SEISMIC RETROFITTING OF BUILDINGSToday, many countries are the victims of severe earthquakes resulting in the

loss of lives of human being, animals, destruction of buildings, and many more. So, it is high time for the civil engineers to take this issue as a serious matter and they must think of some alternatives of saving human lives as well as wildlife.

WHAT IS SEISMIC RETROFITTING? Seismic, by the word itself means something relating to earthquakes or

vibrations of the earth and its crust. It happens due to some disturbances below the earth’s crust which cannot be seen.

Seismic retrofitting is the modification of existing structures to make them more resistant to seismic activity, ground motion or soil failure due to earthquakes. There are several techniques which have come forward nowadays by which an existing structure can be modified and make it less prone to earthquakes.

It is important to keep in mind that there is no such thing to make a structure fully earthquake proof but seismic performance can be greatly enhanced through proper initial design or subsequent modifications.

Common seismic retrofitting techniques fall into several categories:

1. Base Isolators.2. Supplementary dampers.3. Tuned mass dampers.4. Slosh tank.5. Active control system.

WHAT IS THE NEED FOR SEISMIC RETROFITTING OF BUILDINGS?The need for seismic retrofitting in existing buildings can arise due to the

following reasons:

Buildings not designed according to the codes of practice. Deterioration of strength of the buildings. Not considering the safety of buildings while construction.

UNIQUENESS:-The impact of earthquake is sudden with little or no warning to make

preparations against damages and collapse of structures. The common seismic retrofitting techniques have evolved very fast and have changed the belief of people that buildings can be made earthquake proof with proper modifications and thereby saving human lives. Seismic retrofit strategies have been developed in the past few decades following the introduction of new seismic provisions and the availability of advanced materials like fiber reinforced polymers, fiber reinforced concrete and high strength steel.

AIMS AND OBJECTIVES:-

1. Public safety: The goal is to protect human life, ensuring that the structure will not collapse upon its occupants or passerby.

2. Structure survivability: The goal is that the structure, while remaining safe may require extensive repair but not replacement.

3. Structure functionality: Primary structure undamaged and the structure is undiminished in utility for its primary application.

4. Structure unaffected: A high level of retrofit is preferred for historic structures of cultural significance.

BASE ISOLATORS:-

Base isolation is one of the most powerful means of protecting a structure against earthquake forces. It is meant to enable a building to survive a potentially devastating seismic impact through proper initial design or subsequent modifications.

Tomb of Cyrus is said to be the oldest base-isolated structure in the world.

Base Isolation system consists of the following:

1. Isolation units 2. Isolation components

Isolation units:- They consist of shear or sliding units. They are intended to provide the decoupling effect to a building. The first evidence of architects using the principle of base isolation for earthquake protection was discovered in Pasargadae, a city in Iran in 6th century BC.

Isolation components:- These are the connections between isolation units and their parts having no decoupling effect of their own.

How do base isolators work?Base Isolation is a technique developed to prevent or minimize damage to

buildings during an earthquake. So far it has been used in New Zealand, India, Japan, Italy and the USA. When a building is built away (isolated) from the ground, resting on flexible bearings or pads known as base isolators, it will move little or not at all during an earthquake.

How are base isolators constructed?They consist of basic components-a lead plug, rubber and steel, which are

generally placed in layers.Rubber: - The rubber provides flexibility. At the end of an earthquake, the rubber bearings will slowly bring the building back to its original position which takes months to happen.

Lead: - Lead has plastic property. During an earthquake, the kinetic energy of the earthquake is absorbed into heat energy as the lead is deformed.

Steel: - If layers of steel are used with rubber, the bearing can move in the horizontal direction but is stiff in the vertical direction.

SUPPLEMENTARY DAMPERS: -

A Supplementary Damping System (SDS) is essentially an energy dissipation system that is incorporated into the design of a structure to absorb vibration energy, thereby reducing motion.

Dampers for earthquake protection

How do the supplementary dampers work?

Supplementary damping is the most efficient and cost effective way to achieve energy dissipation in buildings. This would inadvertently mean decreasing the energy dissipation demand on the structural components i.e. beams/columns/slabs thereby increasing the survivability of the building structure. Dampers are mechanical devices that look somewhat like huge shock absorbers and their function is to absorb and dissipate the energy supplied by the ground movement during an earthquake so that the building remains unharmed. Whenever the building is in motion during an earthquake tremor or excessive winds, dampers help in restricting the building from swaying excessively and thereby preventing structural damage. The energy absorbed by dampers gets converted into heat which is then dissipated harmlessly into the atmosphere. Dampers thus work to absorb

earthquake shocks ensuring that the structural members i.e. beam and columns remain unharmed.

Benefits:-

Achievement of occupant comfort and safety criteria. � Increased design life (structural durability) through reducing structural

stress and/or fatigue. Decreased construction and maintenance costs. � Increased tenant space.

Dampers used in a building

TUNED MASS DAMPERS : -

A tuned mass damper, also known as a harmonic absorber 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 buildings.

How do tuned mass dampers work?Tuned mass dampers stabilize against violent motion caused by harmonic

vibration. It reduces the vibration of a system with a comparatively lightweight component so that the worst-case vibrations are less intense. Practical systems are tuned to either move the main mode away from a troubling excitation frequency or to add damping to a resonance that is difficult to damp directly.

Location of Taipei101’s largest tuned mass damper

SLOSH TANK: -

In fluid dynamics, slosh refers to the movement of liquid inside another object undergoing motion.

Nowadays, one of the biggest challenges that engineering faces is to reduce structure motion due to external loadings especially in high rise buildings. Slosh tank is one of the inventions that can be installed in different locations and levels into a structure, in order to increase dampening (energy absorbing mechanism) and decrease vibrations. It can either be installed on the top floor of a structure or in some certain floors or even at each floor of a building.

Rectangular slosh tank

Sensors

Control Actuators

Excitation Structure Response

ACTIVE CONTROL SYSTEM: -

The use of active control systems and some combinations of active and passive systems, so called hybrid systems, as a means of structural protection against seismic loads has received considerable attention in recent years. Active/hybrid control systems are force delivery devices integrated with real-time processing evaluators/controllers and sensors within the structure. They act simultaneously with the hazardous excitation to provide enhanced structural behavior for improved service and safety.

An active structural control system consists of the following:a) Sensors located about the structure to measure either external excitations, or

structural response variables, or both.b) Devices to process the measured information and to compute necessary

control force needed based on a given control algorithm.c) Actuators, usually powered by external sources, to produce the required

forces.

Structure with Active Control

Controller Sensors

LIST OF IS CODES REQUIRED FOR SEISMIC DESIGN:

IS 4326:1993 – Earthquake resistant design IS 13827:1993 – Earthquake resistance of earthen buildings IS 13828:1993 – Earthquake resistance of low strength masonry buildings IS 13920:1993 – Ductile detailing of reinforced concrete structures IS 13935 – Seismic strengthening of buildings IS 1893 – Earthquake resistant design of structures