seismic retrofitting techniques

PRESENTED BY

ARITRA BANERJEE

B070543CE

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.

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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.

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When is Seismic Retrofitting Needed ?

The two circumstances are:-

Earthquake damaged buildings, and

Earthquake-vulnerable buildings(with no exposure to

severe earthquakes)

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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.

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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.

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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.

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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.

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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

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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

Classification of Retrofitting Techniques

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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

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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

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Adding Shear Walls and Bracings

Fig: Effect of Adding Shear Walls and Bracings

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Adding Steel Bracings

Fig: RC Building retrofitted by steel bracing

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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

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Jacketing

Fig: Column Jacketing

Fig: Beam Jacketing

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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.

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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

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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

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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

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Types of Base Isolations

Base isolation systems which uses Elastomeric Bearings

Base isolation systems with Sliding System

Fig: Elastomeric Isolators

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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

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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

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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

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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

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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)

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Effectiveness of Base Isolation

Fig: A 3-D Model of a building in SAP2000

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Effectiveness of Base Isolation

Fig: Comparison Stresses in Z direction for Fixed Base (left) and Isolated Base (right)

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Effectiveness of Base Isolation

Fig: Comparison of Shear Stresses in Y-Z direction for Fixed Base(left) and Isolated base (right)

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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.

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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.

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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).

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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).

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Viscous Dampers

Fig: Cross-section of a Viscous Fluid Damper

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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

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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

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Fig: Some Energy Dissipation Devices

Energy Dissipation Devices

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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

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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.

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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.

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THANK YOU…

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ANY QUESTIONS…

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