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EARTHQUAKE RESISTANT
STRUCTURAL SYSTEMS
Dr. K.NagamaniProfessor
Structural Engineering Division
College of Engineering, Chennai-25.
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As is said often quakes dont kill
people, it is the unsafe buildings, whichdo. The Bhuj quakes aftermath is aliving example of this.
codes are not mandatory and hence notadhered to. As a result, even structures
in urban areas like Ahmedabad crashedliterally like a pack of cards.
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Loading Pattern and ResultingInternal Structural Actions
Level 1
The frame
Level j
Level j+1
Roof
OturningMoment
Forces Shear forces
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Types of Control Structures
Conventional structures
Passive vibration Control
Semi-Active and Active vibrationControl
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Configuration
architectural shape and size;
type, size and location ofstructural elements;
type, size and location of non-structural elements.
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PLAN OF
BUILDING
(Asymmetry should be
avoided)
Asymmetricbuildings
undergo torsionand the extreme
corners ofasymmetric
buildings aresubjected to very
large earthquakeforces
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GENERAL SHAPE OF BUILDING
Very slender
buildings should
beavoided
Inverted pendulum type buildings areunstable
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GENERAL SHAPE OF BUILDING
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Projections and large overhangs
Avoid long projected
balcony
Large projections
should be avoided
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Floating columns
Large overhangs, projections and floating columns attract largeearthquake force and therefore likely to damage/collapse due tounstability
S ti f di i il
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Separation of dissimilar
buildings
To avoid
collision,
adjacent
dissimilarbuildings should
be separated by a
minimum gap
Type of construction Minimum gap
per storey(mm)
Load Bearing Building 15
RCC Frame Building 20
Steel Frame Building 30
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Buildings
with softstorey
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Weak beamand strong
columndesign
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Conventional
Structural SystemsThe main vertical resisting systems
for earthquakes are:
shear walls;
braced frames; moment resisting (or rigid) frames.
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Horizontal Diaphragm
Acts as a horizontal I-beam. That is, the diaphragm
itself acts as the web of the beam and its edges act as
flanges
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Shear Walls
Shear walls are vertical walls that are designedto receive lateral forces from diaphragms andtransmit them to the ground. The forces in
these walls are predominantly shear forces inwhich the fibers within the wall try to slide pastone another. When you build a house ofcards, you design a shear wall structure, and
you soon learn that sufficient card "walls" mustbe placed at right angles to one another or thehouse will collapse.
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Shear Walls
B d F
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Braced Frames
Braced frames act similarly to shear walls.The most common material for braced-frame construction is steel in the form ofrolled sections or tubes. Where diagonal
bracing is used, the braces in compressionare sometimes ignored because ofbuckling. Where the bracing is in onedirection only (within the plane of the
braced frame) the diagonal member mustbe proportioned to prevent buckling when incompression.
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Moment Resistant Frames
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Non-structural Components
It is common place for engineers to ignore thestructural effect of these elements. In some
cases the non-structural elements provide
accidental strength to the building. They may, however, interfere adversely with
the structural behaviour of the essential load-carrying structure.
This could lead to unanticipated overstressingof essential load-carrying members.
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Nonstructural Components
Partition walls
Architectural Elements
Mechanical Elements
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HEAVY MASS ON TOP - W.T. COLLAPSE
WHIPPING EFFECT
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Basic Configuration Issues and
Structural Response
The size of a building
The height of a building
horizontal dimensions
height/width ratio to 3 or 4
symmetry
redundancy
soft storey concept is very dangerous
strong column weak beam
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A WEAK COLUMN -
STRONG FLOOR SYSTEM
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A WEAK COLUMN -
STRONG BEAM SYSTEM
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WEAK COLUMN - STRONG BEAM SYSTEM
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WEAK COLUMN-STRONG ROOF SYSTEM
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EXCESSIVE TOP CANTILEVERS
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HEAVY CANTILEVER FRONT FACADE
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HEAVY MASS ON TOP - W.T. COLLAPSE
WHIPPING EFFECT
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Insufficient connection between the RC elevator core
and rest of the building lead to the underutilization of
the lateral strength and stiffness of the elevator core
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Collapse of upper storey of a building
at Gandhidham. It is suspected that this
may have been caused by inadequate
lap lengths in the column
reinforcement.
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No building is earthquake-proof. But a properly engineered tall
building should be able to withstand the maximum credible
earthquake for its area without collapse, and lesser seismic eventswithout major structural damage,
says R. Shankar Nair, Chairman of Council on Tall Buildings
and Urban Habitat, Chicago.
Of course, mistakes do happen, even in the U.S. But if
American standards of design and construction had prevailed in
the Bhuj area (an economic impossibility, of course), there
would have been casualties from the collapse of a few small
buildings and from falling objects, but no large, recently-built
multi-storey building should have collapsed.
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Materials
high ductility;
high strength-to-weight ratio;
homogeneity;
ease in making full-strength
connections.
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Some Advanced Earthquake
Resistant Techniques
Base Isolation
Energy Dissipation Devices
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Energy Dissipation Devices
Friction Dampers
Metallic Dampers
Viscoelastic Dampers Viscous Dampers
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What is Structural Control?
Mechanical system employed to reduce structural vibrations
Control deviceControl algorithms Enhance the safety and habitability of structures Interested in numerous sources of vibration
Winds (Strong gusts and typhoons)Earthquakes (Weak and strong)Machinery Types of structural controlPassive (Very common approach to control)
ActiveSemi-Active
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Concept of Semi- and Active
Control
Two approaches to the employment ofactive and semi-active structural controlsystems:
Feed-back control (most common)
Feed-forward control (least common)
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Passive Structural Control
Tend to be very simple systems
Requires no external power to operateSimply impart forces which are developed inresponse to structures motion
Relatively inexpensive solution to reducingstructural vibrations
Usually compact and non-invasive to
architectural spaces Limits exist on the amount of control attainable
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Passive Control Device Types
Viscoelastic Dampers:
Contains a viscoelastic polymer
sandwiched between two metal plates.
Viscoelastic polymer deforms through
shear action removing energy from thesystem.
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Viscoelastic Polymer BraceDamper
Cylindrical viscous damper (CVD) a
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Cylindrical viscous damper (CVD), adamper using the shearing resistance of aviscous fluid, consists of three concentric
steel tubes filled with viscous fluid.
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CVD
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Lead Extrusion Damper
T i l i t ll ti (b ) f
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Typical installation (brace) ofLead Extrusion Damper
Bingham Material damper, using viscous
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g p , gresistance of a special filler, consists of a fluid filled
cylinder, a piston and a rod.
The Oiles Viscous Wall Damper is a vibration attenuator
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p
using the shear resistance force of a highly viscous fluid.
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Base Isolation
lead-rubber bearings
These are among the frequently-usedtypes of base isolation bearings
B I l t d d Fi d B
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Base-Isolated and Fixed-Base
Buildings
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Lead-Rubber Bearing
B I l t d Fi d B
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Base-Isolated, Fixed-Base
Buildings
BASE ISOLATION SYSTEM
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BASE ISOLATION SYSTEM
Without Base Isolation With Base Isolation
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Wh t i th T d M D S t ?
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What is the Tuned Mass Damper System?
This is a system for absorbing the vibrations within a
building generated by high winds or an earthquake.There are two main systems classified as passive controland active control. The passive control systems, such usa TMD (Tuned Mass Damper) using a weight whichoscillates at the same period as the building does and an
additional damper that connects two relatively movingpoints when the building oscillates, absorbs thevibrations automatically without the need of an electricalcontrol system. The active control systems use acomputer-controlled actuator to realize the best
performance. They are AMD (Active Mass Damper)which suppresses the oscillation of a building byactuating a weight and an ABS(Active Brace System)which controls axial forces of braces and others.
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Function
During an earthquake or strong wind, every
building shakes at its own natural perioddepending on its rigidity and size.TMD-RP and AMD (Active Mass-added Damper)move so that the additional mass of the vibration
control system offsets the motion of the buildingto absorb vibration energy.
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A Second Type of Base Isolation:
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Seco d ype o ase so at oSpherical Sliding Isolation
Systems
Damping Devices and Bracing
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Damping Devices and Bracing
Systems
Examples of Building
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Examples of Building
Applications
San Francisco Airport International Terminal Owner: City & County of San Francisco Engineer: Skidmore, Owings & Merrill Friction PendulumTM seismic isolation of this
new building protects the expansive glassexterior walls and the long span rooftrusses. Use of FrictionPendulumTM bearings, instead of the
equivalent rubber bearings, saved 600 tons ofstructural steel. With over 1.2 million sq. ft. ofsupported space, it is the largest seismicallyisolated building in the world.
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U.S. Court of AppealsOwner: General Services AdministrationEngineer: Skidmore, Owings & Merrill
Seismic retrofit of this historic building usingFriction PendulumTM bearings saved $7.6million in construction costs and 80,000 sq. ft. ofbasement space, compared to the rubber
bearing design. The project won the 1994 GSADesign Award for Engineering, Technology &Innovation
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American River Bridge
Isolation Bearings Lower Construction Costsand Double Seismic Resistance Capacity
The American River Bridge at Lake Natoma in Folsom, California, isf th l t b id t i i i l ti F i ti
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one of the largest new bridges to use seismic isolation. FrictionPendulumTM seismic isolation allows the bridge to elastically resistthe safety level earthquake, with no structural damage.
The use of seismic isolation bearings saved $1 million inconstruction costs, compared to the non-isolated bridge design. Theconstruction cost savings came from a reduction in the size of thedrilled caissons. Seismic force demands for the non-isolated bridgedesign, would have been more than twice the bridges's elasticstrength capacity. Consequently, a non-isolated bridge would havebeen expected to sustain structural damage during the safety leveldesign earthquake event.
The bridge structure consists of two post-tensioned concrete boxframes on piers supported by 8 foot diameter drilled caissons. The48 Friction PendulumTM bearings are located on top of the piersand abutments. The bearings have a 10 inch displacement capacityand support dead plus live loads of up to 4 million pounds. Thebearings were installed pre-displaced so as to accommodateconstruction movements from post-tensioning and concreteshrinkage.
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White River Bridge
Seismic Isolation Bearings Subject ToExtreme Cold Temperatures
The new White River Bridge constructed in the Yukon,C d i t d 9 F i ti P d l TM i i
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Canada, is supported on 9 Friction PendulumTM seismicisolation bearings. It is a 590 feet long, steel girderstructure consisting of 2 spans which carry 2 lanes oftraffic. Use of Friction PendulumTM seismic isolationbearings achieved an elastic structure response for thedesign level earthquake (0.2g peak ground acceleration),at a substantially lower cost than would have beenpossible without isolation bearings.
Because of its location in northern Canada, the White
River Bridge is subjected to extreme temperatures. TheFriction PendulumTM seismic isolation bearings are ableto maintain their design properties over a wide range oftemperatures, including extreme cold conditions. Thebearings maintained their design stiffness and damping
when tested over a temperature range of -94F to+140F. When tested at temperatures as low as -166F,they demonstrated stable performance without incurringbearing damage.
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Emergency Operations CenterOwner: State of WashingtonEngineer: KPFF Consulting Engineers
Friction PendulumTM seismic isolation ofthis essential emergency facility ensurescontinued operations following an
earthquake.
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Citicorp Building NYC-TMD
Built in 1978 with an unusual base
400 ton TMD installed on top
Deflections reduced by 40%
The mass moves in the opposite directionto building movement
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ACTIVE MASS DAMPER(AMD)
During winds and small earthquakes Large mass whose motion is controlled by an
actuator
Velocity feedback system
Mass of AMD is 1% of the building weight
Need large amount of power
20% damping in first mode and 5% in second
mode With the AMD at top, displacement at roof level
is reduced by to 1/3
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Kyobashi Siewa Building
1989, Tokyo, Japan
Extremely narrow building(33mx4m)- 10Stories Steel Construction
First Active structural controlled building inthe world
system: TMD
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system: TMD
Crystal Tower Osaka Prefecture (Completed in1990)Design: Takenaka CorporationTotal Floor Space:85,994 m2Number of floors: 2 floors below ground, 37floors above groundStructural control device:TMD with six 90t weight masses (Utilizing crystalice heat storage tanks)
Object: Vibration control for a building againstheavy wind(Transverse movements in two directions )
Crystal Tower Osaka Prefecture
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Application of vibration control
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Application of vibration control
structure system: AMD
Applause Tower Osaka Prefecture (Completedin 1992)Design: Takenaka CorporationTotal Floor Space:96,793 m2Number of floors: 3 below ground, 34 abovegroundStructural control device: AMD, mass weight480t(Utilizing a heliport)
Object: Suppression of building vibration instrong winds or medium/small earthquakes(Parallel movements in two directions)
A l T
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Applause Tower
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