Seismic retrofitting techniques

transcript

Seminar Presentation on

Seismic Retrofitting Techniques

Submitted to-Mr. S.C.VermaCivil Engg. DepartmentCOER

Submitted By:-Neeraj RanaCivil Engg.-4th yearSection – E (T2)Roll No.- 55

IntroductionEarthquake creates great devastation in terms of life,

money and failures of structures.

Earthquake Mitigation is an important field of study froma

long time now.

Seismic Retrofitting is a collection mitigation techniquesfor

Earthquake Engineering.

It is of utmost importance for historic monuments, areas prone

to severe earthquakes and tall or expensive structures. 1

Seismic RetrofittingDefinition

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.

When is Seismic Retrofitting Needed ?

The two circumstances are:-

Earthquake damaged buildings, and

Earthquake-vulnerable buildings(with no exposureto

severe earthquakes)

Retrofit Performance ObjectivesPublic 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:

remaining safe for exit,

The goal is that

the structure, while may

require extensive repair (but

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 preferredfor

historic structures of high cultural significance.

Need of Retrofitting in ExistingEarthquake 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,butdeficiencies 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.

Basic Concept of RetrofittingThe 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 PhilosophyUnder 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

Some Conventional ApproachesAdding 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 atthe

exterior of the building.

Not preferred in the interior of the structure to

avoid interior mouldings.

Fig: Additional Shear Wall

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

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 confinementTo increase shear strengthTo increase flexural strength

Jacketing

Fig: Column Jacketing

Fig: Beam Jacketing

Retrofit of Structures using InnovativeMaterials

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 RetrofittingMaterial

Fig: A 3-D Model of a Building

(a) Wall Stresses before Retrofitting

(b) Afterinstallation of Steel Window frames

(c) Additional FRP Retrofitting

Fig: A Retrofit Application combining Conventional and Composites Retrofitting

Base Isolation (or Seismic Isolation)Isolation of superstructure from the foundation is known

as base isolation.It is the most powerful tool for passive structuralvibrationcontrol technique

Fig: Base Isolated Structures

Concept of Base IsolationSignificantly 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 IsolationsBase isolation systems which uses Elastomeric Bearings

Base isolation systems with Sliding System

Fig: Elastomeric Isolators

Elastomeric Base Isolation SystemsThis 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 ofthe earthquake ground motionA layer with low horizontal stiffness isintroduced

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 IsolatorsThe structure is supported by bearing pads thathave curved surface and low friction.During an earthquake, the building is free to slide on thebearings.

Fig: Spherical Sliding Base Isolator24

Friction Pendulum Bearing

It increases the natural time period of oscillation bycausing the structure to slide along the concave inner

These are specially designed base isolators which workson the principle of simple pendulum.

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 being2000 poundsBenicia 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

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

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

Advantages of Base IsolationIsolates 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 IsolationExpensive

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,

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 DampersSeismic 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 ofbuildings.Types:- Viscous Dampers (energy is absorbedby silicone-

based fluidpassing 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 amplitudeof 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

Fig: Some Energy Dissipation Devices

Energy Dissipation Devices

Indian Codes for Earthquake DesignIS: 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

ConclusionSeismic 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 ofpractice for professionals related to this field.