Tanta University
Faculty of Engineering
Earthquake Engineering
Base Isolation
Made by : Abdelkader Gamal Abdelkader
Under supervision of: Dr Tamer Elkorany
Earthquake Protective Systems
Earthquake Protective
Systems
Passive Protective Hybrid Protective Active Protective
Systems Systems Systems
Tuned Mass Active Mass
Active Isolation
Damper Damping
Semi-Active
Energy Dissipation Active Bracing
Isolation
Semi-Active Mass
Base Isolation Adaptive Control
Damping
Base Isolation is the most common System
Base Isolation (or Seismic Isolation)
Isolation of superstructure from the foundation is known as
base isolation.
It is the most powerful tool for passive structural vibration
control technique
Fig: Base Isolated Structures
Concept of Base Isolation
Concept of Base Isolation
The concept of base isolation is explained
through an example building resting on
frictionless rollers. When the ground
shakes, the rollers freely roll, but the
building above does not move(As shown in
Fig (a)). Thus, no force is transferred to the
building due to the shaking of the ground;
simply, the building does not experience
the earthquake.
Concept of Base Isolation
Concept of Base Isolation
Now, if the same building is rested on the flexible
pads that offer resistance against lateral
movements fig (b), then some effect of the ground
shaking will be transferred to the building above.
If the flexible pads are properly chosen, the forces
induced by ground shaking can be a few times
smaller than that experienced by the building built
directly on ground, namely a fixed base building
fig (c). The flexible pads are called base-isolators,
whereas the structures protected by means of these
devices are called base-isolated buildings. The
main feature of the base isolation technology is
that it introduces flexibility in the structure.
As a result, a robust medium-rise masonry or reinforced
concrete building becomes extremely flexible. The isolators
are often designed, to absorb energy and thus add damping
to the system. This helps in further reducing the seismic
response of the building. Many of the base isolators look like
large rubber pads, although there are other types that are
based on sliding of one part of the building relative to other.
Also, base isolation is not suitable for all buildings. Mostly low
to medium rise buildings rested on hard soil underneath;
high-rise buildings or buildings rested on soft soil are not
suitable for base isolation.
Consideration for Seismic Isolation
The benefits of using seismic isolation and energy dissipation devices (isolators for
simplicity) for earthquake-resistant design are many:
Isolation leads to a simpler structure with much less complicated seismic analysis as
compared with conventional structures;
Isolated designs are less sensitive to uncertainties in ground motion;
Minor damage at the design level event means immediate reoccupation;
The performance of the isolators is highly predictable, so they are much more reliable than
conventional structural components (e.g. some ductile walls in the Christchurch
earthquakes); and finally,
Even in case of larger-than-expected seismic events, damage will concentrate in the isolation
system, where elements can be easily substituted to restore the complete functionality of
the structure.
Basic elements of seismic isolation
The three basic elements in seismic isolation systems
that have been used to date are :
A vertical-load carrying device that provides lateral flexibility so that the period
of vibration of the total system is lengthened sufficiently to reduce the force
response,
A damper or energy dissipater so that the relative deflections across the flexible
mounting can be limited to a practical design level, and
A means of providing rigidity under low (service) load .
Seismic- isolation design principle
Factor of Safety = Column Strength/Earthquake force > 1
Factor of Safety = Capacity/Demand > 1
Capacity > Demand
The earthquake causes inertia forces proportional to the product
of the building mass and the earthquake ground accelerations.
As the ground accelerations increases, the strength of the
building, the capacity, must be increased to avoid structural
damage.
1. From the above figure:-The top curve shows realistic
elastic forces based on a 5%ground response
spectrum will be imposed on a non- isolated structure .
2. The spectrum shown is for a rock site if the structure
has sufficient elastic strength to resist the level of the
load.
3. The lowest curve shows the forces which the uniform
building code requires a structure to be designed for.
4.And the second lowest curve shows the probable strength
assuming the structure is designed for the UBC(Uniform
Building Code) Forces.
5.The probable strength is 1.5 to 2.0 times higher than the
design strength because of the design load factors. actual
material strength which are greater in practice than those
assumed for design, conservatism in structural design, and
other factors.
6.The difference between maximum elastic force and the
probable yield strength is an appropriate indication of the
energy, which must be absorbed by ductility in the structural
elements.
Types of Base Isolations
Base isolation systems which uses Elastomeric Bearings
Base isolation systems with Sliding System
Fig: Elastomeric Isolators
Elastomeric Base Isolation Systems
This is the mostly widely used Base Isolator.
The elastomer is made of either Natural Rubber or Neoprene.
The structure is decoupled from the horizontal components of
the earthquake ground motion
#A layer with low horizontal stiffness is introduced between
the structure and the foundation
Fig: Steel Reinforced
Elastomeric Isolators
Sliding Base Isolation Systems
It is the second basic type of isolators.
This works by limiting the base shear across the isolator
interface.
Fig: Metallic Roller Bearing
Spherical Sliding Base Isolators
The structure is supported by bearing pads that have
curved surface and low friction.
During an earthquake, the building is free to slide on the
bearings.
Fig: Spherical Sliding Base Isolator
Friction Pendulum Bearing
works whichisolatorsbasedesignedspeciallyareThese
on the principle of simple pendulum.
It increases the natural time period of oscillation by
causing the structure to slide along the concave inner
surface through the frictional interface.
It also possesses a re-centering capability.
Fig: Cross-section of Friction Pendulum Bearing
Friction Pendulum Bearing (Contd.)
Typically, bearings measure 3 feet in dia., 8 inches in height and weight being
2000 pounds
Benicia Martinez Bridge, California is one of the largest bridges to date to
undertake a seismic isolation retrofit.
Largest seismic isolation bearings, measuring 13 feet in diameter, and weighing
40,000 pounds. They have a lateral displacement capacity of 53 inches, a 5
million pound design dead plus live load, and a 5 second period.
Fig: Bearing used in Benicia Martinez Bridge (left) and Benicia Martinez Bridge (right)
Effectiveness of Base Isolation
Fig: A 3-D Model of a building in SAP2000
Effectiveness of Base Isolation
Fig: Comparison Stresses in Z direction for Fixed Base (left) and Isolated Base (right)
Effectiveness of Base Isolation
Fig: Comparison of Shear Stresses in Y-Z direction for Fixed Base(left) and Isolated
base (right)
Advantages of Base Isolation
Isolates Building from ground motion
Lesser seismic loads, hence lesser damage to the structure.
Minimal repair of superstructure.
Building can remain serviceable throughout construction. Does
not involve major intrusion upon existing superstructure.
Disadvantages of Base Isolation
Expensive
Cannot be applied partially to structures unlike other retrofitting
Challenging to implement in an efficient manner
Allowance for building displacements
Inefficient for high rise buildings
Not suitable for buildings rested on soft soil.
Seismic Dampers
Seismic Dampers are used in place of structural elements, like
diagonal braces, for controlling seismic damage in structures.
It partly absorbs the seismic energy and reduces the motion of
buildings.
Types:-
Viscous Dampers (energy is absorbed by silicone-based fluid
passing between piston-cylinder arrangement),
Friction Dampers (energy is absorbed by surfaces with friction
between them rubbing against each other), and
Yielding Dampers (energy is absorbed by metallic components
that yield).
Viscous Dampers
Fig: Cross-section of a Viscous Fluid Damper
Tuned Mass Damper(TMD)
It is also known as an active mass damper (AMD) or harmonic
absorber.
It is a device mounted in structures to reduce the amplitude of
mechanical vibrations.
Their application can prevent discomfort, damage, or outright
structural failure.
They are frequently used in power transmission, automobiles
and tall buildings.
Fig: TMD in Taipei 101
Tuned Mass Damper(TMD) (Contd.)
Taipei 101 has the largest TMD sphere in the world and weighs 660 metric
tonnes with a diameter of 5.5 metre and costs US$4 million (total structure costs
US$ 1.80 billion).
Fig: TMD in Taipei 101
Energy Dissipation Devices
Fig: Some Energy Dissipation Devices
