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DISASTER RESISTANT STRUCTURES


DISASTER  RESISTANT  STRUCTURES
[CASE STUDY: EARTHQUAKE]







By
U.VETRIVEL
ARUNAI COLLEGE OF ENGINEERING

INTRODUCTION:

Buildings are the Worst Killers than the DISASTER” 

A disaster is a natural or man-made (or technological) hazard that has come to fruition, resulting in an event of substantial extent causing significant physical damage or destruction, loss of life, or drastic change to the environment, property and destroys the economic, social and cultural life of people.

In developing countries, losses due to natural disasters are 20 times greater  than in developed countries(as a percentage of GDP).

Over the last few decades disasters of all types (such as earthquake,    flood etc.) have increased in frequency, which means that more people are suffering the consequences.



EFFECTS OF RECENT DISASTERS:
The 2011 Sikkim earthquake also known as the 2011 Himalayan earthquake was a magnitude 6.9 (Mw) earthquake centered within near the border of Nepal and the Indian state of Sikkim, on Sunday, 18 September 2011. In this, 111 people were killed in the earthquake and structural damage occurred in Bangladesh, Bhutan, and across Tibet.

The March 11, 2011 earthquake and tsunami that occurred in Japan followed by a nuclear crisis and shortage of electricity. The human toll has been great with 10,102 persons killed, 17,053 missing, and another 2,777 injured.

The Christchurch earthquake on Tuesday, 22 February 2011, was a powerful natural event that severely damaged New Zealand's second-largest city, killing 184 people with magnitude 6.3 (ML).

SEISMOLOGY:
Seismology is the branch of Geophysics concerned with the study and analysis of Earthquakes and the science of energy propagation through the Earth's crust.

Engineering Seismology is concerned with the solution of engineering problems connected with the Earthquakes. Seismology is extremely important because:
Study of earthquakes gives us important clues about the earth’s interior
Understanding earthquakes allows us to minimize the damage and loss of life.

WHAT IS AN EARTHQUAKE ?
An earthquake is the vibration of Earth produced by the rapid release of accumulated energy in elastically strained rocks.

Energy released radiates in all directions from its source, the focus.

Energy propagates in the form of seismic waves.



CAUSES OF EARTHQUAKE :
The seabed and the land are formed of a crusty skin of light rocks floating on the soft centre of the earth, which is not one solid piece but is made up of lumps, separated by faults and trenches, or pressed together into mountains.

These separate lumps and plates are not static but are moved in slow motion by convection forces in the molten core, gravitational forces from the Sun and Moon and centrifugal forces from the Earth's rotation.

The movement carries on, but the material where they touch is stretched, or compressed, or bent sideways. The material deforms (like stretching or compressing or twisting a bit of plastic).Then it breaks, and there is a sudden movement.

Enormous amounts of energy are released, far more than the biggest Nuclear Bombs. The shock waves from this release of energy shoot out in all directions, like the ripples when you throw a stone in a pond which is an Earthquake.

MOVEMENT OF TECTONIC PLATES :
Earth is divided into sections called Tectonic plates that   float on the fluid-like interior of the Earth. Earthquakes are usually caused by sudden movement of earth plates.

There are seven major Tectonic plates. They move in any direction, while one plate gets contact with other it releases more energy this energy is moved as earthquake to the ground.

These tectonic plates collide or one plate may get under the other. This process of one plate getting under the other is called as subduction.
EARTHQUAKE ZONES IN INDIA :
HOW THE BUILDING FAILS ?
An Earthquake moves the ground. It can be one sudden movement, but more often it is a series of shock waves at short intervals, like our ripples from the pebble in the pond.

All buildings can carry their own weight (or they would fall down anyway by themselves). They can usually carry a bit of snow and a few other floor loads and suspended loads as well, vertically; so even badly built buildings and structures can resist some up-and-down loads.

It is this side-to-side load which causes the worst damage, often collapsing poor .buildings on the first shake. The side-to-side load can be worse if the shocks come in waves, and some bigger buildings can vibrate like a huge tuning fork, each new sway bigger than the last, until failure.


Often more weight has been added to a building or structure at most frequently at greater heights; say another floor and another over that; walls built round open balconies and inside partitions. This extra weight produces great forces on the structure, collapse.

In a lot of multi storey buildings, the floors and roofs are just resting on the walls, held there by their own weight; This can result in a floor or roof falling off its support and crashing down.




IMPACTS OF SHOCK :


GENERAL CONCEPTS OF EQRD :
To be earthquake proof, buildings, structures and their foundations need to be built to be resistant to sideways loads. The lighter the building is, the less the loads. This is particularly so when the weight is higher up. Important aspects in EQRD are :

Symmetry: The building as a whole or its various blocks should be kept symmetrical about both the axes. Asymmetry leads to torsion during earthquakes and is dangerous.

Regularity: Simple rectangular shapes, behave better in an earthquake than shapes with many projections.

Stability of Slope: Hillside slopes liable to slide during an earthquake should be avoided and only stable slopes should be chosen to locate the building.











D. Beam Column Effect:

Moving for higher zones Strong column
and weak beam design proves better.
 Damage of beams         Localized Effect
 Damage of columns      Entire  Structural
                                       Damage               Joint displacement   Column Failure


                                                       








Beams as a Structural Member
Beams  have two types of failure.

FLEXURAL FAILURE which is the propagation on vertical cracks; this can be contracted by provision of longitudinal bar along the length.

SHEAR FAILURE will result in propagation of inclined cracks. To counteract this we hereby provide closed loops called stirrups. The ends are bent to an angle of 135’ to resist the thrust effectively.

Column as a structural member :
Columns can sustain two types of Damage.
Axial-flexural (or combined compression bending) Failure .
Shear failure.

Shear damage is brittle and must be avoided in columns as by providing transverse ties at close spacing which carry the horizontal shear forces and hold concrete and vertical bars together.


G.Shear Wall: Reinforced concrete buildings often have vertical plate-like RC walls called Shear Walls. Their thickness varies from 150mm to 400mm. RC shear walls also perform much better if designed to be ductile.

H. Short Column Behavior: The short column is stiffer as compared to the tall column, and it attracts larger earthquake force. Therefore it cause X-shaped cracks. This behavior is called Short Column Effect.

J. Beam column joint: The points where the beams and columns intersect is a beam column joint. During earthquake the upper bars and lower bars act in a different direction causing elongation or damage of joint. In design practice large column size, having large closed loops are placed inside. Normally we will go for the anchoring of the bars at the ends.

FOUNDATION :
Firm soil:
In firm soil conditions, any type of footing (individual or strip type) can be used. It should of course have a firm base of lime or cement concrete with requisite width over which the construction of the footing may start.

It will be desirable to connect the individual reinforced concrete column footings in Zone A by means of RC beams just below plinth level intersecting at right angles.

Soft soil:
In soft soil, it will be desirable to use a plinth band in all walls and where necessary to connect the individual column footings by means of plinth beams.

Continuous reinforced concrete footings are considered to be most effective from earthquake considerations.

Width of the footing should be wide enough to make the contact pressures uniform, and the depth of footing should be below the lowest level of weathering.

EARTHQUAKE RESISTING STRUCURES :







CONCLUSION :

 Whenever there is an earthquake related disaster in the news with collapsed buildings &other structure all over the place, it shows that earthquake resistant design (EQRD) of structure are still in dark age.

As a civil engineer it is our prime duty to inject new ideas to minimize the effects of earthquakes as far as possible and make our India in a best path of its development towards “DEVELOPED INDIA 2020”….
                  
Thank you…

















 
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