Seismic Retrofitting Techniques.pdf 3d4o3a

PRESENTED BY ARITRA BANERJEE

Introduction Earthquake creates great devastation in 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

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 ersby, 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

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. 6

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 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 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 may sustain severe (even irreparable) damage, but the building should not collapse.

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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

Fig: Additional Shear Wall

avoid interior mouldings.

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Some Conventional 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

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Adding Shear Walls and Bracings

Fig: Effect of Adding Shear Walls and Bracings 13

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 15

Jacketing

Fig: Beam Jacketing

Fig: Column Jacketing 16

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 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 18

Base Isolation (or Seismic Isolation) Isolation) Isolation of superstructure from the foundation is known as base isolation. It is the most powerful tool for ive structural vibration control technique

Fig: Base Isolated Structures 19

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

Types of Base Isolations Base isolation systems which uses Elastomeric Bearings Base isolation systems with Sliding System

Fig: Elastomeric Isolators 21

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

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

Spherical Sliding Base Isolators The structure is ed 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

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

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) 26

Effectiveness of Base Isolation

Fig: A 3-D Model of a building in SAP2000 27

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) 29

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). 32

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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

ing 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( 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 prevent discomfort, damage, or outright structural failure. They are frequently used in power transmission, automobiles and tall buildings.

Fig: TMD in Taipei 101 35

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 36

Energy Dissipation Devices

Fig: Some Energy Dissipation Devices 37

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. 39

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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