Behavior of Building Structure with Base Isolation System

Control Systems for Seismic Protection of Structures – Seminar

Submitted By: Mahesh Chand Sharma

Introduction

Civil engineering structures located in environments where earthquakes or large wind forces are common will be subjected to serious vibrations during their lifetime. These vibrations can range from harmless to severe with the later resulting in serious structural damage and potential structural failure.

Seismic Protection of Structures

The Traditional Technique of a seismic Design  –  ( increase the stiffness of structures by enlarging the section of columns, beams, shear walls, or other elements)

Modern Approach through Structural Controls   – (by installing some devices, mechanisms, substructures in the structure to change or adjust the dynamic performance of the structure)

Basic Principles of Seismic Response Control

Control systems add damping to the structure and/or alter the structure’s dynamic properties. Adding damping increases the structural energy-dissipating capacity, and altering structural  stiffness can avoid resonance to external   excitation, thus reducing structural  seismic response.

Seismic Protection

  1. Passive control systems
  2. Active Control systems
  3. Semi-active control systems
  4. Hybrid  control systems

Passive control systems

The passive control system does not require an external power source and being  utilizes the structural motion to dissipate seismic energy or isolates the vibrations so that response of structure can be controlled. The passive control devices includes

1. Base Isolation

2. Passive Energy Dissipating (PED)   Devices

Base Isolation

Behavior of Building Structure with Base Isolation System

Behavior of Building Structure with Base Isolation System

A building mounted on a material with low lateral stiffness, such as rubber, achieves a flexible base.

During the earthquake, the flexible base is able to filter out high frequencies from the ground motion and to prevent the building from being damaged or collapsing

– deflecting the seismic energy and

–  absorbing the seismic energy

Various Type  of Base Isolation

Elastomeric Bearings:

  •      -Low-Damping Natural or Synthetic Rubber Bearing
  •      – High-Damping Natural Rubber Bearing
  •      – Lead-Rubber Bearing (Low damping natural rubber with lead   core)

Sliding Bearings

  •       – Flat Sliding Bearing
  •       – Spherical Sliding Bearing
Rubber Layers Provide lateral flexibility

Rubber Layers: Provide lateral flexibility

Major Components:

– Rubber Layers: Provide lateral flexibility

– Steel Shims: Provide vertical stiffness to support building weight while limiting lateral bulging of rubber

– Lead plug: Provides source of energy dissipation

Low Damping Natural or Synthetic Rubber Bearings

Linear behavior in shear for shear strains up to and exceeding 100%.

Damping ratio = 2 to 3%

Advantages:

  •     Simple to manufacture
  •      Easy to model
  •      Response not strongly sensitive to rate of loading, history of loading, temperature, and aging.

Disadvantage:

-Need supplemental damping  system

High-Damping Natural Rubber Bearings

  •  Damping increased by adding  extra-fine  carbon black, oils or resins, and other proprietary fillers
  • Maximum shear strain = 200 to 350%
  • Damping ratio = 10 to 20% at shear  strains of 100%
  • Effective Stiffness and Damping depend on:

       – Elastomer and fillers

       – Contact pressure

       – Velocity of loading

       – Load history (scragging)

       – Temperature

Lead-Rubber Bearings

  • damping properties can be improve by plugging a lead core into the bearing
  • damping of the lead-plug bearing varies from 15% to 35%.
  • The Performance  depends on the imposed lateral force
  • The hysteretic damping is developed with energy absorbed by the lead core.
  • Maximum shear strain = 125 to 200%

Sliding Bearings

Sliding Bearings

Sliding Bearings

The imposed lateral force is resisted by the product of the friction coefficient and the vertical load applied on the bearing

Passive Energy Dissipating Devices (PED)

Mechanical devices to dissipate or absorb a portion of structural input energy, thus reducing structural response and possible structural damage.

•Metallic Yield Dampers relies on the principle that the metallic device deforms plastically, thus dissipating vibratory energy
• Friction Dampers – here friction between sliding faces is used to dissipate energy

•Visco-elastic Dampers – Visco-elastic (VE) dampers utilize high damping from VE materials to dissipate energy through shear  deformation. Such materials include rubber, polymers, and glassy substances.

Visco-elastic Dampers

Visco-elastic Dampers

• Viscous Fluid Dampers – A viscous fluid damper consists of a hollow cylinder filled with a fluid. As the damper piston rod and piston head are stroked, The fluid flows at high velocities , resulting in the development of friction
• Tuned Mass Dampers And Tuned Liquid Dampers – A mass that is connected to a structure by a spring and a damping element without any other support,in order to reduce vibration of the structure; Tuned liquid dampers are similar to tuned mass dampers except that the mass-spring-damper system is replaced by the container filled with fluid

Active Control systems

In the active control, an external source of energy is used to activate the control system by providing an analog signal to it. This signal is generated by the computer following a control algorithm that uses measured responses of the structure

Types of Active Control systems

  1. Active Mass Damper Systems -It evolved from TMDs with the introduction of an active control mechanism.
  2. Active Tendon Systems – Active tendon control systems consist of a set of pre-stressed tendons whose tension is controlled by electro-hydraulic servomechanisms
  3. Active Brace Systems

Semi-active Control Systems

It compromise between the passive and active control devices. The structural motion is utilized to develop the control actions or forces through the adjustment of its mechanical properties. The action of control forces can maintained by using small external power supply or even with battery.

  1. Stiffness control devices
  2. Electro-rheological dampers
  3. Magnetorhelogical dampers
  4. Friction control devices
  5. Fluid viscous dampers
  6. Tuned mass dampers
  7. Tuned liquid dampers

Electro-rheological Dampers

Electro-rheological Dampers

Electro-rheological Dampers

ER fluids that contain dielectric particles suspended within  non-conducting viscous fluids. When the ER fluid is subjected to an electric field, the dielectric particles polarize and become aligned, thus offering resistance to the flow.

Stiffness control devices

Modify:

– the stiffness

-the natural vibration characteristics

So create a non-resonant condition during earthquake

Magneto-rheological Dampers

MR fluid contains micron-size, magnetically polarizable particles dispersed in a viscous fluid. When the MR fluid is exposed to a magnetic field, the particles in the fluid polarize, and the fluid exhibits visco-plastic behavior, thus offering resistance to the fluid flow.

Hybrid  control systems

Combine controls system together

  • Passive + Active
  • Passive + Semi-Active

Smart base-isolation

  • Reduce external power requirement
  • Improve reliability

When loss of electric during earthquake, hybrid control can act as a passive control

  • Reduce construction and maintenance costs due to active or semi-active
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