23-08-2012, 11:03 AM
EARTHQUAKE PROTECTION SYSTEM
EARTHQUAKE PROTECTION SYSTEM.pptx (Size: 3.15 MB / Downloads: 92)
What is Structural Control?
Mechanical system employed to reduce structural vibrations
Enhance the safety and habitability of structures
Vibration control of civil structures is more in 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.
Retrofitting reqd. if new seismic activity detected
High strength may result in high acceleration levels, so increasing strength alone wont always work.
Passive Structural Control
Tend to be very simple systems• Requires no external power to operate
Simply impart forces which are developed in response to
structure’s motion
Passive control system include:
Tuned mass dampers
Energy dissipaters
Seismic isolation
These systems have significant application to buildings, bridges and industrial plants
Tuned mass dampers
A tuned mass damper, or harmonic absorber, is a device mounted in structures to prevent discomfort, damage or outright structural failure by vibration. Typically, the dampers are huge concrete blocks mounted in skyscrapers or other structures, and moved in opposition to the resonance frequency oscillations of the structure by means of springs, fluid or pendulums.
Contd…
The TMD consist of
Spring
Oscillating mass
Viscodamper
As main components, or may be
designed as pendulum, also with
A combination of viscodamper.
Millennium Bridge
First pedestrian bridge crossing over the river
thames
It is a 325m steel bridge
The maximum sway of the deck was approximately 70mm.
Research indicated that the movement was caused
by the sideways loads we generate when walking.
When we walk, in addition to our weight, we create a repeating pattern of forces as our mass rises and falls. Also a small sideways force caused by the sway of our mass as our legs are slightly apart.
Contd…
There are two fundamental ways to limit dynamic excitation:
Stiffen the structure: The additional structure required to do this would dramatically change the appearance of the bridge.
Add damping to absorb the energy: It was decided to adopt a damping solution, either active damping or passive damping.
Active damping: It was too complex, expensive and production
times were too long for this
Passive damping: The bridge deploys two forms, Viscous dampers
and Tuned Mass Dampers.
Viscous dampers are located under the deck, around the piers to
control the lateral motions.
The tuned mass dampers are also located beneath the deck and
reduce vertical movements.
Dampers or Energy Dissipaters
Seismic dampers can be used in place of structural elements, such as diagonal braces.
Dampers act like the hydraulic shock absorbers in cars.
Dampers were used since 1960s to protect tall buildings against wind effects. It was only since 1990s, that they were used to protect buildings against earthquake effects.
Four basic types of dampers:
Traditional Viscoelastic dampers
Friction dampers
Metallic dampers
Fluid viscous dampers
Traditional Viscoelastic dampers
These are stacked plates separated by inert polymer materials.
Pose problem over varying temperature.
Not achieved much success in practical applications due to undesirable added spring effect of these devices.
There are no manufacturers that manufacture purely viscoelastic damper.
Friction dampers
Consist of sliding steel plates
work on the principal that when two metal surfaces slide, friction heat is produced and energy gets dissipated.
susceptible to corrosion and cold welding which has a direct effect on the yielding threshold
There are some maintenance problems.
Metallic dampers
Consist of multiple steel plates which yield when a threshold force is reached.
As the metal yields energy is dissipated.
These dampers are required to be replaced after every seismic event.
Over a period of time they have also not been able to catch the momentum as the technology in the other damper field has fast progressed.
Fluid viscous dampers
They have existed for a long time and were
developed and used in the aerospace
industry
Fluid viscous dampers are fluid filled metal
cylinders with pistons and work like shock
absorbers.
superior for both seismic and wind
applications
They absorb energy at all frequency ranges
of the earthquake and also do not need to be
replaced after an earthquake.
Have a great flexibility in design and can be
configured to protect against an earthquake
of any magnitude.
Contd…
Less displacement . . . over 50% reduction in drift in any
cases
Decreased base shear and inter-story shear, up to 40%
Reduced displacements and forces can mean less steel
and concrete. This offsets the damper cost and can
sometimes even reduce overall cost
Only fluid dampers reduce both stress and deflection
Easily installed in a structure as diagonal braces or as
part
Stable, predictable performance at any temperature
Long life, no maintenance
Base isolation
Is a collection of structural elements of a building that should substantially decouple the building's structure from the shaking ground thus protecting the building's integrity.
Structure-foundation interface is occupied by “isolators”
Deformation of system is in the isolators
Typical for high frequency structures, Short and stocky structures
Objectives:
Reducing the stiffness
Increasing the natural period of system
Provision of increased damping to increase the energy dissipation in the
system.
Base isolators
High-damping rubber: Pure rubber isolators are
softer and allow greater movement
Lead-core rubber: lead core isolators absorb some
of the seismic energy by yielding and also force the
isolator back into place quicker.
Friction pendulum: Friction pendulums isolators
permit a lower displacement profile than the
rubber counterparts. They function like a ball on a
curved plate
Types of base isolation
Use of elastomeric bearings
Elastomer made of either natural rubber or neoprene
Structure is decoupled from the horizontal components of the
earthquake ground motion
This gives the structure a fundamental frequency that is much
lower than its fixed-base frequency
sliding system
Transfer of shear across the isolation interface is limited
The friction-pendulum system is a sliding system using a
special interfacial material sliding on stainless steel
Base Isolation in Real Buildings
It has been in increased use since 1980’s
Base isolation has now been used in numerous
buildings in various countries
Base isolation is also useful for retrofitting
important buildings like hospitals and historic
buildings
In India
Two single storey buildings in Killari town were
built with rubber base isolators resting on hard
ground.