Seismic Device

Seismic Devices for Structural Control

Durgesh C. RaiAssistant Professor

Department of Civil EngineeringIndian Institute of Technology Kanpur

Kanpur 208 016

Structural Control Strategies

Supplemental DampingDissipate seismic energy

StructuralControl forBridges

Reduce seismic response

Load DispersionLoad sharing across several spans

Seismic IsolationReduce seismic energy input

Shock Transmission Unit (STU) Load

Lead-plug Rubber Bearing

Dampers (Hysteretic,Viscous, Friction)

Isolators (Friction Pendulum)

Load Dispersion

Various Names• STU (Shock Transmission Unit)• LUD (Lock-up Device)• DCD (Displacement Control Unit)• RCD (Rigid Connection Device)

Function• Create a rigid link at a moveable connection during a

shock load, transmitting the force into the adjacent structure.

• The rigid link affords the possibility of sharing the load with other piers or abutments. The rigid link creates a sharing of the total load through all substructure connections.

• Once the shock load is removed the device reverts to its benign influence on the structure

Displn.

Shock loads

Normal loads

Load Dispersion

Conventional Bridge under horizontal loadEach span and pier carry full load

L L L L

Typical 4 span, simply supported bridge

A B DC E

Fixed bearings Sliding bearings

Horizontal load on support: t

45t traction/brakingin span

Total support horizontal support capacity required for traction/braking = = 45t + 45t + 45t + 45t = 180t

45t braking

Load Dispersion

Conventional Bridge with STU under horizontal loadLoad gets distributed to adjacent spans!

A B C D ESTU STU STU

Addition of 4 STUs(for simplicity assume equal stiffness at all supports)

Total support horizontal support capacity required for traction/braking = = 9t + 9t + 9t + 9t + 9t = 45t

45t traction/brakingin span

Horizontal load on support: t

A B C D E

Shock Transmission Unit

• Controlling migration of the medium around the piston • Fluid flows around it when very slow movements occur,• However ‘solidifies’ at rapid movements and can not squeeze through.• Uses thixotropic properties of material, such as silicon

Spherical orplain bearings

Hole in piston or gap around pistonallows slow passage of silicone compound

Piston cylinder packedwith silicone compound

Anchor pinHydraulic seals

Shock Transmission Unit

• Reduce cost of substructure, e.g., Bassien Creek bridge in Mumbai on NH8. 3100 kN STU reduced well fndn. size from 16.5m OD to 12.5m whereas adjacent well dia increased by 1 m only!

• Converts simple structure into redundant structure; many load paths.

ELEVATION

2'-2" + "A"

Earthquake Ground Motion

Characteristics:PGA, PGV, PGDFrequency distributionDuration

Response Spectra

Flexible systemsLarge displacementSmall accelerationDisplacement controlled

Stiff systemsSmall displacementLarge acceleration Force controlled

Intermediate systemsVelocity controlled

Controlling Motion Variables

Accn.Vel.

Displ.

Force or Displacement ?

Demand Response SpectrumAccelerationDisplacementNatural periodDamping

Increasing Period (flexible isolator)and damping (energy dissipator)

can reduce seismic response

Effect of Isolation and Damping

Isolation lengthens the time-period of the structure reducing force response at the cost of increased displacement.Additional damping to control excessive displacement

Single Degree of Freedom System

Equation of Motion

Multiply both sides by and integrate from initial rest state to any time t

Natural Frequency

Damping Ratioωωωω = k m

ξξξξ ωωωω= c m( )2

mu cu ku mug!! ! !!+ + = −

Seismic Energy

0 0 0mu cu dt F udt mug udt

t t! ! ! ( !! ) !+ + = −! ! !

KineticEnergy

ViscousDampingEnergy

Strain &HystereticEnergy

InputEnergy

W W W W IE D S H+ + + =( )

Dissipative Energy ComponentsW WD H+

Seismic IsolationReduce Energy input

Energy Dissipation DevicesAbsorb energy

Various energies

Kinetic energy Viscous Damping energy

Hysteretic energy

Energy Distribution

Hysteretic Energy

Viscous Damped Energy

Strategies for Earthquake Resistance

Enhance Energy Dissipation Capacity for Fixed Base Structures• Hysteretic Loss or Yielding• Ambient Viscous Damping

Ambient damping Cs and hysteretic capacity of Ks is inadequatefor earthquake resistance

Effect of Damper

Energy Dissipation Device (EDD) will minimize/eliminateinelastic action in primary structural members and reduce non-structural damage

Three Types1) Viscous & Visco-elastic 2) Friction 3) Metallic

Effect of Viscous Damping

The response of higher damped system will be less as effect of individual earthquake pulses will damp out quickly and will not accumulate.

��

0 1 2 3 4 5

Frequency (Hz)

2% of CriticalDamping

��

0 0.05 0.1 0.15 0.2

5%Damping Ratio

Range of InherentAmbient Damping

��

Free Vibration

Harmonic Excitation

• Damping is effective when excitation frequency is in vicinity of natural frequency

��

0 0.5 1 1.5 2 2.5

Frequency Ratio

��

Effective Range

��

Supplemental damping can effectively reduce system response for earthquake type motions.

��

0 5 10 15

Frequency (Hz)

Sylmar Motion(Northridge 1994)

��

0 0.1 0.2 0.3 0.4 0.5

Damping Ratio

5% Normalization

Upper Bound

Lower Bound

22 ground motionsStructures with period 0.5 to 3 sAshour (1987)

Energy Dissipation Devices

Metallic System• Based on hysteresis behaviour of metals where

deformation in post-elastic range is used• Utilizes flexure, shear or extensional modes of

deformation

Desirable Features• Stable hysteretic behaviour• Long term reliability• Resistance to environmental factors

Rubber Bearing Isolator

• Laminated rubber bearing (LRB) for isolation and damping

• Lead-plug LRB for increased stiffness and damping

Arrangement of LRB Isolator

Friction Pendulum Isolators

• Pendulum of long period• Energy dissipation

through friction

Friction Pendulum Isolator

Capacity = 265 kLShell thick. = 150 mm, Diameter = 4.5 mLong. Steel = 0.283%

Friction Pendulum Isolator

Rocker Isolator and Damper

Orificed Viscous Fluid Damper

Dissipates energy through movement of a orificed piston in a highly viscous fluid.

Design Considerations

• Objective is to enhance damping and improve dispersion of loads without making natural time period too large.

• Natural period of the bridge with isolation should be at least twice of the natural period without isolation devices.

• For isolated structure, adequate gaps needed to accommodate associated large displacement.

Design Considerations

Seismic Isolation NOT effective if– Ground around the bridge is not stable. That is

susceptible to liquefaction or lateral spreading.– Superstructure is on a very flexible substructure

Not suitable for long piers.Further increasing period has diminishing returns!Piers may have to be stiffened

– Bridge bearings experience uplift Behaviour rather difficult to predict!

Unseating Prevention Systems

Objective is to supplement the role of seat length– Should operate before unexpected relative

displacement occurs.– If it works as expected, seat width may not be needed– Provide a fail-safe function against unexpected

situation.– Could consists of

Connection of superstructure to substructureProtuberance to both superstructure and substructureConnecting two spans of superstructure together

Providing protuberances on the superstructure and the substructure

Connecting Superstructure to Substructure

Chaining steel girder to substructure

Connecting two superstructures together

Connecting two adjacent plate girders of superstructure

Connecting two adjacent trusses

Connecting two adjacent cross-beams

Connecting two adjacent main girders

Bracing and sharing load

Inelastic deformation permitted in specially detailed regions of structure provide energy dissipation potential. However,

• Results in damage to structural members• Large drifts required cause extensive non-

structural damage

��

-150 0 150

Roof Displn. (mm)

STMF (X-DIag.)

Seismic Device

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