Passive energy dissipation devices

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PED Devices for Seismic Performance enhancement


<ul><li><p>1 </p><p>CHAPTER 1 </p><p>INTRODUCTION </p><p>1.1 PASSIVE ENERGY DISSIPATION SYSTEM </p><p>Passive energy dissipation systems encompass a range of materials and devices for </p><p>enhancing damping, stiffness and strength, and can be used both for seismic hazard </p><p>mitigation and for rehabilitation of aging or deficient structures. In general, such systems are </p><p>characterized by their capability to enhance energy dissipation in the structural systems in </p><p>which they are installed. </p><p>1.1.1 Principles of Operation </p><p>These devices generally operate on principles such as frictional sliding, yielding of </p><p>metals, phase transformation in metals, deformation of viscoelastic (VE) solids or fluids, </p><p>fluid orificing and sloshing. </p><p>1.1.2 Basic Function </p><p>The basic function of passive energy dissipation devices when incorporated into the </p><p>superstructure of a building is to absorb or consume a portion of the input energy, thereby </p><p>reducing energy dissipation demand on primary structural members and minimizing possible </p><p>structural damage. </p><p>Figure 1.1 Conventional Structure </p><p>Figure 1.2 Structure with Passive Energy Dissipation (PED) </p><p>Excitation Structure Response </p><p>Excitation Structure Response </p><p>PED </p></li><li><p>2 </p><p>1.2 CLASSIFICATION OF PASSIVE ENERGY DISSIPATION SYSTEMS </p><p>A large number of passive control systems or PED devices have been developed and </p><p>installed in structures for performance enhancement under earthquake loads. </p><p> A variety of passive energy dissipation devices are available and have been </p><p>implemented worldwide for seismic protection of structures. Passive energy dissipation </p><p>systems are classified herein in three categories as follows. </p><p>1. Rate-dependent system </p><p>2. Rate-independent system </p><p>3. Others </p><p>1.2.1 Rate-Dependent System </p><p> Rate-dependent system consists of dampers whose force output is dependent on the </p><p>rate of change of displacement across the damper. The behaviour of such dampers is </p><p>commonly described using various models of linear viscoelasticity. </p><p> This system is also called as velocity-dependent or viscoelastic system. It may or may </p><p>not impart additional stiffness to the structure. This system works on the principle of fluid </p><p>orificing or deformation of viscoelastic solids. </p><p> Examples: Viscoelastic Fluid dampers and Viscoelastic Solid dampers. </p><p>1.2.2 Rate-Independent System </p><p> Rate-independent systems consist of dampers whose force output is not dependent on </p><p>the rate of change of displacement across the damper but rather upon the magnitude of the </p><p>displacement and possibly the sign of the velocity i.e., the direction of motion. </p><p> The behaviour of such dampers is commonly described using various nonlinear </p><p>hysteretic models. This system is also called as displacement-dependent or hysteretic system. </p><p>It always adds stiffness to the structure. This system works on the principle of yielding of </p><p>metals or sliding friction. </p><p> Examples: Metallic dampers and Friction dampers. </p></li><li><p>3 </p><p> Energy dissipation systems which cannot be classified by one of the above basic </p><p>systems depicted are classified as other systems. These systems work on the various principle </p><p>of operation and can be further classified as follows. </p><p>1. Re-centering System </p><p>2. Dynamic Vibration Absorbers </p><p>1.2.3 Re-centering System </p><p> This system utilizes either a preload generated by fluid pressurization or internal </p><p>springs, or a phase transformation to produce a modified force-displacement response that </p><p>includes a natural re-centering component. </p><p> Examples: Pressurized fluid dampers, Preloaded spring-friction dampers, and Phase </p><p>transformation dampers. </p><p> 1.2.4 Dynamic Vibration Absorbers </p><p> In these systems, supplemental oscillators involving mass, stiffness and damping are </p><p>introduced in order to significantly enhance performance, the dynamic characteristics of the </p><p>supplemental oscillators must be tuned to those of the primary structure. The objective of </p><p>incorporating a dynamic vibration absorber into a structure is basically to reduce energy </p><p>dissipation demand on the primary structural members under the action of external forces. </p><p>The reduction, in this case, is accomplished by transferring some of the structural\vibrational </p><p>energy to the absorber </p><p> Examples: Tuned mass dampers and Tuned liquid dampers </p><p>Explanations on these various dampers are given in the following chapters. </p></li><li><p>4 </p><p>CHAPTER 2 </p><p>METALLIC DAMPERS AND FRICTION DAMPERS </p><p>2.1 METALLIC DAMPERS </p><p> Metallic dampers are hysteretic systems that dissipate energy with no significant rate </p><p>dependence and utilize the yielding of metals as the dissipative mechanism. The mechanism </p><p>involved in energy dissipation in metallic dampers can be categorized as one form of internal </p><p>friction. One of the effective mechanisms available for the dissipation of energy input to a </p><p>structure from an earthquake is through inelastic deformation of metals. </p><p> Many of these devices use mild steel plates with triangular or X shapes so that </p><p>yielding is spread almost uniformly throughout the material. Single round hole metallic </p><p>damper and double X shaped metallic damper are commonly used. </p><p> Figure 2.1 X-shaped Plate Damper Figure 2.2 Triangular Plate Damper </p><p> The idea of utilizing supplemental metallic hysteretic dampers within the </p><p>superstructure is to absorb a large portion of the seismic energy during earthquakes. The </p><p>performance objectives of using metallic dampers within the superstructure are energy </p><p>dissipation and strength enhancement. </p><p> Other configurations of steel yielding devices include bending type of honeycomb and </p><p>slit dampers and shear panel type. Two major types of metallic dampers are </p><p> Buckling-Restrained Brace (BRB) dampers </p><p> Added Damping and Stiffness (ADAS) dampers. </p></li><li><p>5 </p><p>2.1.1 Buckling-Restrained Brace Dampers </p><p> A BRB damper consists of a steel brace usually having low-yield strength with a </p><p>cruciform cross section that is surrounded by a stiff steel tube. The region between the tube </p><p>and brace is filled with a concrete-like material and a special coating is applied to the brace to </p><p>prevent it from bonding to the concrete. Thus, the brace can slide with respect to the </p><p>concrete-filled tube. The confinement provided by the concrete-filled tube allows the brace to </p><p>be subjected to compressive loads without buckling i.e., the damper can yield in tension or </p><p>compression with the tensile and compressive loads being carried entirely by the steel brace. </p><p> Under compressive loads, the damper behaviour is essentially identical to its </p><p>behaviour in tension. Since buckling is prevented, significant energy dissipation can occur </p><p>over a cycle of motion. In many cases, BRB dampers are installed within a chevron bracing </p><p>arrangement </p><p>Figure 2.3 Typical Arrangement of BRB Damper </p><p>Figure 2.4 Sectional View of BRB Damper </p></li><li><p>6 </p><p>2.1.2 Added Damping and Stiffness Dampers </p><p> An ADAS damper consists of a series of steel plates wherein the bottom of the plates </p><p>are attached to the top of a chevron bracing arrangement and the top of the plates are attached </p><p>to the floor level above the bracing. </p><p> As the floor level above deforms laterally with respect to the chevron bracing, the </p><p>steel plates are subjected to a shear force. The shear forces induce bending moments over the </p><p>height of the plates, with bending occurring about the weak axis of the plate cross section. </p><p> The geometrical configuration of the plates is such that the bending moments produce </p><p>a uniform flexural stress distribution over the height of the plates. Thus, inelastic action </p><p>occurs uniformly over the full height of the plates. For example, in the case where the plates </p><p>are fixed-pinned, the geometry is triangular. In the case where the plates are fixed-fixed, the </p><p>geometry is an hourglass shape. </p><p> To ensure that the relative deformation of the ADAS device is approximately equal to </p><p>that of the story in which it is installed, the chevron bracing must be very stiff. ADAS damper </p><p>will be damaged after an earthquake and may need to be replaced. </p><p>Figure 2.5 Typical Arrangement of X-plate Metallic Damper (ADAS) </p><p>The advantages and disadvantages of friction dampers are as follows. </p><p>Advantages </p><p> Stable hysteretic behaviour </p><p> Long-term reliability </p></li><li><p>7 </p><p> Insensitivity to environment factors like temperature, humidity etc. </p><p> Materials and behaviour familiar to practicing engineers </p><p> Inexpensive </p><p>Disadvantages </p><p> Devices damaged after earthquake; may require replacement </p><p> Nonlinear behaviour; may require nonlinear analysis </p><p>2.2 FRICTION DAMPERS </p><p> Friction dampers are hysteretic systems that dissipate energy with no significant rate </p><p>dependence and utilize the mechanism of solid friction that develops between two solid </p><p>bodies sliding relative to one another to provide the desired energy dissipation. </p><p> Several types of friction dampers have been developed for the purpose of improving </p><p>seismic response of structures. Damping using frictional dampers is considered to be the most </p><p>effective and economic solution for seismic upgrade. </p><p> In late seventies, frictional dampers were developed inspired with the principle of </p><p>friction brakes in automobiles. They usually consist of series of steel plates specially treated </p><p>to develop most reliable friction. The plates are clamped together with high strength steel </p><p>bolts. </p><p> During severe seismic excitations, friction dampers slip at a predetermined optimum </p><p>load before yielding occurs in other structural members and dissipate a major portion of the </p><p>seismic energy. </p><p> This allows the building to remain elastic or at least yielding is delayed to be available </p><p>during maximum credible earthquakes. </p><p> Another feature of friction damped buildings is that their natural period varies with </p><p>the amplitude of vibration. Hence the phenomenon of resonance is avoided. The performance </p><p>objectives are energy dissipation and strength enhancement. </p><p> Nowadays, several frictional dampers are being used. They are available for tension </p><p>cross bracing, single diagonal bracing and for chevron bracing. A short description on various </p><p>types of friction dampers as follows. </p></li><li><p>8 </p><p>2.2.1 Slotted-Bolted Friction Damper </p><p> The slotted-bolted damper consists of steel plates that are bolted together with a </p><p>specified clamping force. The clamping force is such that slip can occur at a pre-specified </p><p>friction force. At the sliding interface between the steel plates, special materials are utilized to </p><p>promote stable coefficients of friction. </p><p>Figure 2.6 Slotted-Bolted Friction Damper Assembly </p><p>2.2.2 Pall Cross-Bracing Friction Damper </p><p> The Pall cross-bracing friction damper consists of cross-bracing that connects in the </p><p>centre to a rectangular damper. The damper is bolted to the cross-bracing. Under lateral load, </p><p>the structural frame distorts such that two of the braces are subject to tension and the other </p><p>two to compression. </p><p> This force system causes the rectangular damper to deform into a parallelogram, </p><p>dissipating energy at the bolted joints through sliding friction. </p><p>Figure 2.7 Pall Cross-Bracing Friction Damper </p></li><li><p>9 </p><p>2.2.3 Sumitomo Friction Damper </p><p> Sumitomo friction damper is a cylindrical friction damper that dissipates energy via </p><p>sliding friction between copper friction pads and steel. The copper pads are impregnated with </p><p>graphite to lubricate the sliding surface and ensure a stable coefficient of friction. </p><p>Figure 2.8 Sumitomo Friction Damper </p><p>2.2.4 Energy Dissipation Restraint </p><p> The design is similar to the Sumitomo concept, since this device also includes an </p><p>internal spring and wedges encased in a steel cylinder. However, there are several novel </p><p>aspects of the Energy Dissipation Restraint (EDR) that combine to produce very different </p><p>response characteristics. </p><p>Figure 2.9 Energy Dissipation Restraint </p></li><li><p>10 </p><p>The EDR utilizes steel compression wedges and bronze friction wedges to transform the axial </p><p>spring force into normal pressure acting outward on the cylinder wall. Thus, the frictional </p><p>surface is formed by the interface between the bronze wedges and the steel cylinder. Internal </p><p>stops are provided within the cylinder in order to create the tension and compression gaps. </p><p> Consequently, unlike the Sumitomo device, the length of the internal spring can be </p><p>altered during operation, providing a variable frictional slip force. </p><p>The advantages and disadvantages of friction dampers are as follows. </p><p>Advantages </p><p> Simple and foolproof in construction </p><p> Insensitivity to environment factors like temperature, humidity etc., </p><p> Large energy dissipation per cycle </p><p> Compact in design and can be easily hidden within drywall partitions </p><p> Do not need regular inspection, maintenance, repair or replacement before and </p><p>after the earthquake </p><p>Disadvantages </p><p> Sliding interface conditions may change with time (reliability concern) </p><p> Strong nonlinear behaviour, may excite higher modes and require nonlinear </p><p>analysis </p><p> Permanent displacements if no restoring force mechanism provided </p><p> Adds Large Initial Stiffness to System </p></li><li><p>11 </p><p>CHAPTER 3 </p><p>VISCOELATIC FLUID DAMPERS VICOELASTIC SOLID DAMPERS </p><p>3.1 VISCOELASTIC FLUID DAMPERS </p><p> Viscoelastic Fluid dampers are viscoelastic systems that dissipate energy with </p><p>significant rate dependence and utilize the fluid orificing and deformation of viscoelastic </p><p>fluids as the dissipative mechanism. </p><p> A Viscoelastic Fluid damper generally consists of a piston within a damper housing </p><p>filled with a compound of silicone or similar type of oil, and the piston may contain a number </p><p>of small orifices through which the fluid may pass from one side of the piston to the other. </p><p>Thus, it dissipates energy through the movement of a piston in a highly viscoelastic fluid </p><p>based on the concept of fluid orificing. Viscoelastic fluid dampers are commonly installed </p><p>either within chevron bracing or diagonal bracing. </p><p> As the damper piston rod and piston head are stroked, fluid is forced to flow through </p><p>orifices either around or through the piston head. The resulting differential in pressure across </p><p>the piston head (very high pressure on the upstream side and very low pressure on the </p><p>downstream side) can produce very large forces that resist the relative motion of the damper. </p><p> The fluid flows at high velocities, resulting in the development of friction between </p><p>fluid particles and the piston head. The friction forces give rise to energy dissipation in the </p><p>form of heat. </p><p>3.1.1 Orifice Fluid Damper </p><p> It contains compressible silicone oil which is forced to flow via the action of a </p><p>stainless steel piston rod with a bronze head. The head includes a fluidic control orifice </p><p>design. In addition, an accumulator is provided to compensate for the change in volume due </p><p>to rod positioning. Alternatively, the device may be designed with a run-through piston rod </p><p>to prevent volume changes. High strength seals are required to maintain closure over the </p><p>design life of the damper. These uniaxial devices, which were originally developed for </p><p>military and harsh industrial environments, have recently found application in seismic base </p><p>isolation systems as well as for supplementa...</p></li></ul>


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