2. Introduction
Earthquake creates great devastation in terms of life, money
and failures of structures.
Earthquake Mitigation is an important field of study from a
long time now.
Seismic Retrofitting is a collection mitigation techniques for
Earthquake Engineering.
It is of utmost importance for historic monuments, areas prone
to severe earthquakes and tall or expensive structures.
1
3. Seismic Retrofitting
Definition
It is the modification of existing structures to make them
more resistant to seismic activity, ground motion, or soil
failure due to earthquakes.
The retrofit techniques are also applicable for other
natural hazards such as tropical cyclones, tornadoes, and
severe winds from thunderstorms.
2
4. When is Seismic Retrofitting Needed ?
The two circumstances are:-
Earthquake damaged buildings, and
Earthquake-vulnerable buildings(with no exposure to
severe earthquakes)
3
5. Retrofit Performance Objectives
Public safety only: The goal is to protect human life, ensuring that
the structure will not collapse upon its occupants or passersby, and
that the structure can be safely exited. Under severe seismic
conditions the structure may be a total economic write-off, requiring
tear-down and replacement.
Structure survivability: The goal is that the structure, while
remaining safe for exit, may require extensive repair (but not
replacement) before it is generally useful or considered safe for
occupation. This is typically the lowest level of retrofit applied to
bridges.
4
6. Retrofit Performance Objectives (Contd.)
Structure functionality: Primary structure undamaged and the
structure is undiminished in utility for its primary application.
Structure unaffected: This level of retrofit is preferred for
historic structures of high cultural significance.
5
7. Need of Retrofitting in Existing
Earthquake Vulnerable Buildings
Buildings have been designed according to a seismic code, but the
code has been upgraded in later years;
Buildings designed to meet the modern seismic codes, but
deficiencies exist in the design and/or construction;
Essential buildings must be strengthened like hospitals, historical
monuments and architectural buildings;
Important buildings whose services are assumed to be essential just
after an earthquake like hospitals;
Buildings, the use of which has changed through the years;
Buildings that are expanded, renovated or rebuilt.
6
8. Problems faced by Structural
Engineers are :-
Lack of standards for retrofitting methods
Effectiveness of each methods varies a lot depending upon
parameters like type of structures, material condition,
amount of damage , etc.
7
9. Basic Concept of Retrofitting
The aim is at (CEB1997):-
Upgradation of lateral strength of the structure;
Increase in the ductility of the structure
Increase in strength and ductility
8
10. Earthquake Design Philosophy
Under minor but frequent shaking, the main members of the
building that carry vertical and horizontal forces should not be
damaged; however building parts that do not carry load may sustain
repairable damage;
Under moderate but occasional shaking, the main members may
sustain repairable damage, while the other parts of the building
may be damaged such that they may even have to be replaced after
the earthquake; and
Under strong but rare shaking, the main members may sustain
severe (even irreparable) damage, but the building should not
collapse. 9
12. Some Conventional Approaches
Adding New Shear Walls
Frequently used for retrofitting of non
ductile reinforced concrete frame buildings.
The added elements can be either cast‐in‐place
or precast concrete elements.
New elements preferably be placed at the
exterior of the building.
Not preferred in the interior of the structure to
avoid interior mouldings.
Fig: Additional Shear Wall
11
13. Some Conventional Approaches (Contd.)
Adding Steel Bracings
An effective solution when large openings are required.
Potential advantages for the following reasons:
higher strength and stiffness,
opening for natural light,
amount of work is less since foundation cost may be minimized
adds much less weight to the existing structure
12
14. Adding Shear Walls and Bracings
Fig: Effect of Adding Shear Walls and Bracings
13
18. Retrofit of Structures using Innovative
Materials
Current research on advanced materials has mainly concentrated
on FRP composites.
Studies have shown that externally bonded FRP composites
can be applied to various structural members including
columns, beams, slabs, and walls to improve their structural
performance such as stiffness, load carrying capacity, and
ductility.
17
19. Effectiveness of FRPC as a Retrofitting
Material
Fig: A 3-D Model of a Building (a) Wall Stresses
before
Retrofitting
(b) After
installation of
Steel Window
frames
(c) Additional
FRP
Retrofitting
Fig: A Retrofit Application combining Conventional and
Composites Retrofitting
18
20. 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
19
21. Concept of Base Isolation
Significantly Increase the Period of the Structure and
the Damping so that the Response is Significantly
Reduced.
Fig: Spectral Response for a Typical Base Isolation System
20
22. Types of Base Isolations
Base isolation systems which uses Elastomeric Bearings
Base isolation systems with Sliding System
Fig: Elastomeric Isolators
21
23. 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
22
24. 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
23
25. 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
24
26. Friction Pendulum Bearing
These are specially designed base isolators which works
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
25
27. 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)
26
29. Effectiveness of Base Isolation
Fig: Comparison Stresses in Z direction for Fixed Base (left) and Isolated Base (right)
28
30. Effectiveness of Base Isolation
Fig: Comparison of Shear Stresses in Y-Z direction for Fixed Base(left) and Isolated
base (right)
29
31. 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.
30
32. 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.
31
33. Codes and Guidelines for Base Isolation
International Code Council, Uniform Building Code, Vol. 2, USA,
1997.
International Building Code, IBC 2006.
NZS1170.5:2004, Structural Design Actions, Part 5: Earthquake
Actions – New Zealand, Standards New Zealand.
FEMA-273, NEHRP Guidelines for the Seismic Rehabilitation of
Buildings(1997).
FEMA-274, NEHRP Commentary on the Guidelines for the Seismic
Rehabilitation of Buildings(1997).
32
34. 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).
33
36. 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
35
37. 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
36
38. Fig: Some Energy Dissipation Devices
Energy Dissipation Devices
37
39. Indian Codes for Earthquake Design
IS: 1893-2002 (part-1) Criteria for Earthquake Resistant Design of Structures (Part 1 :
General Provision and Buildings) - Code of Practice
IS: 4326-1993 Earthquake Resistant Design and Construction of Buildings – Code of
Practice
IS: 13920-1993 Ductile Detailing of Reinforced Concrete Structures subjected to
Seismic Forces – Code of Practice
IS: 13935-1993 Repair and Seismic Strengthening of Buildings – Guidelines
IS: 13828-1993 Improving Earthquake Resistance of Low Strength Masonary
Buildings - Guidelines
IS: 13827-1993 Improving Earthquake Resistance of Earthen Buildings – Guidelines
38
40. Conclusion
Seismic Retrofitting is a suitable technology for protection
of a variety of structures.
It has matured in the recent years to a highly reliable
technology.
But, the expertise needed is not available in the basic level.
The main challenge is to achieve a desired performance
level at a minimum cost, which can be achieved through a
detailed nonlinear analysis.
Optimization techniques are needed to know the most
efficient retrofit for a particular structure.
Proper Design Codes are needed to be published as code of
practice for professionals related to this field.
39
41. References
Agarwal, P. and Shrikhande, M., 2006, Earthquake Resistant Design of
Structures, 2nd Edition, Prentice-Hall of India Private Limited, New Delhi.
Cardone, D. and Dolce, M., 2003, Seismic Protection of Light Secondary
Systems through Different Base Isolation Systems, Journal of Earthquake
Engineering, 7 (2), 223-250.
Constantinou, M.C., Symans, M.D., Tsopelas, P., and Taylor, D.P., 1993,
Fluid Viscous Dampers in Applications of Seismic Energy Dissipation and
Seismic Isolation, ATC-17-1, Applied Technology Council, San Francisco.
EERI, 1999, Lessons Learnt Over Time – Learning from Earthquakes
Series: Volume II Innovative Recovery in India, Earthquake Engineering
Research Institute, Oakland (CA), USA.
Murty, C.V.R., 2004, IITK-BMTPC Earthquake Tip, New Delhi.
40
