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A study on seismic response of reinforced structures retrofitted with fluid viscous dampers in shear walls

Master's Thesis 2015 90 Pages

Engineering - Geotechnology

Excerpt

CONTENTS

Acknowledgement

Abstract

List Of Tables

List Of Figures

Chapter 1 INTRODUCTION
1.1 General
1.2 Modes of failure of Reinforced concrete shear walls
1.3 Different retrofitting techniques for RC shear walls
1.4 Motivation of study
1.5 Objective of work
1.6 Organization of report

Chapter 2 LITERATURE REVIEW
2.1 Introduction
2.2 Introduction to the project title
2.3 Literature review
2.4 Summarized outcome of the literature review
2.5 Conclusion

Chapter 3 METHODOLOGY
3.1 Introduction
3.2 Methodology followed
3.3 Model description

Chapter 4 RESULT ANALYSIS
4.1 Introduction
4.2 Results and discussions for 9 storey structure with different configuration
4.3 Results for 9, 18 and 27,3d core 27 storey structures

Chapter 5 CONCLUSION AND FUTURE SCOPE OF WORK
5.1 Brief summary of work
5.2 Conclusion
5.3 Future scope of work

REFERENCES

PROJECT DETAILS

ACKNOWLEDGMENT

I owe my deepest gratitude to my guide Mr. Avinash A R, Assistant Professor, Department of Civil Engineering, MIT, Manipal, for his guidance, encouragement and valuable suggestions at each and every step of the project.

I would also like to extend my thanks to Dr. Kiran Kamath, Professor, Department of Civil Engineering, MIT, Manipal, for his timely advice and suggestions.

I am grateful to Dr. Mohandas Chadaga, Professor and Head, Department of Civil Engineering, MIT, Manipal, for his encouragement and support during the course of this project.

It is also my duty to thank other teaching and non-teaching staff of Civil Engineering Department, MIT, Manipal, for their co-operation, suggestions and their assistance during my work.

I am also thankful to all other persons who helped me directly or indirectly during the entire course of my project.

SACHIN VIJAYA KUCKIAN

ABSTRACT

Devastating effects of the earthquakes on tall structures have been observed very well during recent seismic events. Structures therefore should be strong enough to resist any future earthquakes. Thus any structure which is not strong enough to resist future earthquakes needs to be strengthened and the process of making the structure more resistant to future earthquakes is known as seismic retrofitting.

Reinforced concrete (RC) shear walls are widely used in medium- to high-rise buildings to provide the lateral strength, stiffness and energy dissipation capacity required to resist lateral loads arising from wind or earthquakes. In the past few decades, there has been considerable advancement in the design of RC walls for new construction. There is an essential need to upgrade the seismic performance of existing RC shear walls so that they can meet the requirements of the new performance-based seismic design techniques. Several retrofit techniques using different materials are reported in the literature.

The present study investigates the seismic behavior of multi storey building using damping devices strategically located within the lateral load resisting elements. It concentrates on a retrofitting strategy with passive energy dissipation device known as Fluid Viscous Damper (FVD) which will be applicable to new design as well as retrofitting existing buildings to ensure seismic safety by fitting damping devices which can transform a wall panel into a damping element. The first study involves analysis of a nine storey model having cut outs and the use of the dampers of different configuration in these structures. The second study involves the use the diagonal brace configuration dampers provided in the cut out sections of 2D 9, 18, 27 storey structures and 3D 27 storey with core wall structure at three consecutive storey levels each. For the second study the cut out locations are varied depending on their relative positions. The relative position is the ratio of total height of the structure to the upper edge of topmost cut-out. These structures were initially modelled and time history analysis was performed on the structure without FVD and the structure retrofitted with FVD. Three different ground motions were used for the analysis. Results of the un- retrofitted structures are then compared with retrofitted structure in terms of peak storey displacements, roof accelerations and pseudo spectral accelerations.

Study shows that there has been a significant reduction in seismic demands for a structure retrofitted with FVD in terms of peak storey displacements, pseudo spectral accelerations and roof accelerations when the dampers are placed at lower three cut outs i.e. with high relative position. It is also observed that damping coefficient value obtained is least for upper toggle- brace configuration out of the four different damper configurations and with maximum reduction compared to other configurations. For modeling and analysis purpose the software SAP2000® is used.

Through the study it could be concluded that FVD significantly reduces the seismic demands of the structure in terms of peak storey displacements, pseudo spectral accelerations and roof accelerations. This suggests that FVDs can be efficiently used in retrofitting. Also damping coefficient value obtained is least for upper toggle-brace configuration out of the four different damper configurations suggesting this is the most efficient configuration for retrofitting.

LIST OF TABLES

4.1 Results and discussions for the 2D 9 storey structure

4.2 Geometric details of connecting elements of Toggle brace configurations

4.3 Damping coefficient for diagonal configuration

4.4 Damping coefficient for chevron configuration

4.5 Damping coefficient for upper toggle configuration

4.6 Damping coefficient for lower toggle configuration

4.7 Comparison of reduction of damping coefficients for different damper configurations

4.8 Time period and Reduction (%) for 9 storey models

4.9 Damping coefficient for 9 storey models

4.10 Time period and Reduction (%) for 18 storey structure

4.11 Damping coefficient for 18 storey models

4.12 Time period and Reduction (%) for 27 storey structure

4.13 Damping coefficient for 27 storey models

4.14 Damping coefficient for 27 storey models 3d core structure

4.15 Reduction in terms of Time history and Response spectrum analysis

LIST OF FIGURES

1.1 Different characteristic to be improved by retrofit

1.2 Flexural failure of RC wall

1.3 Shear failure of RC walls

1.4 Retrofitted RC wall using steel bracings

1.5 RC wall strengthened using SMA bars at failure and its hysteretic behaviour

1.6 Fluid Viscous Dampers

1.7 Construction of a Fluid Viscous Damper

3.1 Accelerograms of different earthquakes

3.2 Absolute Acceleration Response Spectra for different earthquakes (5% damping)

3.3 Experimental setup showing an Upper –Toggle brace configuration

3.4 Different FVD configurations

3.5 Flow chart of the procedure for obtaining damping co-efficient of FVD

3.6 Maxwell model used by SAP

3.7 Damper locations for types A, B, C, and D with diagonal brace configuration

3.8 Damper locations for types A, B, C, and D with chevron brace configuration

3.9 Damper locations for types A, B, C, and D with upper toggle brace configuration

3.10 Damper locations for types A, B, C, and D with lower toggle brace configuration

3.11 Details of damper placement and dimensions of shear wall

3.12 Damper locations for 9 storey 2D structure

3.13 Damper locations for 18 storey 2D structure

3.14 Damper locations for 27 storey 2D structure

3.15 Plan for 27 storey core wall

3.16 Isometric view of 27 storey with core wall

4.1 Plot of reduction of peak storey displacements for Type A with different configurations for LA03,LA06 and LA

4.2 Plot of reduction of peak storey displacements for Type B with different configurations for LA03,LA06 and LA

4.3 Plot of reduction of peak storey displacements for Type C with different configurations for LA03,LA06 and LA

4.4 Plot of reduction of peak storey displacements for Type D with different configurations for LA03,LA06 and LA

4.5 Peak roof deflection reduction for Types A-D for LA 03, LA06, and LA14 for Upper toggle configuration

4.6 Peak roof deflection reductions for Types A-D for LA 03, LA06, and LA14 for Chevron configuration

4.7 Peak roof deflection reductions for Types A-D for LA 03, LA06, and LA14 for Lower toggle configuration

4.8 Peak roof deflection reductions for Types A-D for LA 03, LA06, and LA14 for Diagonal configuration

4.9 Time v/s Peak acceleration plot for Type A for LA03 for upper toggle configuration

4.10 Time v/s Peak acceleration plot for Type A for LA06 for upper toggle configuration

4.11 Time v/s Peak acceleration plot for Type A for LA14 for upper toggle configuration

4.12 Time v/s Peak acceleration plot for Type D for LA03 for upper toggle configuration

4.13 Time v/s Peak acceleration plot for Type D for LA06 for upper toggle configuration

4.14 Time v/s Peak acceleration plot for Type D for LA14 for upper toggle configuration

4.15 Relative position v/s deflection reduction (%) plot of 9 storey for LA

4.16 Relative position v/s deflection reduction (%) plot of 9 storey for LA

4.17 Relative position v/s deflection reduction (%) plot of 9 storey for LA

4.18 Relative position v/s deflection reduction (%) plot of 18 storey for LA

4.19 Relative position v/s deflection reduction (%) plot of 18 storey for LA

4.20 Relative position v/s deflection reduction (%) plot of 18 storey for LA

4.21 Relative position v/s deflection reduction (%) plot of 27 storey for LA

4.22 Relative position v/s deflection reduction (%) plot of 27 storey for LA

4.23 Relative position v/s deflection reduction (%) plot of 27 storey for LA

4.24 Relative position v/s deflection reduction (%) plot of 27 storey 3d core for LA for time history analysis

4.25 Relative position v/s deflection reduction (%) plot of 27 storey 3d core for LA for time history analysis

4.26 Relative position v/s deflection reduction (%) plot of 27 storey 3d core for LA for time history analysis

4.27 Relative position v/s deflection reduction (%) plot of 27 storey 3d core for LA for response spectrum analysis

4.28 Relative position v/s deflection reduction (%) plot of 27 storey 3d core for LA for response spectrum analysis

4.29 Relative position v/s deflection reduction (%) plot of 27 storey for 3d core LA for response spectrum analysis

4.30 Time v/s Peak acceleration plot for 27 storey with relative position (H/h1) of 9.52 for LA

4.31 Time v/s Peak acceleration plot for 27 storey with relative position (H/h1) of 9.52 LA

4.32 Time v/s Peak acceleration plot for 27 storey with relative position (H/h1) of 9.52 for LA

4.33 Pseudo spectral accelerations for 27 storey with relative position (H/h1) of 9.52for LA

4.34 Pseudo spectral accelerations for 27 storey with relative position (H/h1) of 9.52 for LA

4.35 Pseudo spectral accelerations for 27 storey with relative position (H/h1) of 9.52for LA

4.36 Time v/s Peak acceleration plot for 27 storey with relative position (H/h1) of 1.01 for LA

4.37 Time v/s Peak acceleration plot for 27 storey with relative position (H/h1) of 1.01 for LA

4.38 Time v/s Peak acceleration plot for 27 storey with relative position (H/h1) of 1.01 for LA

4.39 Pseudo spectral accelerations for 27 storey with relative position (H/h1) of 1.01 for LA

4.40 Pseudo spectral accelerations for 27 storey with relative position (H/h1) of 1.01 for LA

4.41 Pseudo spectral accelerations for 27 storey with relative position (H/h1)

CHAPTER 1

INTRODUCTION

1.1 General

Reinforced Concrete (RC) walls are classified according as load bearing walls, non- load bearing walls, shear walls, flexural shear walls, and squat shear walls. Shear walls are part of the lateral force resisting system that carries vertical loads, bending moments about the wall major axis, and shear forces parallel to the wall length. Shear wall system is one of the most common and effective lateral load resisting systems that are widely used in medium- to high-rise buildings. Shear walls can provide adequate strength and stiffness needed for the building to resist wind and earthquake loadings, provided that a proper design is considered, that cares for both wall strength and ductility. Many of the existing RC buildings with shear wall system that are located in seismically active zones are designed according to older design codes, in which the ductility requirements were not enforced. These buildings might be seismically deficient according to the new codes due to lack of strength and ductility. Therefore, retrofitting of such buildings becomes a necessity and cannot be overlooked.

Different retrofit techniques can be used to upgrade the seismic performance of RC shear walls. The expected mode of failure for a specific existing wall determines the appropriate retrofitting technique that can be used.These retrofitting techniques aim to improve the wall‘s strength, stiffness, ductility, or a combination of these. Increasing the wall energy dissipation capacity is a main aspect for a proper retrofitting due to the nature of dynamic load excitation. Control of the wall permanent deformations is another important target, which can be achieved by using re -centering materials such as shape memory alloys (SMA). Most of the tests conducted on RC shear walls identify their existing and retrofitted performance using roof displacement, base shear, moment-rotation, energy dissipated, and displacement time history relationships. Figure 1 shows different wall characteristics to be improved by retrofit. Different materials could be used for retrofit such as steel, concrete, fiber- reinforced polymers (FRP) composites, and SMA.

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Figure 1.1 Different characteristic to be improved by retrofit (a) Stiffness, Strength, and/or ductility (b) Energy dissipation capacity (c) Permanent deformation control

1.2 Modes of failure of Reinforced concrete shear walls

There are several modes of damage/failure of RC shear walls that were observed from post-earthquake events‘ reconnaissance or reported from controlled experimental research work. It is important to be able to predict and evaluate the expected response of an existing RC wall in order to be able to choose the most suitable and effective retrofitting technique that meets a target performance. The following subsections identify the most common failure modes of RC shear walls.

1.2.1 Flexural failure

In this mode of failure, considerable flexure cracks appear near the bottom part of the tensile zone of the wall, yielding of tensile steel or compression steel may occur, crushing of concrete in the compression zone could happen at the ultimate stages. The compression steel also might buckle if the concrete cover in the compression zone spalled off. This type of failure occurs when the flexural capacity of the RC wall is lower than its shear capacity, which is usually the case for high-rise walls. Figure 2 shows the crack pattern for a wall failed in a flexure manner (Greifenhagen and Lestuzzi, 2005).

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Figure 1.2 Flexural failure ofRC wall. (Greifenhagen and Lestuzzi 2005)

This mode of failure was reported in the experimental work conducted by Lefas and Kotsovos (1990),indicated the importance of higher mode effects for high-rise walls that result in higher shear forces and bending moments in upper region of the wall. This would lead to the formation of a plastic hinge in that region. Therefore, for existing low-rise shear walls, it might be needed to rehabilitate the lower part of the wall only (in the expected location of the plastic hinge region), while for the high-rise walls, it might be needed to rehabilitate other region that might experience plastic hinge formation at higher level (due to higher mode effects that might not have been considered in the original design ofthe wall). Predicting such behaviour is important in the design of the rehabilitated wall to avoid the wall failure at higher levels.

1.2.2 Shear failure

This mode of failure occurs usually for shear walls with low aspect ratio or with inadequate shear capacity. Shear failure is brittle in nature which would reduce the energy dissipation capacity of the wall/structure when subjected to a severe ground motion. For this reason, the main aim for all seismic design codes is to avoid such a mode of failure by ensuring that the shear capacity of the wall exceeds its flexural capacity.

a) Diagonal tension and diagonal compression

Due to principal tensile stresses, inclined shear cracks starts to appear, and hence the shear force acting on the wall is resisted by the compression struts formed between the cracks and the tension in the web reinforcement steel. Diagonal tension failure occurs when insufficient horizontal or diagonal reinforcement is used (yielding of shear reinforcement). If the shear reinforcement was sufficient to transfer high shear forces through the shear cracks, diagonal compression failure could occur due to high compression forces in the diagonal compression struts. For that mode of failure and in case of cyclic loading, the web starts to have X-shaped cracks, and then followed by a brittle failure of the concrete web. The concrete compressive strength is the main factor that affects the capacity of the wall that will experience this mode of failure. Figure 1.3 (a) shows the shear failure of a RC wall tested by Lopes (2001).

b) Sliding shear failure

Sliding shear failure occurs when the wall has sufficient horizontal reinforcement and relatively small amount of vertical reinforcement in the wall web. In this mode of failure, a continuous horizontal crack originating from flexure will be formed at the base of the wall or at the construction joint (i.e. the weak plane). In this case, the wall section will resist the acting shear forces by the dowel action of the vertical reinforcement and by the friction between the concrete surfaces. For walls with low axial load value, the friction between the concrete layers will not be high, and hence this mode of failure could be critical. To increase the capacity of RC walls against sliding, the amount of vertical web reinforcement could be increased, or the concrete surface could be intentionally roughened at locations of construction joints to full amplitude of at least 5 mm as recommended by the CSA (2004). Figure 1.3(b) shows the sliding shear failure of the RC wall tested by Riva et al. (2003).

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Figure 1.3 Shear failures of RC walls, (a) Diagonal compression (Lopes 2001), (b) Sliding shear (Riva et al. 2003).

1.2.3 In-planesplittingfailure

In-plane splitting failure was noticed in lightweight RC walls under high compression forces that can result from lateral loads or higher gravity loads (Mosalam et al. 2003). This type of failure occurs suddenly and without any indication. This failure can be prevented by proper confinement of the wall.

1.2.4 Rocking failure

This type of failure occurs when the overturning moment acting on the wall due to lateral loads is greater than the stabilizing moment of the axial load acting on the wall about the foundation corner. This behaviour is common for masonry walls, where the bond between the masonry blocks is lost at one plane, and then the wall starts to rock about this plane. This could occur also in case of RC precast walls, when the connection between the wall and the foundation is lost. Taghdi et al. (2000) found that RC walls might experience rocking behaviour at a late stage of their testing. They stated that although the rocking behaviour would dissipate the earthquake energy, but still the lateral load resistance of the wall could be insufficient to resist the lateral loads, and hence retrofit would be necessary.

1.3 Different retrofit techniques for RC shear walls

Retrofit of an existing RC wall includes either the repair, rehabilitation or strengthening terms. The rehabilitation and strengthening terms are used when the performance of the existing wall does not satisfy the existing requirements of the design code and needs to be enhanced. However, the term strengthening is used when the wall was not subjected to any damage, while the term rehabilitation is used when the wall has already been damaged and its resistance needs to be restored and improved as well. If the damaged wall‘s performance was satisfying before the damage occurred, and it is needed to restore its capacity without any additional resistance, then the term repair will be representative. There are several factors that control the choice of the retrofitting technique for RC shear walls, some of these factors are

- The deficiency in the existing wall and its expected mode of failure.
- The goal of intervention (e.g. increased stiffness, strength, ductility, etc).
- Consequences of wall rehabilitation (e.g. increased demand on foundation, etc).
- The allocated budget for retrofit.
- Physical constraints (e.g. architectural requirements, accessibility of the building during the retrofitting process, etc).

1.3.1 Concrete replacement

Concrete replacement is the simplest and cheapest technique that can be used to restore strength and ductility of RC walls (Fiorato et al. 1983). In this technique, the damaged concrete is removed, the aggregate of the old concrete is exposed and the surface of the old concrete should be cleaned to remove any loose material and to ensure a strong bond between the old concrete and the new one. If the reinforcing steel bars in the compression zone were slightly buckled after concrete crushing, they should be straightened (Lefas and Kotsovos 1990). The formwork of the web is prepared the new concrete is mixed and poured from one side of the wall. After the removal of formwork, the new concrete should be cured. Therefore, repairing the shear wall by concrete replacement is causing disturbance to the building function, and hence it is not suitable if the building has to be accessible during repair.

1.3.2 Concrete jacketing

In this technique, the wall dimensions are increased by adding new concrete to the original web. Additional reinforcement could be used to increase the strength and ductility of the wall. The new reinforcement can be vertical and horizontal bars that form the reinforcement mesh or it can be diagonal bars. The new reinforcement should be anchored to the wall foundation. One way of anchoring is by placing the reinforcement in holes that are drilled in the foundation, and then it is grouted with epoxy. The new concrete is casted with the new dimensions and cured after solidification. Fiorato et al. (1983) tested two RC walls, one rehabilitated using diagonal bars after removal of the damaged web concrete in the plastic hinge region and the other one is rehabilitated by increasing the web thickness (jacketing). The tests showed that the strength and deformation capacities of the rehabilitated walls had increased, while their initial stiffness was almost half that of the original walls. It should be noted that, in some cases when the wall foundation is not over-designed, it will be needed to strengthen the foundation as well in order to be able to carry the additional weight of the wall and the increased lateral load expected to be carried by the wall.

1.3.3 Retrofitting using steel bracing

Steel bracings are mostly used for rehabilitation of nominally-ductile moment resisting frame structures. They can provide the adequate strength, stiffness and ductility required for the structure, provided that a special attention should be directed to their connections with the existing structure. Steel bracings can be also used to enhance the seismic performance of RC shear walls. In that case, the steel bracing can be anchored to the RC wall at small intervals to minimize the buckling length, which will increase the capacity of the bracing member compared to the case of retrofitting the moment resisting frames that is governed mainly by buckling of the compressed bracing member. Taghdi et al. (2000) tested a RC wall that is retrofitted using this technique. Figure 1.4 shows the retrofitted wall at 1.0 %drift.

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Figure1.4 Retrofitted RC wall using steel bracings at 1.0 %drift (Taghdi et al. 2000)

1.3.4 Retrofitting usingcompositematerials

Fibre-reinforced polymer (FRP) composite materials have received an increasing attention in the past few decades as a potential material for retrofitting of existing structures due to their high strength, light weight, ease of application, and their high resistance to corrosion. FRP laminates, sheets or rods can be used, and the fibres might be prestressed to increase the efficiency of retrofit. The use of FRP composites offers also a faster and easier retrofit alternative, especially when the evacuation of the entire building during the retrofit is not possible, in that case FRP will provide the required strength without interrupting the use of the building. Yet, some of the characteristics of FRP composites such as long-term performance, performance under dynamic excitations, etc., are still under investigation.

1.3.5 Retrofitting using shape memory alloys (SMA)

Shape memory alloys have recently an increasing attention in civil infrastructure researches and seem to have a brilliant future. However, the reported tests on the use of SMA for seismic retrofit of RC walls have been very limited and still more tests are needed. SMA has the ability to undergo large deformations, then it can restore its original shape when the applied stress is removed (super-elastic effect) or when it is heated (shape memory effect). This phenomenon can be very useful in the seismic applications in buildings; such as dampers, bracings, etc. In addition to that, SMA has an excellent resistance against corrosion. Effendy et al. (2006) tested two low-rise RC walls with boundary elements retrofitted using two different types of SMA bracings as shown in Figure 1.5.

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Figure 1.5 RC wall strengthened usmg SMA bars at failure and its hysteretic behaviour (Effendy et al. 2006).

1.3.6 Retrofitting using dampers

a) Viscoelastic dampers
b) Friction dampers
c) Tuned mass dampers
d) Fluid viscous dampers

a) Viscoelastic dampers

Viscoelastic dampers, made of bonded viscoelastic layers (acrylic polymers), have been developed by 3M Company and used in wind vibration control applications. The behaviour of viscoelastic dampers is controlled by the behaviour in shear of the viscoelastic layers. In general, this material exhibits viscoelastic solid behaviour with both its storage and loss moduli being dependent on frequency and temperature (Constantinou and Symans 1992).

b) Friction dampers

These work by dissipating the earthquake energy in the form of friction. A frictional device located at the intersection of cross bracing. When seismic load is applied, the compression brace buckles while the tension brace induces slippage at the friction joint. This, in turn, activates the four links which force the compression brace to slip. In this manner, energy is dissipated in both braces while they are designed to be effective in tension only.

c) Tuned mass dampers

Tuned mass dampers (TMD) employ movable weights on some sort of springs. These are typically employed to reduce wind sway in very tall, light buildings. Similar designs may be employed to impart earthquake resistance in eight to ten storey buildings that are prone to destructive earthquake induced resonances.

d) Fluid Viscous dampers

These dampers consist of some viscous fluid, during an earthquake event they absorb and dissipate the energy in the form of heat. These devices originated in the early 1960's for use in steel mills as energy absorbing buffers on overhead cranes. Variations of these devices were used as canal lock buffers, offshore oil rig leg suspensions, and mostly in shock isolation systems of aerospace and. military hardware.

Construction of FVD

Photograph of FVD is shown in Figure 1.6. FVD is commonly used as passive energy dissipation devices for seismic protection of structures. Figure 1.7 shows the construction of FVD these dampers consist of a hollow cylinder filled with fluid, the fluid typically being silicone based. 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. Mechanical properties and operation of these dampers were exclusively studied and well documented by Constantinou and Symans (1992). Advantages and disadvantages of these FVD has been studied and documented by Hwang (1998).

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Figure 1.6 Fluid Viscous Dampers

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Figure 1.7 Construction of a Fluid Viscous Damper

Equation for FVD

The governing equation for force in FVD is, given in FEMA 273 (1997) is as follows,

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Where, F = Force in damper

C 0 = Damping co-efficient for the device

α = Velocity exponent for the device (for linear FVD, α = 1)

D = Relative velocity between each end of the device. And sgn is the signum function that defines the sign of the relative velocity term.

Application of FVDs in the past

Some large scale previous applications of FVD include,

- The West Seattle Swing Bridge. Fluid dampers with a built-in hydraulic logic system could provide damping at two pre-determined levels. The logic system can determine if the bridge condition is normal or faulted. Under normal conditions, damping is very low. When a fault occurs, due to motor runaway, excessive current or wave loadings, or earthquakes, the device senses the higher than normal velocity and absorbs significant energy.
- The New York Power Indian Point 3 Nuclear Power Plant. Each nuclear generator is connected to the containment building walls by eight 1.34 MN capacity fluid dampers. The dampers are specifically designed for seismic pulse attenuation.
- The Virginia Power North Ana Nuclear Station. This is an application similar to that of the Indian Point 3 Plant, except that the dampers have 8.92 MN capacity.
- Suppression of wind induced vibration of launching platforms such as those of the Space Shuttle and the Atlas Missile.

Advantages of FVD

Following are few disadvantages of FVD,

- Dynamic amplification can be reduced considerably due to the high damping capacity of the dampers.
- If desired the entire structure can be made to behave totally in elastic manner by using FVD.
- Forces generated by FVD are out of phase with column forces, therefore no additional column forces generated during an earthquake event.

Disadvantages of FVD

Following are few disadvantages of FVD,

- FVD are mechanical devices and difficult to manufacture.
- Frequent maintenance is required as there is a chance of breaking of seal over the period of time which may lead to loss of the fluid.

1.4 Motivation for the study

The trend of using energy dissipating devices such as FVD for seismic retrofitting is gaining popularity nowadays. Significant research work has been done previously in the field of passive energy dissipating devices especially on FVDs.

However, the vast majority of applications was realised within the frame structures, while investigations on use of FVDs within the cut outs of shear wall is still limited. For this reason the aim of this research is to investigate the behavior of multi-storey frame shear wall building structures under earthquake loads with damping devices strategically located within the cut outs of shear wall.

One of the drawbacks observed during the literature study is that majority of the previous works include complicated mathematical formulations which are difficult to follow and apply in the field. Thus it is essential to have an easier design procedure which will help practicing engineers to adopt FVD for retrofitting.

The study uses a strategy which was used by Rastogi et.al (2011) for retrofitting, which is easy to follow as rather than developing a complex program for the purpose, it uses time tested and industry leading software SAP2000®.

The first study helps to obtain an optimum damper configuration which will reduce the damping coefficient and thereby reducing the cost of retrofitting and second study helps to find out the optimum location of the dampers to be used in the cut out sections of shear wall.

1.5 Objective of the work

The objectives of the work are as follows,

- Main objective of the work is to study the seismic demands of non-retrofitted structures and structures which are retrofitted with FVD in cut out sections of shear wall using the software SAP. Results of these structures are compared with each other in terms of percentage reduction of storey displacements, roof accelerations and pseudo spectral acceleration.
- Secondary objective of the study is to study different damper configurations to obtain an efficient configuration.
- Tertiary objective of the study is to study different damper location and to obtain an efficient damper placement.

1.6 Organisation of the report

The entire report has been organized in five chapters. A brief outline of each chapter is given below,

Chapter 1 introduces into the area of the work. It explains the present day scenario in the field of energy dissipating devices. It also explains the motivation for the present work and it also includes the failures in shear wall and retrofitting techniques. It also gives a brief insight into the methodology followed in the study. The importance of the end result is also explained in this chapter.

Chapter 2 explains the literature which is available in the pertaining field. It explains the reason behind the selection of the project title. A review is done on the available literature related to the field of retrofitting and energy dissipating devices. Outcome of the literature review is then summarized followed by a general discussion.

Chapter 3 concentrates on the methodology used in the study. It explains the methodology in detail, various assumptions made, and details of the structures used in the study

Chapter 4 lists the results of the study in the form of graphs and tables. Explanations are also given to these graphs and tables. Significance of these results is discussed as well.

Chapter 5 deals with the conclusions which are drawn based on the results of the study. It explains the objective of the study and a brief detail of the methodology adopted. The chapter explains the significance of the results as well. A few suggestions regarding the future work is also given in this chapter.

CHAPTER 2

REVIEW OF LITERATURE

2.1 Introduction

This chapter deals with the backgrounds which lead to the present work. The chapter also lists various research works which have been previously conducted by various authors pertaining to the topic. It also summarizes the general outcome of the literature study.

2.2 Introduction to the project title

The project is basically a study, which is done to understand the behaviour of a structure having shear wall retrofitted with FVD. The approach used in the study is analytical one rather than experimental one and which is the reason for selecting the title as ―Seismic response of reinforced structures retrofitted with fluid viscous dampers in shear walls‖.

2.3 Literature review

Fiorato et al. (1983), tested two RC walls, one rehabilitated using diagonal bars after removal of the damaged web concrete in the plastic hinge region and the other one is rehabilitated by increasing the web thickness (jacketing). The tests showed that the strength and deformation capacities of the rehabilitated walls had increased, while their initial stiffness was almost half that of the original walls. It should be noted that, in some cases when the wall foundation is not over-designed, it will be needed to strengthen the foundation as well in order to be able to carry the additional weight of the wall and the increased lateral load expected to be carried by the wall.

Lefas and Kotsovos (1990), worked on RC wall retrofitting technique by concrete replacement, were he concluded that repairing the shear wall by concrete replacement caused disturbance to the building function, and hence it is not suitable if the building has to be accessible during repair.

Constantinou and Symans (1992), conducted and documented experimental and analytical study of seismic response of structures with supplemental fluid viscous dampers. In the report they have discussed about different energy dissipating devices such as friction dampers, visco-elastic dampers, viscous walls and FVD. In the report they have verified mechanical properties of FVD through an experiment. They have also studied the responses of a structure without and with dampers. They developed mathematical models of different structures and these models were later validated by performing earthquake simulator testing on experimental models of the same. They found that Fluid dampers are capable of achieving and surpassing the benefits offered by active control systems with the additional benefits of low cost, no requirements for power, longevity and reliability.

Elnashai and Pinho (1997), studied the effect of rehabilitation scheme used for retrofitting shear walls using steel plates on the enhancement of a certain property (e.g. wall stiffness, strength or ductility) without altering the other properties.

Taghdi et al. (2000), studied the rocking type of failure occurs when the overturning moment acting on the wall due to lateral loads is greater than the stabilizing moment of the axial load acting on the wall about the foundation corner. This behaviour is common for masonry walls, where the bond between the masonry blocks is lost at one plane, and then the wall starts to rock about this plane. This could occur also in case of RC precast walls, when the connection between the wall and the foundation is lost. They stated that although the rocking behaviour would dissipate the earthquake energy, but still the lateral load resistance of the wall could be insufficient to resist the lateral loads, and hence retrofit would be necessary.

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Details

Pages
90
Year
2015
ISBN (eBook)
9783668992764
Language
English
Catalog Number
v496594
Grade
9.44

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Title: A study on seismic response of reinforced structures retrofitted with fluid viscous dampers in shear walls