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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