In Collaboration In Collaboration with NUST with NUST 15-16 May 2007 15-16 May 2007 PAKISTAN ENGINEERING COUNCIL PAKISTAN ENGINEERING COUNCIL Two Days Short Course on Two Days Short Course on EARTHQUAKE EARTHQUAKE ENGINEERING ENGINEERING
Mar 29, 2015
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!