in collaboration with nust 15-16 may 2007 in collaboration with nust 15-16 may 2007 pakistan...

In Collaboration with NUSTIn Collaboration with NUST15-16 May 200715-16 May 2007

PAKISTAN ENGINEERING COUNCILPAKISTAN ENGINEERING COUNCIL Two Days Short Course onTwo Days Short Course on

EARTHQUAKEEARTHQUAKE ENGINEERINGENGINEERING

DAMPERS AND BASE ISOLATION

SEQUENCE

• UNDERLYING CONCEPT

• DAMPERS AND THEIR TYPES

• BEARINGS AND THEIR TYPES

• APPLICATIONS

Earthquake protection - present

Shear walls are made of reinforced concrete and add strength and stiffness to the building. Shear walls can well handle short duration earthquakes, but if the duration of an earthquake is longer, the stresses developed in the shear walls become so high that they may fail and cause building collapse. Shear walls in a structure can raise the structural cost by 7-10 %

Earthquake protection - present Braced frames and moment-resistant frames maintain structural

integrity by flexibility

Earthquake protection - concept• Approach - force vs energy

– Use of seismic forces– Based on acceleration– Review resulting lateral drift/movement

• Conflict - for stiff soil site, larger the acceleration– Larger the seismic force– Stronger the structure– Stiffer the structure– Higher the seismic acceleration/forces

Earthquake protection - concept• Strong stiff structure is good conceptually and

lateral drift is also minimum• Contradiction comes from energy dissipation

which comes from larger displacements• Dissipation of seismic forces require large

displacements, drift control requires smaller displacements

• Useful seismic systems should have predictable stable and non degradable cyclic behavior

• Base isolation, Moment frames with dampers and Dual-resistance systems with built-in redundancy

Earthquake protection - trendsA building can itself dissipate or dampen the energy from earthquakes. However, the capacity of buildings to dissipate energy before they begin to deform and damage is quite limited. By equipping a building with devices which have high damping capacity, we can greatly decrease the seismic energy entering the building, and thus decrease building damage. Damping devices are usually installed as part of bracing systems. This arrangement provides the column with additional support. Most earthquake ground motion is in a horizontal direction; so, it is a building's columns which normally undergo the most displacement relative to the motion of the ground.

Earthquake protection - trendsThe most common advanced technique is base isolation. The structure is supported by a series of bearing pads which are placed between the building and its foundation. The bearing pads are made from layers of rubber sandwiched together with layers of steel. The bearing is very stiff and strong in the vertical direction, but flexible in the horizontal direction.

Braced frames and moment-resistant frames

DAMPERS

Fluid dampersThe fluid damper consists of a stainless steel piston

with bronze orifice head. It is filled with silicone oil.

The piston head utilizes specially shaped passages

which alter the flow of the damper fluid and thus alter

the resistance characteristics of the damper.

Linear friction dampersLinear friction dampers consist of sliding steel plates

and work on the principal that when two metal surfaces

slide, friction heat is produced and energy gets

dissipated. These types of dampers are susceptible to

corrosion and cold welding which has a direct effect on

the yielding threshold.

Rotational friction dampersThe rotational friction damper dissipates energy by friction of two plates rotating around a friction pad.

It is the same principle as when a car is braked and its kinetic energy is dissipated through the rotational movement in the disc brake around a friction pad. The harder the brake is clamped, the more energy is dissipated.

Energy dissipation various methodsThe hysteresis (force-deflection curve) shows the

efficiency of a damper during an earthquake. The (blue)

area inside the curve is the total dissipated energy. The

curve shows that the rotational friction damper

dissipates more energy than any other damper today.

Friction Yielding metal

Viscoelastic Fluid

CONCEPT OF BASE ISOLATION

0.0

a)

feasiblebuildings

PERIOD

SP

EC

TR

AL

AC

CE

LE

RA

TIO

N

0.5

isolationrange

3.0Tf Ti2.0 2.51.0 1.5

20%10%5%

b)

Tf

PERIOD

5%

10%

20%

1.0 1.5 2.0 2.5 3.0

SP

EC

TR

AL

DIS

PL

AC

EM

EN

T

Ti0.50.0

period shift

applied flexible isolators

applied dam pers

Rigid body motionInter story drift

SOFT FIRST STORY-IZMIT

Strong bending moment

Anti-EQ design to make substructure (piers) of bridges safe for this bending moment.

Fixed support

Movablesupports

The most suitable approach to seismic protection by replacing conventional bearings by HDRB

To protect superstructure

To protect substructure

ELASTOMERIC - BASED SYSTEMS

Base Isolation

Separate Type

Standard LaminatedRubber Bearing

+Steel-bar Damper

+Lead Plug Damper

+Hydraulic Damper Lead rubber bearing

High damping rubberbearing

Built-in type

MECHANICAL CHARACTERISTICS OF ELASTOMETRIC BEARINGS

Horizontal stiffness KH=GA/tr

G- Shear modulus of elastomer, A is a full cross section area, tr -total thickness of rubber

Vertical stiffness Kv=EcA/tr

Ec-instantaneous compression modulus of rubber steel composite and iscontrolled by shape factor S, for circular pad of radius R and thickness t, S=R/2t, for

square pad of site a and thickness t, S=a/4tFor single circular pad Ec=6GS2 For square pad Ec= 6.73GS2

Natural rubber/ high damping bearingsAt 100% shear strain damping is increased between 10%-20%.Damping is neither viscous nor hysteretic (between).

Many steel shims (verticalload capacity & stiffness and prevent lateral bulging of rubber.

Thick steel endplate

Composite material rubber+steel shims

Rubber sheets are vulcanized andbonded onto thin steel Plates under pressureand heat

Neoprene becomes extremely stiff at -40 C and natural rubber at -55 C

LEAD RUBBER BEARING(the most frequently used)

Energy dissipation core (reduced EQ forces & displacement by energy dissipation)

Internal rubber leria+steel reinforces plate

Cover rubberProtecting steel plates from corrosion

Thick steel endplate

Steel plates in bearing force lead plug to deformed in shear.

Lead must fit tightly in element bearing,lead plug is slightly larger than hole (1%) and forcing it in.

Friction pendulum (spherically shaped sliding bearing)-the biggest in the world

Spherical stainless steelsurface

Slider

Lateral force that slidethe structure depend ofcurvature and vertical load

Energy dissipation is generatedby friction between slider andspherical surface

Coefficient of friction is not constant (varies with time and temperature)

Laminated Rubber Bearing (low-damping natural) + Steel Rod Damper

Material in shearquite linear up toShear strain of 100%damping 2-3%

Purpose to have linear viscous dynamic model (rubber fitted the model damper was not exactly

linear viscous element)

AdvantageSimple to manufactureDisadvantage require elaborate connections and metallic dampers are prone to low-cycle fatigue

RUBBER BEARING,STEEL DAMPERS AND OIL DAMPER

Pseudo Dynamic Test

-150 -100 -50 0 50 100 150-150

-100

-50

0

50

100

150

Load-Displacement Hysteresis Loop of Pier (Type SI)

Late

ral L

oa

d (

kN)

Displacement (mm)

Elastic Behavior of PierElastic Behavior of PierPseudo Dynamic TestPseudo Dynamic Testof Seismically Isolated Bridgeof Seismically Isolated Bridge

Dynamic Field Test

Low level test High level test

Imposed initial displacement Yield displacement of isolator Design displacement of isolator

Isolator behavior Linear Nonlinear

tonf

sec

Loading Stabilization Quick Release

Po

t1 t2

"PESTALOZZI“ school first base isolated

building in the world

4-story, 8-unit apartment building in West Java, Indonesia completed 1997.

Implementation to Seismic RetrofitExisting Retrofitted

Steel Bearings

Dampers(LRB or HDR)

Friction Bearings Concrete Block

OLDOLD

NEWNEW

• Seismic Retrofit of Dangsan Railway Bridge (1999)– Replacement of the old bridge due to cracks in superstructure– New superstructure – Increasing seismic capacity using seismic isolator– Reuse of foundations– Retrofit of concrete piers

ACTIVE MAGNETO-RHEOLOGICAL FLUID DAMPER

Steel cross beam

Horizontal actuator

Moving platen

Vertical force = 53 400 kN Longitudinal force=8 900 kN Lateral force= 4 450 kN

Vert. Dis=0.127mLon. Dis.=1.22mLat. Dis.=0.61m

Prestressed reaction wall

Energy Dissipation

Appropriate Scale

FUTURE!