PRESENTED PRESENTED B BY Y ARITRA BANERJEE BANERJEE
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.
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 retr The retrof ofit it tech techni niqu ques es are are also also appl applic icaable ble for for othe other r natural hazards such as tropical cyclones, tornadoes, and severe winds from thunderstorms.
When is Seismic Retrofitting Needed ? The two circumstances are:Earthquake
damaged buildings, and
Earthquak Earthquake-v e-vulner ulnerable able
severe earthquakes)
buildings( buildings(with with no exposur exposuree to
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.
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.
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.
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.
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
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
Classification of Retrofitting Techniques
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
Some Conventional Approaches Approaches (Contd. (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
Adding Shear Walls and Bracings
Fig: Effect of Adding Shear Walls and Bracings
Adding Steel Bracings
Fig: RC Building retrofitted by steel bracing
Some Conventional Approaches (Contd.) Jacketing (Local Retrofitting Technique) Most
popular method for strengthening of building columns
Types-1.
Steel jacket, 2. Reinforced Concrete jacket, 3.
Fibre Reinforced Polymer Composite(FRPC) jacket Purpose
for jacketing:
To increase concrete confinement
To increase shear strength
To increase flexural strength
Jacketing
Fig: Beam Jacketing
Fig: Column Jacketing
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.
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
Base Isolation (or (or Seismic Isolation 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 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
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 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
Friction Pendulum Bearing (Contd. 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.
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).
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:Dampers (energy is absorbed by silicone-based fluid passing between piston-cylinder arrangement),
Viscous
Dampers (energy is absorbed by surfaces with friction between them rubbing against each other), and
Friction
Dampers (energy is absorbed by metallic components that yield).
Yielding
Viscous Dampers
Fig: Cross-section of a Viscous Fluid Damper
Tuned Mass Damper( Damper (TMD) 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 outright structural failure.
prevent
discomfort,
damage,
or
They are frequently used in power transmission, automobiles and tall buildings.
Fig: TMD in Taipei 101
Tuned Mass Damper( Damper (TMD TMD)) ((Contd. 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
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
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.
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.
