Civil Engineering Applications of Vibration Control (Structural Control)

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Civil Engineering Applications of Vibration
Control (Structural Control)

• Naresh K. Chandiramani, Associate Professor
• Room 141, Dept. of Civil Engineering

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Structural Control

• Vibration control of civil structures is more
  recent as compared to machines aerospace
  vehicles.
• Earthquakes and wind loads - main sources of
  structural vibrations.
• Control vibrations by changing rigidity, mass,
  damping, shape, or applying passive or active
  control forces.
• gt 20 full scale active control appl. in Japan
• Passive base isolation used in USA.
• Retrofitting reqd. if new seismic activity
  detected
• High strength may result in high acceleration
  levels, so increasing strength alone wont always
  work.

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Structural control versus Mechanical Aerospace
control

• Environmental disturbances (wind, earthquake
  excitations) occur over wide range of frequency
  and amplitudes, i.e., they are uncertain, whereas
  mechanical loads are usually deterministic.
• Civil structures (without control) are stable and
  may get destabilized with active control, whereas
  aerospace structures require active control for
  stabilization.
• Performance specifications for civil structures
  are coarse (e.g., peak amplitude, time for motion
  to settle down).

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Mathematical model of structure.
Fig. 1 (a) Mathematical model, (b) Schematic of
a building

• In a simplified model, the masses correspond to
  slab masses and stiffnesses correspond to column
  stiffnesses (i.e, the force required per unit
  lateral displacement of column)

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Passive control Base isolation
Fig. 2 (a) Schematic of base isolated building,
(b) Model, (c) Rubber bearing
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Passive control Base isolation

• Structure mounted on a suitably flexible base
  such that the high frequency component of ground
  motion is filtered out and the fundamental
  vibration period is lengthened. This results in
  deformation in the isolation system only, thus
  keeping the structure above almost rigid.
  However, if the earthquake excitation contains a
  major component of this fundamental period, there
  will be large sidesway (albeit almost rigid)
  motions.
• San Fransisco city hall (retrofitted, 530 rubber
  bearings), International terminal at SF airport
  (267 Friction pendulum sliding bearings).
• Not suitable for tall slender buildings (subject
  to high wind loads). For these auxiliary dampers
  (viscous, viscoleastic) are deployed (eg. WTC).

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Passive control Tuned Mass Damper (vibration
absorber)
Fig. 3 (a) TMD schematic, (b) Response
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Passive control Tuned Mass Damper (vibration
absorber)

• TMD, usually having mass about 1 that of
  structure, fitted to top of building. It is tuned
  to reduce vibration for given frequency range.
• Absorber mass takes up vibratory energy, leaving
  the main mass (building) almost static.
• Not very useful for earthquake excitations which
  occur over wide frequency range.
• Main system properties (stiffness-k1, mass-m1)
  known, absorber system properties (stiffness-k2,
  mass-m2) to be designed such that absorber
  frequency equals excitation frequency (w2w).
• Examples John Hancock Tower (Boston), Citicorp
  Building (New York).

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Passive control Untuned viscous absorber
Fig. 4 (a) Model of untuned viscous absorber,
(b) Response
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Types of passive control devices

• Metallic yield damper relies on the principle
  that the metallic device deforms plastically,
  thus dissipating vibratory energy. Used in
  earthquake applications.
• Friction devices here friction between sliding
  faces is used to dissipate energy. When used in
  base isolation systems, the friction coefficient
  has conflicting requirements. It should not be
  too large otherwise shear forces from ground
  during a strong earthquake will transmit to the
  structure. Also it should not be too small or the
  entire structure will move due to small/medium
  wind/earthquake loads. These devices can also be
  fitted between two storeys to damp their relative
  motion. Used in earthquake applications.

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Types of passive control devices

• Viscous/ Viscoelastic devices Example is fluid
  in a cylinder with piston having an orifice.
  These can also be semi-active (eg., variable
  orifice, variable viscosity). Used in earthquake
  and wind applications.
• Tuned mass dampers problems are size of the mass
  to be used and its displacement relative to the
  structure, in order that damping is effective.
• Liquid sloshing dampers, Impact dampers.

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Classification of Control Methods

• Active/Feedback control
• External source of power drives actuators (i.e.,
  provides input voltage) .
• Voltages required are computed by controller
  using certain algorithms with inputs from
  sensors.
• Sensors measure motion (strains, displ, vel,
  accl.)
• Actuators apply forces to structure, thereby
  adding or dissipating energy.
• Examples of sensors are acceleromters, strain
  gauges.
• Examples of actuators are tendons, solenoids,
  piezoelectric stacks, active mass dampers (AMD).
• Destabilization possible.
• External power may not be available during
  earthquake.

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Classification of Control Methods

• Passive control
• No external power required.
• Passive control device (TMD, Base Isolator)
  imparts forces that are developed directly as a
  result of motion of structure (i.e., no actuator
  involved).
• Total energy (structure passive device) cannot
  increase, hence inherently stable.
• Relatively inexpensive.
• Reliable during earthquake
• Not as effective as active, hybrid, semi-active
  control.

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Classification of Control Methods

• Hybrid control
• Uses active passive devices.
• Advantages of both active and passive systems are
  present and their limitations are reduced.
• Essentially an active control system
• Examples viscous damping with AMD, base
  isolation with actuators, TMDAMD).

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Classification of Control Methods

• Semi-active control
• Uses devices where input power requirements are
  orders of magnitude less than fully active
  devices. In fact in some cases battery power is
  sufficient.
• These devices usually dont add energy to the
  system, hence stability ensured.
• These devices can be viewed as controllable
  passive devices (eg., Magneto-Rheological Fluid
  damper where voltage input applied to change
  viscosity depending on motion measured by
  sensors, variable orifice damper, controllable
  friction devices, variable stiffness devices).
  

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Active Control

• The goal is to design a control system to keep
  stresses/strains/displ./accel. (called outputs)
  at certain locations below specified bounds
  (peak, rms) when disturbances (wind, earthquake)
  below specified bound are applied.
• Designer decides choice of outputs based on
  comfort (e.g. accelerations) and safety (e.g.
  stresses).

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Active Control
Fig. 5 Schematic of an active control system
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Active control
Fig. 6 Implementation of control
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Active control
The above provides a simple comparison between
active and passive control. In passive control,
the additional stiffness is chosen and fixed,
i.e., like a re-designed structure. In passive
control the actuator applies a force to the
original structure, the force being proportional
to the displacement measured by sensor (which is
proportional to the sensors output voltage) . In
active control the main task of design is
determining the proportionality constant
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Active Control

• Outline of Design Process
• Develop mathematical model of the structure and
  the chosen sensors and actuators.
• Adopt a mathematical model for the disturbances
  (i.e., wind, earthquake load).
• Decide performance specifications (eg., peak
  accleration, time to settle down after
  disturbance applied, etc).
• Choose type of control algorithm (i.e., how to
  obtain the proportionality constant and hence
  actuation voltages/forces from sensor voltages).
  Then design controller so that performance specs
  are met. Examples of control algorithms are
  proportional, integral, derivative, PI, PID,
  optimal control, robust control, etc.

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Active control with TMD
Fig. 7 Schematic of AMD applied to building
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Active control with TMD
Fig. 8 AMD on Kyobashi Seiwa building
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Active control

• First full scale application of active control to
  a building was done on Kyobashi Seiwa building
  (Japan) in 1989 (Fig. 7,8). Two AMDs were used.
  Primary one weighs 4t and damps transverse
  motion. Secondary one weighs 1t and damps
  torsional motion.
• Can also use Magnetorheological fluid dampers
  (semi-active), active tendons, etc. (Fig. 9, 10,
  11)

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Semi-Active control with MRD
Fig. 9 Control using MR dampers (a) two dampers
(b) single damper
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Actuators MR Damper
Fig. 10 Magnetorheological damper
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Active control with Tendons
Fig. 11 Active tendons used in control
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Active Control with Tendons