level above the bracing. As the floor level above deforms laterally with respect to the chevron bracing, the steel plates are subjected to a shear force. The shear forces induce bending moments over the height of the plates, with bending occurring about the weak axis of the plate cross section. The geometrical configuration of the plates is such that the bending moments produce a uniform flexural stress distribution over the height of the plates. Thus, inelastic action occurs uniformly over the full height of the plates. For example, in the case where the plates are fixed-pinned, the geometry is triangular. In the case where the plates are fixed-fixed, the geometry is an hourglass shape. To ensure that the relative deformation of the ADAS device is approximately equal to that of the story in which it is installed, the chevron bracing must be very stiff. ADAS damper will be damaged after an earthquake and may need to be replaced.
Figure 2.5 Typical Arrangement of X-plate Metallic Damper (ADAS)
The advantages and disadvantages of friction dampers are as follows. Advantages
Stable hysteretic behaviour
Long-term reliability 6
Insensitivity to environment factors like temperatur e, humidity etc.
Materials and behaviour familiar to practicing engineers
Inexpensive
Disadvantages
Devices damaged after earthquake; may require replacement Nonlinear behaviour; may require nonlinear analysis
2.2 FRICTION DAMPERS
Friction dampers are hysteretic systems that dissipate energy with no significant rate dependence and utilize the mechanism of solid friction that develops between two solid bodies sliding relative to one another to provide the desired energy dissipation. Several types of friction dampers have been developed for the purpose of improving seismic response of structures. Damping using frictional dampers is considered to be the most effective and economic solution for seismic upgrade. In late seventies, frictional dampers were developed inspired with the principle of friction brakes in automobiles. They usually consist of series of steel plates specially treated to develop most reliable friction. The plates are clamped together with high strength steel bolts. During severe seismic excitations, friction dampers slip at a predetermined optimum load before yielding occurs in other structural members and dissipate a major portion of the seismic energy. This allows the building to remain elastic or at least yielding is delayed to be available during maximum credible earthquakes. Another feature of friction damped buildings is that their natural period varies with the amplitude of vibration. Hence the phenomenon of resonance is avoided. The performance objectives are energy dissipation and strength enhancement. Nowadays, several frictional dampers are being used. They are available for tension cross bracing, single diagonal bracing and for chevron bracing. A s hort description on various types of friction dampers as follows.
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2.2.1 Slotted-Bolted Friction Damper
The slotted-bolted damper consists of steel plates that are bolted together with a specified clamping force. The clamping force is such that slip can occur at a pre-specified friction force. At the sliding interface between the steel plates, special materials are utilized to promote stable coefficients of friction.
Figure 2.6 Slotted-Bolted Friction Damper Assembly 2.2.2 Pall Cross-Bracing Friction Damper
The Pall cross-bracing friction damper consists of cross-bracing that connects in the centre to a rectangular damper. The damper is bolted to the cross-bracing. Under lateral load, the structural frame distorts such that two of the braces are subject to tension and the other two to compression. This force system causes the rectangular damper to deform into a parallelogram, dissipating energy at the bolted joints through sliding friction.
Figure 2.7 Pall Cross-Bracing Friction Damper 8
2.2.3 Sumitomo Friction Damper
Sumitomo friction damper is a cylindrical friction damper that dissipates energy via sliding friction between copper friction pads and steel. The copper pads ar e impregnated with graphite to lubricate the sliding surface and ensure a stable coefficient of friction.
Figure 2.8 Sumitomo Friction Damper 2.2.4 Energy Dissipation Restraint
The design is similar to the Sumitomo concept, since this device also includes an internal spring and wedges encased in a steel cylinder. However, there are several novel aspects of the Energy Dissipation Restraint (EDR) that combine to produce very different response characteristics.
Figure 2.9 Energy Dissipation Restraint 9
The EDR utilizes steel compression wedges and bronze friction wedges to transform the axial spring force into normal pressure acting outward on the cylinder wall. Thus, the frictional surface is formed by the interface between the bronze wedges and the steel cylinder. Internal stops are provided within the cylinder in order to create the tension and compression gaps. Consequently, unlike the Sumitomo device, the length of the internal spring can be altered during operation, providing a variable frictional slip force. The advantages and disadvantages of friction dampers are as follows. Advantages
Simple and foolproof in construction
Insensitivity to environment factors like temperature, humidity etc.,
Large energy dissipation per cycle
Compact in design and can be easily hidden within drywall partitions
Do not need regular inspection, maintenance, repair or replacement before and after the earthquake
Disadvantages
Sliding interface conditions may change with time (reliability concern)
Strong nonlinear behaviour, may excite higher modes and require nonlinear analysis
Permanent displacements if no restoring force mechanism provided
Adds Large Initial Stiffness to System
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CHAPTER 3 VISCOELATIC FLUID DAMPERS VICOELASTIC SOLID DAMPERS 3.1 VISCOELASTIC FLUID DAMPERS
Viscoelastic Fluid dampers are viscoelastic systems that dissipate energy with significant rate dependence and utilize the fluid orificing and deformation of viscoelastic fluids as the dissipative mechanism. A Viscoelastic Fluid damper generally consists of a piston within a damper housing filled with a compound of silicone or similar type of oil, and the piston may contain a number of small orifices through which the fluid may pass from one side of the piston to the other. Thus, it dissipates energy through the movement of a piston in a highly viscoelastic fluid based on the concept of fluid orificing. Viscoelastic fluid dampers are commonly installed either within chevron bracing or diagonal bracing. As the damper piston rod and piston head are stroked, fluid is forced to flow through orifices either around or through the piston head. The resulting differential in pressure across the piston head (very high pressure on the upstream side and very low pressure on the downstream side) can produce very large forces that resist the relative motion of the damper. The fluid flows at high velocities, resulting in the development of friction between fluid particles and the piston head. The friction forces give rise to energy dissipation in the form of heat. 3.1.1 Orifice Fluid Damper
It contains compressible silicone oil which is forced to flow via the action of a stainless steel piston rod with a bronze head. The head includes a fluidic control orifice design. In addition, an accumulator is provided to compensate for the change in volume due to rod positioning. Alternatively, the device may be designed with a run-through piston rod to prevent volume changes. High strength seals are required to maintain closure over the design life of the damper. These uniaxial devices, which were originally developed for military and harsh industrial environments, have recently found application in seismic base isolation systems as well as for supplemental damping during seismic and wind-induced vibration.
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Figure 3.1 Orifice Fluid Damper 3.1.2 Viscous Damping Wall
In this design, the piston is simply a steel plate constrained to move in its plane within a narrow rectangular steel container filled with a viscous fluid. For typical installation in a frame bay, the piston is attached to the upper floor, while the container is fixed to the lower floor. Relative inter-story motion shears the fluid and thus provides energy dissipation.
Figure 3.2 Viscous Damping Wall 3.1.3 Types of Arrangements
The Viscoelastic fluid dampers can be installed by any of the following arrangements 3.1.3.1 Chevron Bracing Arrangement
The chevron bracing arrangement is attractive since the full capacity of the damper is utilized to resist lateral motion. However, the bracing is subjected primarily to axial forces and thus, to be effective, the bracing must have high axial stiffness. Excessive flexibility in the brace reduces the effectiveness of the damper. Note that, in many installations, the chevron bracing arrangement is inverted such that damper is located near the floor rather than near the ceiling. This facilitates installation and future inspection of the dampers.
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Figure 3.3 Chevron Bracing Arrangement 3.1.3.2 Diagonal Bracing Arrangement
The diagonal bracing arrangement may be less effective since only a component of the damper force (the damper axial force multiplied by the square of the cosine of the angle of inclination) resists lateral motion. However, the bracing is subjected only to axial forces and thus is inherently stiff. As an example, for a damper inclined at 45 degrees, the damper effectiveness is reduced by 50% due to the inclination. For convenient access, the damper is commonly installed near the bottom corner of the structural framing and is pin-connected to the framing.
Figure 3.4 Diagonal Bracing Arrangement
3.1.3.3 Toggle Bracing Arrangement
For stiff structures, the motion of the damper can be amplified via a mechanical linkage known as a toggle brace system.
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Figure 3.5 Toggle Bracing Arrangement
Viscoelastic fluid dampers generally exhibit minimal stiffness over a range of frequencies that often includes the fundamental natural frequency of building or bridge structures. Thus, such dampers generally have minimal influence on the fundamental natural frequency of the structure. The advantages and disadvantages of viscoelastic fluid dampers are as follows Advantages
High force and displacement capacity No added stiffness at lower frequencies
Moderate frequency and temperature dependency
May be able to use linear analysis
Disadvantages
Possible fluid seal leakage (reliability concern)
3.2 VISCOELASTIC SOLID DAMPERS
Viscoelastic solid dampers are viscoelastic systems that dissipate energy with significant rate dependence and utilize the shear deformation of viscoelastic solids as the dissipative mechanism. Viscoelastic solid dampers used in civil engineering structural applications are usually copolymers or glassy substances that dissipate energy when subjected to shear deformation. These dampers consist of viscoelastic layers bonded with steel plates or solid
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thermoplastic rubber sheets sandwiched between steel plates. The steel plates are attached to the structure within chevron or diagonal bracing As one end of the damper displaces with respect to the other, the viscoelastic material is sheared. The shearing action results in the development of heat which is dissipated to the environment. By their very nature, viscoelastic solids exhibit both elasticity and viscosity i.e., they are displacement and velocity dependent. 3.2.1 General Characteristics
Some characteristics of viscoelastic solid dampers are:
They have no threshold or activation force level, thus they dissipate energy for all levels of earthquake excitation and wind even while the structure remains elastic or at the early stages of cracking
They can be manufactured to add significant damping to building frames for improved structural response. The hysteretic characteristics of dampers· are functions of shear strain level, excitation frequency, damping material type, thickness and temperature.
They make a substantial contribution to the initial stiffness of the structure. While the stiffening effect may lead to better control of lateral deformations, the same stiffening may lead to larger seismically induced forces from the input ground motions.
When mounted in a structure, shear deformation and hence energy dissipation takes place when the structural vibration induces relative motion between the outer steel flanges and the centre plate.
Figure 3.6 Cross-section of Viscoelastic Solid damper
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Figure 3.7 Longitudinal-section of VE Solid damper configuration
If the viscoelastic damper material properties are not strongly frequency or temperature-dependent over the expected range of frequencies of motion, linear analysis may be used if the structure remains elastic. Nonlinear analysis is generally applicable for seismic analysis of structures with viscoelastic solid dampers since, under strong earthquakes, it may be difficult to completely eliminate inelastic response.
Figure 3.8 Viscoelastic Solid Damper of a Bracing Member Advantages
Economically feasible
Activated at low displacements
Provides restoring force
Simple modelling
Disadvantages
Lower Force and Displacement Capacity
Moderate frequency and temperature dependency 16
Possible debonding and tearing of VE material (reliability concern)
Necessity for nonlinear analysis in most
practical cases (as it has been shown that it is generally not possible to add enough damping to eliminate all inelastic response)
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CHAPTER 4 TUNED MASS DAMPERS 4.1 GENERAL
Tuned mass dampers are dynamic vibration absorbers which consist of an auxiliary mass-spring-dashpot system anchored or attached to the main structure. The mass moves relative to the structure and is attached to it by spring and viscous damper in parallel. The objective of incorporating a tuned mass damper into a structure is basically the same as that with other energy dissipation devices discussed above, to reduce the energy dissipation demand on the primary structural members under the action of external forces. This reduction is a accomplished by transferring some of the structural vibration energy to the tuned mass damper (TMD) which, in its simplest form, consists of an auxiliary mass-spring dashpot system anchored or attached to the main structure. When the structure vibrates, it excites the TMD and the kinetic energy is transferred from the structure to the structure to the TMD and is absorbed by the damping component of the device. A number of practical considerations must be observed in the engineering design of a TMD system. First and foremost is the amount of added mass that can be practically placed in the building. The TMD travel relative to the building is an important design parameter. A large movement often needs to be accommodated· for a reasonable reduction of response of the building. Another major technique associated with a sliding mass arrangement is to provide a low-friction bearing surface so that the mass can respond to the building movement at low levels of excitation. This becomes more critical when TMD functions are used as an additional damper to improve occupant comfort. Finally, cost is an issue which must be addressed in the evaluation of a TMD for a specified application
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Figure 4.1 A Building with Tuned Mass Damper
The principle of a tuned mass damper is based on the fact that by setting its natural frequency equal to one of the natural frequencies of the structure, its mass always opposes the motion of the structure and hence suppresses or reduce the structure’s vibratory motion. During an earthquake, TMD will move against the direction of main structural vibration and an inertia force will be acted on the structure to reduce the response of the structure. One TMD can attenuate only the first mode response of a structure with its frequency tuned to the fundamental frequency of the structure. First several modes of a high rise structure are primary and the anticipated response reduction cannot be achieved if only the first mode is controlled.
Figure 4.2 Modes of the Structure 19
A short description of the various types of tuned mass dampers follows. 4.1.1 Translational Tuned Mass Damper
The Configuration of a unidirectional translational tuned mass damper has mass which rests on bearings that function as rollers and allow the mass to translate laterally relative to the floor. Springs and dampers are inserted between the mass and the adjacent vertical support members, which transmit the lateral force to the floor level and then into the structural frame. Bidirectional translational dampers are configured with springs/dampers in two orthogonal directions and provide the capability for controlling structural motion in two orthogonal planes.
Figure 4.3 Translational Tuned Mass Damper 4.1.2 Pendulum Tuned Mass Damper
PTMDs replace the translational spring and damper system with a pendulum, which consists of a mass supported by a cable which pivots about a point.They are commonly modelled as a simple pendulum.
Figure 4.4 Pendulum Tuned Mass Dampers
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For small angular oscillations they will behave similarly to a translational TMD and can be modelled identically with an equivalent stiffness and equivalent damping ratio. Pendulum tuned mass damper can significantly reduce the excessive floor vibrations during earthquakes. The bearing support structure used in the translational TMD assembly is expensive and susceptible to wear over the lifespan of the system. As a result PTMD designs can be less expensive to manufacture and last longer The advantages and disadvantages of viscoelastic fluid dampers are as follows Advantages
Easy to design and construct No external power required for their operation
Respond to small level of excitation
Properties can be adjusted in the field
Can be considered in new design as well as in upgrading work
A single unit can be effective in reducing vibrations induced by small earthquakes, wind and traffic.
Economically cost effective
Require low maintenance
Disadvantages
Large mass and space are needed for their effectiveness and installation respectively
Effectiveness largely depend on the accuracy of their tuning
Only effective to control the response of a structure in one of its modes. Several dampers are needed, thus, when the response of the structure is important in more than one mode.
Friction limits its effectiveness to react to low level excitations Special features are needed if a damper is to control low level excitations
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CHAPTER 5 TUNED LIQUID DAMPERS 5.1 GENERAL
Tuned Liquid dampers are dynamic vibration absorbers which involves the dissipation of energy either through the sloshing of liquids in a container or via the passage of liquids through orifices. The basic principles involved in applying a tuned liquid damper (TLD) to reduce the dynamic response of structures is quite similar to that of tuned mass damper. In particular, a secondary mass of liquid (usually water) is introduced into the structural system and tuned to act as a dynamic vibration absorber. In the case of TLDs, the damper response is highly nonlinear due either to liquid sloshing or the presence of orifices. Tuned liquid damper absorbs structural energy by means of viscous actions of the fluid and wave breaking. Different shapes of container, such as rectangular or circular can be used as TLD implementations. TLD is more effective when it is placed at the top storey of the structure. To increase the energy dissipation of the sloshing liquid, the flow-damping devices such as screens or posts in the container are required. 5.1.1 Tuned Sloshing Damper
Tuned Sloshing Dampers are generally rectangular type or circular type and are installed at the highest floor according to building type. The structure of a TLD is quite simple; it is a liquid tank partially filled with liquid (or water) A TSD can be classified as shallow water type or deep water type depending on height of water in the tank. If the height of water ‘h’ against the length of the water tank in the direction of excitation ‘L’ (or diameter ‘D’ in case of circular tank) is less than 0.15 it can be classified as shallow water type else as deep water type if is more than 0.15. The depth of the liquid in a container could be deep or shallow, depending on the natural frequencies of the structure under control. Shallow water type has a large damping effect for a small scale of externally excited vibration, but it is very difficult to analyse the
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system for a large scale of externally excited vibration as sloshing of water in a tank exhibits nonlinear behaviour.
Figure 5.1 Tuned Sloshing Damper dimensions
In case of deep water type, the sloshing exhibits linear behaviour for a large scale of externally excited force. Tuned sloshing damper dissipates energy through the liquid boundary layer friction, the free surface contamination, and wave breaking. Generally tuning the fundamental sloshing frequency of the tuned sloshing damper to the structures natural frequency causes a large amount of sloshing & wave breaking at the resonant frequencies of the combined TSD-Structure system, this dissipate a significant amount of energy. 5.1.2 Tuned Liquid Column Dampers
The structure of a TLCD is also quite simple, it is a long U-shape vessel partially filled with fluid (or water). The U-shape vessel is a long circular or rectangular tube. The length of the tube should be at least ten times greater than the diameter or the in-plane dimension of the cross section. The cross section of a TLCD can either uniform or non-uniform, but it is usually symmetrical about the vertical centre line of a TLCD. The ends of the vertical columns of a TLCD are usually open.
Figure 5.2 Tuned Liquid Column Damper dimensions
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Tuned Liquid Column Dampers (TLCDs) dissipates structural vibration by combined action involving the motion of the liquid mass in the tube, where the restoring force is due to the gravity acting upon the liquid and the damping effect as a result of l oss of hydraulic pressure due to the orifices installed inside the container. In simple, the fluid flow and oscillation inside the U-shape vessel due to resonance can provide a force opposite to the direction of the vibration and can reduce the vibration of the structure effectively The damping capacity of TLCD can be controlled through controlling orifice opening. This allows us to actively control the damping in TLCD system, and frequency of a TLCD can be tuned by adjusting the liquid column in the tube.
Figure 5.3 A Tall Structure with TLCD 5.1.2.1 Types of Tuned Liquid Column Dampers
Liquid Column Vibration Absorber (LCVA)
Double Tuned Liquid Column Damper (DTLCD)
Hybrid Tuned Liquid Column Damper (HTLCD)
Pressurized Tuned Liquid Column Damper (PTLCD)
5.1.2.1.1 Liquid Column Vibration Absorber
The major difference between a Tuned Liquid Column damper and a Liquid Column Vibration Absorber is that, the cross section of the LCVA is not uniform. Since it has different dimensions for vertical and horizontal portions of container, it has benefits of easy
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tuning and wide range of natural frequency, as the natural frequency of the LCVA is determined not only by the length of the liquid column but also the geometric configuration. 5.1.2.1.2 Double Tuned Liquid Column Damper
One of the major disadvantages of TLCD and LCVA system is their unidirectional nature of action, and hence they can be applied effectively to the structure, which oscillates in only one predominant plane, but not to the structure that oscillates in bidirectional plane. To overcome this difficulty, a system has been proposed, named Double Tuned Liquid Column Damper (DTLCD), which consists of two TLCD in orthogonal directions.
Figure 5.4 Double Tuned Liquid Column Damper 5.1.2.1.3 Hybrid Tuned Liquid Column Damper
A Hybrid fluid dynamic system, named Hybrid Tuned Liquid Column Damper (HTLCD), has also been provided to overcome the above difficulty. This system consists of a unidirectional TLCD fixed on the surface of a rotatable circular platform whose motion is controlled by an electrical-mechanical system.
Figure 5.5 Hybrid Tuned Liquid Column Damper 25
This hybrid system is passive in generation of control force to attenuate the displacement amplitudes, whereas active in searching the right direction. 5.1.2.1.4 Pressurized tuned liquid Column Damper
By implementing a static pressure inside two sealed air chambers at two ends of a TLCD, a new kind of TLCD is formed, whose frequency can be adjusted by both the length of its liquid column and the pressure inside its two air chambers. This is called Pressurized tuned liquid Column Damper
Figure 5.6 Pressurized tuned liquid Column Damper
The advantages and disadvantages of Tuned liquid dampers are as follows Advantages
Low initial cost and virtually free of maintenance
Ease of frequency tuning
Containers can be utilized for building water supply
The water in the tank can be used for fire fighting
Ease to apply as a retrofitting tools to existing structures
Disadvantages
All the water mass does not participate in counteracting the structural motion
The phenomenon of beating where a fraction of the energy absorbed by TLD returns back to the structure after the excitation stops
The mechanism to achieve the optimal damping ratio of a TLD is complicated
Highly non-linear behaviour of the sloshing motion, especially under large excitation amplitudes
Due to the low density of water, a relatively large space is required in order to achieve the desired mass for the secondary system. 26
CONCLUSION The objective of this report is to present a brief overview of different passive energy dissipation systems which have been proposed and used for structural applications. The main points of the report are summarized below. 1. Significant reductions in response can be achieved using supplemental damping devices. 2. Passive energy dissipation devices enhance energy dissipation in structures and reduce the energy dissipation demand on structural components. This category of dampers include: metallic, friction, viscoelastic fluid and solid dampers. Most of these devices show stable behaviour and are effective in reducing the seismic response. 3. The performance of tuned systems in reducing the seismic response are also effective 4. The acceptance of innovative systems in structural engineering is based on a combination of performance enhancement versus construction costs and longterm effects. Continuing efforts are needed in order to facilitate wider and speedier implementation.
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