S.L.

KUNALAN

V.MANIVANNAN

(II YEAR / IV SEMESTER) DEPARTMENT OF CIVIL ENGINEERING TAGORE ENGINEERING COLLEGE, RATHINAMANGALAM, VANDALUR POST,

CHENNAI-60048

Sekarkuna.kuna@gmail.com
PH NO: 9791758508 ABSTRACT:

vmani.2212@gmail.com
PH NO: 9791189654

Natural disasters such as earthquake, landslides and floods affect many countries in the world. The disasters not only affect the economy of nations but also hinder their development. The Engineers do not attempt to make earthquake proof buildings that will not get damaged even during the rare but strong earthquake; such buildings will be too robust and also too expensive. Instead, the Engineering intention is to make buildings earthquake resistant; such buildings resist the effects of ground shaking, although they may get damaged severely but would not collapse during the strong earthquake. Thus, safety of people and contents is assured in earthquake resistant buildings, and there by a disaster is avoided. This is a major objective of seismic design codes through out the world. This paper also has some of the measures to be taken while constructing a building in earthquake prone areas.

INTRODUCTION:
Earthquakes are natural hazards under which disasters are mainly caused by damage to or collapse of building and other man-made structures. Experience has shown that for new constructions, establishing resistant regulations and their implementation is the critical safe guard against earthquake- induced damage. As regards existing structures, it is necessary to evaluate and strengthen them based on evaluation criteria before an earthquake. Building design must be such that the building has adequate strength, high ductility, and will remain as one unit, even while subjected to very large deformation. How ever we can do much to reduce disasters provided we design and build or strengthen the buildings so as to

minimize the looses based on the knowledge of the earthquake performance of different building types during an earthquake. The study of damage therefore provides an important step in the evolution of strengthening measures for different type of buildings.

EARTHQUAKE EFFECTS:
There are four basic causes of earthquake induced damage: Ground shaking Ground failure Tsunami Fire

GROUND SHAKING:
The principle cause of earthquake induced damage is ground shaking. As the earth vibrates, all the buildings on the ground surface will respond to that vibration in varying degrees. Earthquake induced accelerations, velocities and displacement can damage or destroy a building unless it has been designed and constructed or strengthened to be earthquake resistant. However, experiences from past strong earthquakes have shown that reasonable and prudent practices can keep a building safe during an earthquake.

GROUND FAILURE:
Earthquake induced ground failure has been observed in the form of ground rupture along the fault zone, landslides, settlement and soil liquefaction .soil liquefaction can occur in lower density saturated sands of relatively uniform size. The phenomenon of liquefaction is particularly important for dams, bridges, underground pipelines, and buildings standing on such ground.

TSUNAMI:

Tsunamis are seismic sea waves are generally produced by a sudden movement of the ocean floor. As the water waves approach land, their velocity decreases and their height increases from 5 to 8 m, or even more. Obviously, tsunamis can be devastating for buildings built in coastal areas.

FIRE:
When the fire following an earthquake starts, it becomes difficult to extinguish it, since a strong earthquake is accompanished by the loss of water supply and traffic jams. Therefore, the earthquake damage to buildings directly due to earthquake. In case of the 1923 kanto earthquake 50% of Tokyo and 70% of the total number of houses were burnt and more than 10,000 people were killed by the fire.

What are the seismic effects on structure?

INERTIA FORCES IN STRUCTRES
Earthquake causes shaking of the ground. So a building resting on it will experience motion at its base. From Newtons first law of motion, even though the base of the building moves with the ground, the roof has a tendency to stay in its original position

This is much like the situation that you are faced with when the bus you are standing in suddenly starts; your feet move with the bus, but your upper body tends to stay back making you fall

backwards!!

HORIZONTAL AND VERTICAL SHAKING:

Earthquake causes shaking of the ground in all directions-along the two horizontal directions(x and y, say), and the vertical direction (z say). Also, during earthquake, the ground shakes randomly back and forth along each direction.How ever, horizontal shaking along x and y directions (both + and directions of each) remains a concern. Structure designed for gravity loads, in general, may not be able to satisfy sustain the effects of horizontal earth quake shaking. Hence, it is necessary to ensure adequacy of the structure against horizontal earthquake effects.

IMPORTANCE OF ARCHITECTURALFEATURES:
The importance of the configuration of a bulding was aptly summarized by late Henry degenkoib, a noted earthquake Engineer of USA, as:

If we have a poor configuration to start with, all the engineer can do is to provide a band-aid-improve a basically poor solution as best as he can. Conversely, if we start-off with a good configuration and reasonable framing system, even a poor engineer cannot harm its ultimate performance too much.
ADJANCY OF BUILDING:
When two buildings are too close to each other, they may pound on each other during strong shaking. With increase in building heights do not match (fig 4), the roof of the shorter building may pound at the mid-height of the column of the taller one; this can be very dangerous

DAMAGE IN BUILDINGS: UN AVOIDABLE

Design of buildings to resist earthquakes involves controlling the damage to acceptable levels at a reasonable cost. Different types of damage (mainly visualized though cracks; especially so min concrete and masonry buildings) occur in buildings during earthquakes. For instance, in a reinforced concrete frame building with masonry filler walls between columns, the cracks between vertical columns and filler walls are acceptable, but diagonal cracks running through the columns are not (fig 3). In general, qualified technical professionals are knowledgeable of the causes and severity of damage in earthquake resistant buildings

HOW

DO

EARTHQUAKES

AFFECT

REINFORCED

CONCRETE BUILDINGS?
In recent times, reinforced concrete buildings have become common in India, particularly in towns and cities. Reinforced concrete consists of two primary materials, namely concrete with reinforcing steel bars. The inertia forces travel downwards- through slab and beams to columns and walls, and then to the foundations from where they are dispersed to the ground. As inertia forces accumulate downwards from top of the building, the columns and walls at lower storeys experience higher earthquake induced forces(fig 1) and are therefore designed to be stronger then those in stroyes above.

HOW TO REDUCE EARTHQUAKE EFFECTS ON BUILDING?

Two basic technologies are used to protect buildings from damaging earthquake effects. These are BASE ISOLATION and SEISMIC DAMPERS. The idea behind base isolation is to detach (isolate) the building from the ground in such away that earthquake motions are not transmitted to through the building, or at least greatly reduced. Seismic dampers are special devices introduced in the building to absorb the energy provided by the ground motion to the building (much like the way shock absorbers in motor vehicles absorb the impacts due to undulations of the road).

BASE ISOLATION:

The flexible pads are called base isolators, whereas the structures protected by means of these devices are called base-isolated buildings. The main feature of the base isolation technology is that it introduces flexibility in the structure. The isolators are often designed to absorb energy and thus add damping to the system. This help in further reducing the seismic response of the building. Base isolators are not suitable for all buildings. Most suitable candidates for base-isolation are low to medium rise buildings rested on hard soil underneath; high rise building or buildings rested on soft soil are not suitable for base isolation

SEISMIC DAMPERS:
Another approach for controlling seismic damage in buildings and improving their seismic performance is by installing seismic dampers in place of structural elements, such as diagonal braces. These dampers act like the hydraulic shock absorbers in cars- much of the sudden jerks are absorbed in the hydraulic fluids and only little is transmitted above to the chassis of the car. When seismic energy is transmitted through them, dampers absorb part of it, and thus damp the motion of the building. Dampers were used since 1960s to protect tall buildings against wind effects.

Commonly used types of seismic dampers include viscous dampers (energy is absorbed by silicone- based fluid passing between piston - cylinder arrangement), friction dampers (energy absorbed by surfaces with friction between them rubbing against each other), yielding dampers (energy is absorbed by metallic components that yield).fig 3.

CONCLUSION:
To find fault is easy but finding remedy for those fault is difficult. Though there may be disasters disturbing and distracting us from our normal life, we should form the alternative solution to get rid of such problems. Such problems should not affect us in the mere future. Sometimes many illiterates become victims of such disasters before they know what is happening. Even the educated people are not aware of the disaster damages. So each and every Civil Engineer should design the future projects by considering the preventive methodologies against the damages due to these disasters.

REFERENCES:

HANSON, RD, and SOONG, T.T. (2001), seismic design with supplemental energy dissipation devices, EARTHQUAKE ENGINEERING, Research Institute, Oakland (ca), USA. SKINNER, RI.Robinson, W.H and Mc VERRY, GH , (1999), an introduction to seismic isolation, JOHN WILEY&SONS, New York seismic isolation.