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AIM:
To study different types of building forms which resists damages
caused by an earthquake.
HYPOTHESIS:
"EARTHQUAKE RESISTANT BUILDING FORMS HELPS TO IMPROVES THE
STABILITY OF THE BUILDING STRUCTURE AND CAUSES LESS DAMAGE
AND
ENSURE BETTER LIVING"
OBJECTIVES:
To analyze what is an earthquake, the consequences and various
seismic zones in
Rajasthan.
To study the various earthquake zones in rajasthan (zone-4,
zone-3, zone-2) and to
formulate various design guidelines related to forms and
geometrical arrangement of
building elements for a better earthquake resistant
structure.
To analyze the various geometrical forms, which resists damage
to buildings when an
earthquake occurs.
To analyze the need of symmetrical arrangements of building
forms for better stability.
To analyze the pattern of attack of earthquake on building.
SCOPE:
To identify the best suitable geometrical forms which resist
damages caused by an
earthquake.
To verify newer techniques and mediums for better living.
This study helps architects and engineers, in developing
building forms which are better
in resisting earthquakes.
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NEED OF STUDY:
The main aim behind the selection of this topic is to study the
various building forms/
geometrical form which resist less damage to building structure
and ensure better living.
According to the BSI 2002, the earthquake is repeatedly taking
place in least active zones and
hence the consequences for the same are required to be
taken.
List of earthquakes in rajasthan in last 20 years:-
RAJASTHAN map showing seismic zones
Figure 1: Rajasthan Map showing earthquake zones
Source- Disaster Management and Relief Department
- Government Of Rajasthan
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METHODOLOGY :
Hypothesis
Objectives
Literature Study
Primary Study Secondary Study
To study the effects of
earthquake taken place in India.
1. Study the buildings and their
form, resting earthquake
2. Guidelines for earthquake.
Collection of Data
Cross Classification Analysis
Outcomes
Conclusion
Aim
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INTRODUCTION TO EARTHQUAKE:
Earthquakes occur due to slippage of rocks in the earth's crust
or in the upper part of the mantle,
Consequent to these sudden movements strong vibrations occur on
the ground in a short span of
time. The tremendous amount of energy suddenly releasing during
an earthquake which
accumulates slowly due to geological process.
According to the elastic rebound theory, energy is stored in the
rocks up to the elastic limit may
be for hundreds or thousands of years. Eventually the rocks snap
or rupture at the weakest point,
relieving the enormous strains built up over the years. This
stored up energy is released in the
form of seismic waves, which radiate outward from the point
where the rocks are fractured.
Earthquakes are identified by their location (Longitude and
Latitude), depth of the focus and the
energy released/size of the earthquakes. The most common
measures of the size of the
earthquakes are magnitude and intensity.
SOURCE: http;/alabamaquake.com
Figure 2: FIGURE SHOWING HOW EARTHQUAKE ORIGNATES
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Definitions
Focus Point
Origin point of the earthquake lying below the earth's surface
is known as Focus Point of the
earthquake, where slip starts.
Epicentre
The point just vertically above the focus on the earth surface
is known as the Epicentre.
P-Waves
These waves are Primary waves which are fastest among all the
waves and generally travel with
a speed between 6 to 14 km per second inside the earth. The
speed of the waves remains
unaffected when passing through solid sections of the earth but
slow down when passing through
liquid portions. These are longitudinal waves and create a
"Push-pull" effect on rock mass like
sound waves.
S-Waves
These waves are Secondary or Shear waves and also travel inside
the earth at speeds of 0.58
times that of P wave (generally 3 to 8 km per second). These
waves travel easily through solid
sections but loose their identity when passing through liquid
portions. These waves are
transverse waves and cause earth to move at right angles to the
direction of the wave. These
wave are of most destructive nature.
SOURCE:http;/alabamaquake.com
Figure 3: EARTHQUAKE WAVE (P AND S)
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L (Surface) Waves
These waves are love waves and always travel near the surface of
the earth and travel at a speed
of 0.9 times that of S wave (3 to 5 km per second). These waves
in association with s-wave also
cause maximum damage.
SOURCE:http;/geo.utep.edu.com
Figure 4: L-WAVE OF EARTHQUAKE
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Magnitude
Magnitude is a measure of the amount of energy released in an
earthquake. It is most commonly
determined on Richter scale devised by an American seismologist
in 1935. In this method, the
magnitude is determined from the maximum amplitude (of S wave)
recorded on a particular type
of seismograph.After applying a distance factor the value is
extrapolated at the epicentre. It is a
fixed number and given on a logarithmic scale. An increase of
one unit represents an increase in
amplitude of ground shaking by ten times and energy released
thirty times. Richter scale is open-
ended, however maximum magnitude is obtained around 9.
Intensity
The intensity is the effect of earthquake on the ground and the
objects in the affected area. It is
assigned on the basis of damage that depends upon the magnitude,
depth of focus, distance from
the epicentre and the ground condition. It varies from place to
place. It is given on grade I to XII
on Modified Mercalli (MM) or Medvedev - Sponheaer - Karnik (MSK)
scale.
Tectonic plates
Tectonic plates are made of elastic but brittle rocky material.
And so, elastic strain energy is
stored in them during the relative deformations that occur due
to the gigantic tectonic plate
actions taking place in the Earth. But, when the rocky material
along the interface of the plates in
the Earths Crust reaches its strength, it fractures and a sudden
movement takes place there (the interface between the plates where
the movement has taken place (called the fault) suddenly slips
and releases the large elastic strain energy stored in the rocks
at the interface. For example, the
energy released during the 2001 Bhuj (India) earthquake is about
400 times (or more) that
released by the 1945 Atom Bomb dropped on Hiroshima!!
Figure 5: Movement of tectonic plates
Source: http;/geo.utep.edu.com
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Past history of earthquake in Rajasthan :
Though the state of Rajasthan has not had a major earthquake in
recent years, small to
moderate earthquake have been felt in the state. Several faults
have been identified in this
region out of which many show evidence of movement during the
Holocene epoch. The
Cambay Graben terminates in the south-western part of the state.
The Konoi Fault near
Jaiselmer trends in a north-south direction and was associated
with the 1991 Jaiselmer
earthquake. Several active faults criss-cross the Aravalli range
and lie parallel to each other.
The most prominent of them is the north-south trending Sardar
Shahr Fault and the Great
Boundary Fault which runs along the Chambal River and then
continues in the same
direction into Uttar Pradesh. However, it must be stated that
proximity to faults does not
necessarily translate into a higher hazard as compared to areas
located further away, as
damage from earthquakes depends on numerous factors such as
subsurface geology as well
as adherence to the building codes.
Table showing the recent earthquakes in rajasthan Source:
Disaster management and Relief department, Govt. of Rajasthan
Time period Place Density
Dec, 2012 Jaipur 3.6 magnitude
2 years ago Daosa 4.0 magnitude
4 years ago Sadri 4.6 magnitude
5 years ago Jaisalmer 5.1 magnitude
8years ago Basi 4.2 magnitude
8 years ago Govindgarh 4.0 magnitude
11 years ago Chomu 4.5 magnitude
12 years ago Nim ka thana 4.1 magnitude
16 years ago Phalodi 3.5 magnitude
18 years ago Pokaran 5.2 magnitune
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Earthquake are qualitatively classified by the destruction they
cause. Generally earthquakes of
magnitude greater than 5 only cause damages while the magnitude
of major earthquake is 7 or
more. A qualitative classification of earthquakes can be seen in
the table below.
Magnitude (M) Classification Annual frequency of
occurrence
M8 Great Earthquake 1
M7 and
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Phases and origin
An earthquake has phases and parts. The earths plates are
constantly moving away from each other, collide or slide one under
the other. At shallower levels, where the rock is less elastic
and
prevents movement, energy builds up until it reaches a
saturation point and is suddenly released,
causing an earthquake or a tremor.
The precise point where it begins to release energy is the focus
or hypocenter of the earthquake.
The point on the earths surface directly above the focus is the
epicentre. Usually thats the point
with the highest damage. After a great earthquake, the rocks of
the area around the outbreak
continue to move as they adjust to new positions, causing a lot
of earthquakes known as
aftershocks.
The energy released by an earthquake is transmitted at high
speed in all directions through the
surrounding rocks. Like another type of energy, it spreads
through waves. In this case, they are
called seismic waves.
Figure 7: Origin of earthquake and seismic waves
Source : http;/geo.utep.edu.com
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Scales
There are two ways to measure the force of the earthquakes. One
of the scales is called Mercalli
scale and the other is Ritcher scale.
The Mercalli Scale is a scale of 12 degrees developed to assess
the intensity of earthquakes
through the effects and damage to various structures. It was
named after Italian physicist
Giuseppe Mercalli. Low levels of the scale are associated with
how people feel the tremor, while
higher grades are associated with structural damage
observed.
The levels are:
1-Instrumental, 2-Weak, 3-Slight, 4-Moderate, 5-Rather Strong,
6-Strong, 7-Very Strong, 8-
Destructive, 9-Violent, 10-Intense, 11-Extreme,
12-Cataclysmic.
The seismic scale of Richter, also known as local magnitude
scale, is an arbitrary logarithmic
scale that assigns a number to quantify the energy released in
an earthquake, named in honour of
the American seismologist Charles Richter. The measurement is
performed using data supplied
by seismographs, instruments to measure surface energy waves.
The levels are: Less than 2.0 Micro, 2.0-3.9 Minor, 4.0-4.9 Light,
5.0-5.9 Moderate, 6.0-6.9 Strong, 7.0-7.9 Major, 8.0-9.9 Great,
+10.0 Massive.
Modified Mercalli Scale:
The Mercalli scale modified by American scientists describes the
effects of the earthquake as given in the table below:
Class of Earth
quakes
Description
I Not felt except by very few under especially favourable
circumstances.
II Felt only by few person at rest, especially on upper floors
of buildings and
delicately suspended objects may swing.
III Felt quite noticeably indoors, especially on upper floors of
buildings but many
people do not recognize it as an earthquake; standing motorcars
may rock
slightly. Vibration may be felt like passing of a truck.
IV During the day felt indoors by many, outdoors by a few; at
night some are
awakened; dishes, windows, doors disturbed; walls make cracking
sound;
sensation like heavy truck striking the building; and standing
motor cars
rocked visibly.
V Felt by nearly everyone; many awakened; some dishes, windows
etc. broken;
a few instances of cracked plaster; unstable objects overturned;
disturbance of
trees, poles, and other tall objects noticed and pendulum clocks
may stop.
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VI Felt by all; many frightened and run outdoors; some heavy
furniture moved; a
few instances of fallen plaster or damaged chimneys and damage
slight.
VII Everybody runs outdoors; damage negligible in buildings of
good design and
construction; slight to moderate in well built ordinary
construction;
considerable in poorly built or badly designed structures; some
chimney
broken; noticed by persons driving motor cars.
VIII Damage slight in specially designed structures;
considerable in ordinary but
substantial buildings with partial collapse; very heavy in
poorly built
structures panel walls thrown out of framed structure; heavy
furniture
overturned sand and mud ejected in small amounts; changes in
well water and
person driving motor cars disturbed.
IX Damage considerable in specially designed; well designed
framed structures
thrown out of plinth; very heavy in substantial buildings with
partial collapse;
buildings shifted off foundations; ground cracked conspicuously
and
underground pipes broken.
X Some well built wooden structures destroyed; most masonry and
framed
structures with foundations destroyed; ground badly cracked.
Rails bent.
Landslides. Shifted sand and mud; water splashed over banks.
XI Few, if any masonry structures remain standing; bridges
destroyed; broad
fissures in ground; underground pipelines completely out of
service, Earth
slump; land slips in soft ground and rails bent greatly.
XII Total damage; waves seen on ground surface; objects thrown
upward into the
air.
Source: Disaster management and Relief department, Govt. of
Rajasthan
Effects :-
The effects of an earthquake can be many different, some example
are explained below.
Movement and ground rupture.
They are the main effects of an earthquake on the Earths surface
due to friction of tectonic plates, causing damage to buildings or
structures that are rigid in the area affected by the
earthquake. Damage to buildings depends on the intensity of de
movements, the distance
between the structure and the epicenter and the geological and
geomorphologic conditions that
enable better wave propagation.
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Figure 8: Ground rapture
Source: - Building Materials and Technology Promotion
Council, New Delhi, India
TSUNAMI: Tsunamis are huge ocean waves that travel large amount
of water moving towards the coast. In
the open sea the distance between the crest of the waves are
close to 100 km. The periods range
from five minutes to an hour. According to depth of water,
tsunamis can travel at speed of 600 to
800 km/h. They can travel long distances across the ocean, from
one continent to another.
OTHER EFFECTS:
Land slides, Liquification, fire.
BUILDINGS AGAINST EARTHQUAKES: The behaviour of a building
during earthquakes depends critically on its overall shape, size
and
geometry, in addition to how the earthquake forces are carried
to the ground. Hence, at the
planning stage itself, architects and structural engineers must
work together to ensure that the
unfavourable features are avoided and a good building
configuration is chosen. The importance
of the configuration of a building was aptly summarised by Late
Henry Degenkolb, 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.
Even when a building designed and constructed to meet all the
requirements required by the
rules of earthquake resistant design and construction, there is
always the possibility of an
earthquake even stronger than they have been provided and must
be resisted by building without
damage occurring.
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In tall buildings with large height-to-base size ratio, the
horizontal movement of the floors during
ground shaking is large. In short but very long buildings, the
damaging effects during earthquake
shaking are many. And, in buildings with large plan area like
warehouses, the horizontal seismic
forces can be excessive to be carried by columns and walls.
Figure 9: Oversizes buildings do not perform well during
earthquake
Source: - Building Materials and Technology Promotion
Council, New Delhi, India
Figure 10: Basic Geometric Shapes that resists earthquake
Source: IITK, Kanpur
Buildings with one of their overall sizes much larger or
much
smaller than the other two, do not perform well during
earthquakes.
In general, buildings with simple geometry in plan have
performed well during strong earthquakes. Buildings with re-
entrant corners, like those U, V, H and + shaped in, have
sustained significant damage. Many times, the bad effects of
these
interior corners in the plan of buildings are avoided by making
the
buildings in two parts. For example, an L-shaped plan can be
broken up into two rectangular plan shapes using a
separation
joint at the junction. Often, the plan is simple, but the
columns/walls are not equally distributed in plan. Buildings
with
such features tend to twist during earthquake shaking.
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Basics principles of building Regular shape: The geometry of the
building must be simple in plan and elevation. Complex
shapes, irregular or asymmetrical cause bad behaviour when the
building is rocked by an
earthquake. Irregular geometry bring on the structure undergoes
torsion or attempt to turn in a
disorderly manner. The lack of uniformity makes it easier in
some corners are presented intense
concentrations of power that can be hard to resist.
SOURCE :- CRITERIA FOR EARTHQUAKE RESISTANT
DESIGN OF STRUCTURES BIS 2002
A. Building on slopy ground B.Buildings with walls on two/one
sides (in plan)
.
Building Materials and Technology Promotion
Council, New Delhi, India
Figure 11: Buildings have unequal vertical members; they cause
the building to twist about a vertical axis
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Damages in buildings, infrastructures and people The main cause
of damage caused by earthquakes is shaking itself. This shock
causes the
collapse of numerous objects and the collapse of the buildings.
The collapse of building cause in
its habitants trapped in the rubble, often perish by being
crushed. Also falling objects can cause
numerous injuries; even death if it is very heavy objects
(furniture, heavy lamps, suspended
ceiling, etc) or cutting (pieces of glass windows).
Most accidents can be caused by earthquakes are due mainly to
the following types of effects:
- Effects on buildings and infrastructure:
resistant features. Destruction and partial collapse of
buildings (falling from ceilings, walls,
partitions,balconies, exterior walls, cracks in walls, etc).
Fires caused by a short, exhaust gas and
flammable materials. Flooding from broken dams, water pipes,
etc.
,
tiles, pots, etc. Fall of broken glass and ceramic tiling,
especially dangerous when they fall from
upper floors. Fall of furniture, hanging objects, etc.
and installations. Partial damage to the roads (roads, bridges,
tunnels, railways, etc) due to
settlements landslides and mudslides. Fall of utility poles and
power lines.
Pounding can occur between adjoining buildings due to horizontal
vibrations of the two
buildings.
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LITERATURE STUDY :-
PRIMARY STUDY
EARTHQUAKE ZONES IN RAJASTHAN:
As per the BMPTC Atlas the State of Rajasthan State falls under
earthquake zones II, III and IV.
Some area of Districts of Jalore, Sirohi, Barmer and Alwar
districts fall in zone IV where as
many parts of Bikaner, Jaisalmer, Barmer, Jodhpur, Pali, Sirohi,
Dungarpur, Alwar, Banswara,
fall in zone III. A table showing zones and likelihood of
earthquakes of different intensity and
magnitude is shown below.
S.
No.
Seismic Zone Intensity
MSK
Magnitude District
1 IV [High Damage
Risk Zone]
VII-VIII 6.0 - 6.9 Some area of Barmer [Chohtan Block],
Jalore [Sanchore Block] Alwar [Tijara
Block] and Bharatpur [Block Nagar,
Pahari]
2 III [Moderate
Damage Risk Zone]
VI-VII 5.0 - 5.9 Parts of Udaipur, Dungarpur, Sirohi,
Barmer, Jaisalmer, Bikaner, Jhunjhunu,
Parts of Sikar, Jaipur, Dausa,
Bharatpur.
3 II [Low damage
Risk Zone]
IV-VI 4.0 - 4.0 Ganganagar, Hanumangarh, Churu,
Jodhpur, Pali, Rajasamand, Chittorgarh,
Jhalawar, Baran, Kota, Bundi,
Sawaimadhopur, Karauli, Dholpur,
Banswara, some area of Bikaner,
Udaipur, Jhunjhunu, Sikar, Jaipur.
Source: Disaster management and Relief department, Govt. of
Rajasthan
The earthquake of Kutch in 2001 was felt in many parts of
Rajasthan as well. Its effect was felt
more severely in the Western District namely Jalore, Barmer and
Jaisalmer. Many buildings in
these districts like schools, rest houses and privately owned
buildings had developed huge cracks
and had been rendered unsafe. Many other buildings developed
cracks making them unsafe for
further use without proper retrofitting. Many of the public
buildings mainly schools are still lying
in dilapidated conditions.
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Case Study 1 : Gujarat Earthquake
1. 2001 Bhuj Earthquake
The Bhuj earthquake in Gujarat, India occurred on the 26 January
2001 and caused massive
destruction to property and loss of life. This earthquake had a
moment magnitude Mw = 7.9
USGS and struck the Kutch region of India at 8.46am local time,
with the shaking lasting for a
few minutes. Kutch has a population of about 1.3 million people.
Other major cities in Gujarat eg
Ahmedabad and Jamnagar, which are hundreds of kilometres away,
were also effected by the
earthquake.
In Kutch, major towns such as Bhuj (pop 150,000), Anjar (pop
50,000), Bhachau (pop 40,000),
and Rapar (pop 25,000) were almost totally destroyed and many
villages surrounding these
towns were badly damaged. To date over 20,000 persons are
reported dead and over 167,000
injured, predominantly from the Kutch region. The reported
deaths will increase as towns are
cleared, an operation which will take many years.
Most people were killed or badly injured because of:
a) poorly constructed buildings either totally or partially
collapsing
b) walls collapsing within narrow streets, burying people
escaping into them
c) untied roofs and cantilevers falling onto people
d) free standing high boundary walls, parapets and balconies
falling due to the severe shaking
e) gable walls falling over
f) the failure of modern reinforced structures with large open
spaces at ground to first floor level,
for example garage or shop spaces, collapsing and burying
occupants (soft storey collapses)
g) inhabitants not knowing how to respond to the shaking and
collapse of walls around them.
Figure 12: Earthquake waves showing the area affected
Source: Earthquake Engineering Research Institute, EERI Web site
at www.eeri.org.
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Generally, commercial buildings were worst affected by the
earthquake because of poor
workmanship, use of materials and inadequate attention to
detailing.
Low-rise rubble masonry buildings were totally destroyed near to
the epicentre, but some
survived (though badly damaged) when further away. These were
also older forms of
construction. Cutstone masonry and more modern reinforced
concrete framed buildings faired
much better, although damaged to varying extents. These later
building types are largely built by
owner-occupiers and hence better care was taken in the materials
used and their workmanship.
Many lessons can be learnt from those non-engineered low rise
buildings which survived.
Large earthquakes can still cause damage to buildings even if
designed to the relevant Indian
codes and this Guide. However, the seismic measures taken are
intended to absorb damage in a
controllable way and save lives. They are not intended to ensure
that a building always survives
intact. If seismic measures had been taken into account in the
design of buildings the loss to life
would have been significantly reduced as many buildings would
have not collapsed.
Damage to buildings were caused by a combination of affects:
Old decaying buildings predating modern construction
practices
New Buildings not being designed to Indian earthquake codes
Lack of knowledge, understanding or training in the use of these
codes by local engineers
Unawareness that Gujarat is a highly seismic region
Buildings erected without owners seeking proper engineering
advice
Improper detailing of masonry and reinforced structures
Poor materials, construction and workmanship used, particularly
in commercial buildings
Buildings having poor quality foundations or foundations built
on poor soils
A majority of building structures in Gujarat can be divided into
the following two broad
categories: (i) load bearing masonry and (ii) reinforced
concrete frames with unreinforced
masonry infill walls.
Load bearing masonry:
A majority of buildings in the Kachchh region are built in
unreinforced load bearing masonry. A
large number of such buildings also exist in areas outside
Kachchh, including inurban centers
such as Ahmedabad. The types of masonry units used include (i)
random rubble stones, (ii)
rough dressed stones, (iii) clay bricks, and (iv) solid or
hollow concrete blocks. The units are
assembled with mud mortar, lime mortar, or cement mortar. The
stone blocks used in load
bearing masonry are generally quite large, the commonly used
dimensions being 400 mm by 600
mm by 225 mm thick. The roof structure consists of either
Manglore clay tiles laid on timber
planks supported by purlins and rafters made from wooden logs or
a reinforced concrete slab.
When the building has more than one storey, the floors and roofs
are generally reinforced
concrete slabs.
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Figure 13: A village house in Kachchh; stone masonry with
manglore roof
Source: Earthquake Engineering Research Institute, EERI Web site
at www.eeri.org.
Many buildings in Kutch of up to 2 storeys in height are made of
random rubble masonry
construction. The 26 January 2001 earthquake caused massive
damage to these buildings. A
great many partially or completely collapsed, especially close
to the epicentre in Bhuj, Anjar,
Bachau and Sukhpur, where the destruction was almost total.
Towns and villages that are further
from the epicentre of the earthquake were less affected but only
in the sense that total collapse
was not as widespread. For example, near the villages of Kera or
Naranpur buildings of this
nature were still standing with sometimes only partial
collapse.
Figure 14: Destruction of heavy stone masonry walls that had no
reinforcement and were not tied to each other
Source: Earthquake Engineering Research Institute, EERI Web site
at www.eeri.org.
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Figure 15: Partial collapse of gable wall for a single storey
random masonry wall in kera
Source: Earthquake Engineering Research Institute, EERI Web site
at www.eeri.org.
During the earthquake, many buildings easily separated at
corners and T-junctions resulting in
walls overturning and roofs collapsing, which killed thousands
of people. This was because the
random rubble walls were made of uneven stone and the stones
were laid on either weak soil or
mortar bedding. Under the heavy seismic shaking, the tensile
strength of the mortar (and rubble) was easily exceeded, and walls
bulged or totally collapsed.
Figure 16: Heavily damaged single storey rubble masonry wall
with concrete roof in Manukawa & Sukhpur. Note: Walls survived
due to diaphragm action from roof. Cantilever beams embedded in
walls also helped this.
Note: window openings are also not close to corners. Source:
Beneficial effects of masonry infill walls on seismic performance
of RC frame buildings. 12th World
Conference on Earthquake Engineering, Auckland, New Zealand,
Paper No.1790.
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NON-ENGINEERED CUT-STONE MASONARY WALL BUILDINGS:
Generally, cut-stone and concrete blockwork buildings are built
with more care and attention
than rubble masonry structures but again were not seismically
designed. Older buildings had
timber floors and roof, while newer construction have concrete
floors with a flat concrete roof or
a clay tiled timber roof. Many were damaged but did not
collapse. Damage varied from slight to
heavy damage.
The masonry buildings which performed the best, have the
following features in common:
Cut-stones were bedded in cement mortar
Roofs were properly fixed to the top of the walls.
Window openings were sensibly sized in relation to the total
wall length;
Buildings were symmetrical with no concentrated masses;
Many had cross walls at sensible spacing, although it was
unclear whether they were
adequately tied at T and L junctions;
Foundations were typically founded at 0.5 to 1.0m depth,
probably on firm to medium dense soils or rock.
Figure 17: Cut-stone building in Bhuj
Source: Earthquake Engineering Research Institute, EERI Web site
at www.eeri.org.
An old government building (predating 1900s) made with solid cut
stone masonry walls is
shown in Figure 17. This building received slight to moderate
damage although it is in the centre
of Bhuj and all around, rubble buildings have totally
collapsed.
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Figure 18 shows a two-storey modern cut-stone wall building near
Bhuj, in town called
Mirzapur.
The building has cut-stone walls about 0.225 to 0.3m thick and
has a 1st level concrete floor and
a pitched timber roof. The window openings are not close to the
edge and are also sensibly
spaced. This is probably one of the main reasons why it survived
with so little damage. Even so
some vertical bending cracking has happened near to the corners,
again due to out of plane shear
forces.
Figure 18: Modern cut-stone masonry building in Mirzapur
Source: Earthquake Engineering Research Institute, EERI Web site
at www.eeri.org.
Many buildings which did not collapse suffered from severe
diagonal cracking at their corners,
some with partial collapse at corners, primarily because of
window openings being too close to
the corner and because of lack of toothing between returns.
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NON-ENGINEERED REINFORCED CONCRETE BUILDINGS:
In the last 10 to 15 years reinforced concrete frame structures
have become a common
construction feature of domestic buildings in Kutch. These are
usually frames of concrete
column and slab construction with either a flat concrete roof or
a pitched timber roof to keep the
interior of the building cool in the summer. They are usually up
to 2 to 3 storeys in height. These
buildings were designed to support the vertical weight of the
structure. The majority were
damaged in the earthquake because they were not designed to
resist horizontal forces caused by
seismic loading.
Figure 19: The inset shows large deformations were concentrated
at
column heads, which caused many soft storey failures, as per
picture. Buildings if designed with uniform deflections as per left
diagram
of insert would have survived without collapse. Source : Gujarat
Relief Engineering Advice Team (GREAT)
Figure 19 shows a building, which collapsed because part of the
floor area was converted to an
opening for car parking. The building was subjected to torsion
about its centre of rigidity and
failed because of soft storey behaviour with large deformations
and rotations concentrated at the
top of the columns.
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Figure 20 : Soft storey second floor collapse in Sukhpur
Source : Gujarat Relief Engineering Advice Team (GREAT)
Figure 20 shows a building where the owner had a middle floor
supported on columns with large
internal open spaces, and hardly any masonry infill walls. Under
seismic loading, large
deformations occurred at the top and bottom of the columns and a
soft storey collapse occurred,
the upper floor storey falling onto the first storey. This shows
that soft storey collapses do not
always occur at ground floor.
Often, the owner retained an local architect and sometimes a
local structural engineers practice
to design the building. Even so, no buildings were designed for
seismic shaking. If it were not for
buildings having non-structural infill wall panels many more
buildings might have experienced total collapse.
Seismic shear force and deformations would have been
concentrated at the column heads, causing soft storey
failures as occurred in many multi-storey structures with
large
openings at ground level.
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Example of a 3-Storey reinforced concrete frame structure, which
is severely
damaged in Kundanpur (near Kera) Kutch:
An example of a recently completed reinforced concrete frame
building with block work
masonry infill walls which was severely damage, caused by a
catalogue of poor design practices
is described below. The owner of this property had retained the
service of a local engineer to
design his building.
a) Poor building configuration (resulting in torsion during
earthquakes). The ground
floor plan was asymmetrical (L-shaped internally) relative to
the floors above. As a result,
the whole building at ground floor level has twisted clockwise
under the heavy mass from
the floors above. Severe damage has occurred to the walls and
columns at ground floor
level. The reason for the L shape plan at ground level was
because the owner wanted a large
open plan living room area.
b) Discontinuous columns. Figure 21 shows that the external
columns along the wall are
not continuous with the columns at first floor level and above.
Only the corner columns are
continuous through all the floors. This was a building where the
owner decided during
construction that the engineer had not allowed enough columns
and he decided to place a
few more between the walls. Unfortunately, they were placed
randomly along the walls as
shown.
c) Large window openings. Figure 21 also shows that the window
openings between
columns are large, exceeding the limit of 33% of total wall
length as advised by the Indian
codes for a three storey plus roof structure.
d) Short column failures. Short column failure (diagonal
cracking) can be seen to have
occurred over the mid height of all the external concrete
columns (these were 225mm
square) and through the masonry columns. This is because when
infill walls with wide
openings are attached to columns, the portion of column that
will deform under lateral
seismic loading becomes very short compared to its normal
height. Such short columns
become much stiffer and attract much larger shear forces
resulting in severe diagonal
tension and cracking failure in the columns.
Under the action of the seismic shear and torsional effects, the
damage to this building was
largely concentrated at ground floor level with upper floors
remaining intact and undamaged.
The first floor concrete slab and beams were undamaged by the
earthquake.
The foundation plans show walls were on concrete strip
foundations, 0.75m wide, founded at a
depth of 0.9m below ground. The external canopy columns were on
1.2m square pad foundations
located at the same depth. The building was founded on a mixture
of weak weathered sandstone
rock at one end and medium dense to dense sand at the other end.
The owner stated that the
foundations had not failed. Photos and videos examined by the
authors confirmed this was
correct. There was no evidence of the structure experiencing
significant total and differential
settlement.
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Figure 21: Floor plans
Source : Gujarat Relief Engineering Advice Team (GREAT)
Figure 22: Building under construction one year prior to
earthquake
Source : Gujarat Relief Engineering Advice Team (GREAT)
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Figure 23: Damage to completed building after earthquake
Figure 24: Large window openings close to corners and short
column failures
Figure 24 a : Diagonal cracking at corner column caused by
twisting
of frame and short column failure.
Source : Gujarat Relief Engineering Advice Team (GREAT)
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Performance of reinforced concrete frame buildings
A large number of reinforced concrete frame buildings located in
Ahmedabad suffered serious
damage or collapsed. As stated earlier, Ahmedabad is about 300
km from the epicenter. At such
a distance the intensity of ground motion would not be expected
to be large. The fact that a
number of buildings in Ahmedabad suffered damage could be
attributed to several factors. Many
buildings were founded on deep sediments deposited by the
Sabarmati river. This may have
amplified the ground motion experienced by such buildings.
Another important factor contributing to the damage was the use
of open first storey combined
with poor detailing and indifferent quality of construction.
Almost all buildings with open first
storey suffered some damage. In some cases the buildings
collapsed, while in some others the
damage was so severe that the buildings had to be written off.
At the time of our visit, which is
about 7 weeks after the earthquake, the rubble from the
collapsed building had been cleared but
the severely damaged buildings had not been pulled down.
Figure 25: A block of damaged reinforced concrete frame
buildings in Ahmadabad
Source : Gujarat Relief Engineering Advice Team (GREAT)
A typical example of a framed building with open first storey is
shown in Fig. 25, which shows
what was once a complex of four identical five-storey blocks.
Each block had a reinforced
concrete frame construction with an open first storey and brick
infill walls in upper storeys. Two
of the four blocks, which were located in the foreground of the
picture, completely collapsed
killing several residents. The other two blocks that are seen
standing in the picture suffered
severe damage. The owners have decided to pull them down.
Temporary supports have been
provided to the buildings in their lowest storey so that the
useful contents of the buildings could
be salvaged.
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Any number of examples can be cited of the damage suffered by
the open first storeys in
multistory reinforced concrete buildings in Ahmedabad. A
particularly tragic case was of a ten-
storey building known as Shikhara. The building was in the shape
of an H. It had been completed
only recently and was not fully occupied. One of the open arms
of the H collapsed during the
earthquake causing the death of 89 persons. Details of the
building are shown in Figs. 26 and 27.
The collapse was evidently caused by the failure of the columns
in the open first storey. The
first-storey columns in parts of the building that remain
standing are severely damaged. Attempts
have been made to repair these columns, as shown in Fig. 26, but
the residents are unwilling to
return to the building.
The technique used for repairs to the columns of the first
storey can be observed from Fig. 27.
The columns are being prepared for concrete jacketing. In the
present case they have been
encased in four vertical angle sections, one at each corner. The
angles are tied together by
welding horizontal steel bars. Forms will be erected around this
assembly and concrete will be
poured from an open space at the top of the forms to complete
the concrete jacket.
Figure 26: One wing of the Shikhara building detached itself
from the building and collapsed.
Source : Gujarat Relief Engineering Advice Team (GREAT)
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Figure 27: Repairs to damaged columns in the first storey of the
shikhra building
Source : Gujarat Relief Engineering Advice Team (GREAT)
Another large reinforced concrete frame building whose failure
attracted much publicity was the
Mansi building located in downtown Ahmedabad. The building is 12
stories tall and consists of
two identical but separate blocks. A part of one of the two
blocks completely collapsed killing
22 people. The open first-storey columns of the parts that
remain standing are heavily damaged.
The building has been abandoned and its fate remains to be
decided. Figures 28 and 29 show
some details of the damaged building. An observation of the
remaining parts of this building
indicates that the most likely cause of the collapse was the
soft first storey. The masonry infills in
the upper stories of the building make the building stiff,
attracting significantly higher
earthquake forces. The high shears imposed on the first-storey
columns have caused damage to
the visible hinge regions at the top of the columns, as well as
shear failure in some of the
columns, as seen in Figure 29.
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Figure 28: The portion of the Mansi building that collapsed
detached itself from the block seen in the
foreground; the other block in the background is still standing,
but its first-storey columns are heavily damaged.
Figure 29: Shear failure of a first-storey column in the Mansi
building.
Source : Gujarat Relief Engineering Advice Team (GREAT)
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Concrete frame buildings with open first storeys and masonry
infill walls in the upper levels
located in the epicentral region of Bhuj, Anjar, and Gandhidham
suffered a worst fate. First, the
ground motion was more intense in these areas; second, the
infills were in most cases made with
heavier stone blocks rather than in clay bricks. Some examples
of damaged or collapsed
buildings are shown in Figs. 3032. Figure 30 shows the collapsed
open first storey of a four
storey concrete frame building in Bhuj in which the upper
storeys have come down as a rigid
body. Figure 31 shows a similar building also in Bhuj. In this
case the columns on one side of the
building failed and the building came down to rest on its side.
Figure 32 shows some columns in
the first storey of a building in Anjar. The loss of concrete
cover and the lack of sufficient hoop
reinforcement have caused the columns in the open storey to be
severely damaged in the hinge
region.
Figure 30: The open first storey of this building in Bhuj was
crushed bringing the upper three storeys down.
Figure 31: The columns on one edge of the open first storey of
this building in Bhuj collapsed bringing the
building down on its side.
Source : Gujarat Relief Engineering Advice Team (GREAT)
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Figure 32: Failure of column through plastic hinging and
buckling of longitudinal reinforcement due to loss of
concrete cover and insufficient hoop reinforcement.
Source : Gujarat Relief Engineering Advice Team (GREAT)
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Summary and conclusions from the case study:
The moment magnitude Mw 7.7 earthquake that struck the Kachchh
region of the province of
Gujarat in India at 8:46 a.m. on 26 January 2001 caused
tremendous loss of life and property.
The epicenter of the earthquake was located at 50 km northeast
of the town of Bhuj. The
earthquake was felt over a large part of India, and while the
greatest damage due to the
earthquake occurred in the region of Kachchh, many other parts
of Gujarat, including the major
urban center of Ahmedabad, were quite severely affected. The
official estimate of casualties is
20 000. The number of injured is reported to be 166 000. The
earthquake caused extensive
ground movement, cracking, liquefaction, and lateral spreading
in the region of Kachchh. About
370 000 houses and huts were completely destroyed, while another
931 000 were partially
destroyed. The total financial loss is estimated at $7.1 billion
(around 379,850,000,000 Indian
rupees).
Important conclusions that can be drawn from the present survey
can be summarized as follows:
1. There is a need for a study of the type of
earthquakeresistant construction that would be
suitable for the rural areas and smaller urban centers of
developing countries. Most of the
destruction caused by earthquake has taken place in such
countries, and in the present age of
global interaction and global economy it is incumbent upon
developed countries such as Canada
to undertake such a study.
2. The beneficial effect of masonry infill walls in reinforced
concrete frames in resisting
earthquake forces was evident in the performance of various
buildings during the Gujarat
earthquake. The infills prevented the collapse of many buildings
even though such infills were
neither reinforced nor positively tied to the boundary elements.
A comprehensive study is
required to assess the effectiveness of infill panels in
providing resistance to earthquake forces.
3. Experience during the Gujarat earthquake has shown that
building codes and standards should
form the basis of regulations governing building design, so that
they have a legal standing.
Although India has a comprehensive set of codes and standards
governing earthquakeresistant
design, they do not have a legal standing and are thus only
advisory in nature. A consequence of
this was that the designers in Gujarat had little incentive to
conform to the codes and standards,
and even the engineered buildings did not conform to the
recommendations of the relevant codes
and standards.
4. The Gujarat earthquake reestablished the need for designing
the lifeline structures and
essential facilities to ensure their survival during such
events, so that the services necessary for
rescue and recovery are not adversely affected. Widespread
failure of power in the district of
Kachchh was caused because a large number of control room
buildings in the electric substations
collapsed, damaging the control equipment and batteries. A
number of hospital buildings,
telephone exchange buildings, civil administration buildings,
and water service buildings were
damaged or destroyed, seriously hampering the rescue and relief
operations.
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SECONDARY STUDY:-
China Country of many earthquakes.
China locates between the two largest seismic belts, i.e. the
circum-Pacific seismic belt
and the circum-Indian seismic belt. Squeezed by the Pacific
plate, the Indian plate and the
Philippine plate, the seismic fracture zones are well developed
in this area. Ever since we entered
the 20th century, more than 800 earthquakes of more than
magnitude 6 have happened in China.
Earthquakes have happened in almost all the provinces,
municipalities and autonomous regions
except in Guizhou, Zhejiang and Hong Kong.
Earthquakes occurring in China were characterized by their high
frequencies, seismic intensity,
shallow epicenter and wide distributions. China, as a matter of
fact, is a country with many
earthquakes. Ever since 1900, over 550,000 people died in
earthquakes in China, which takes up
53% of the total casualties in earthquakes around the world.
Ever since 1949, more than 100
destructive earthquakes have happened in the provinces,
municipalities and autonomous regions
of China, among which 14 of them are provinces in East China.
These earthquakes caused the
death of more than 270,000 people, which took up 54% of the
total death toll caused by natural
disasters in China. The earthquake stricken districts cover an
area of 300,000 square kilometers
and more than 7 million rooms were destroyed by earthquakes. The
earthquakes and other
natural calamities are becoming the main threats to China in
peaceful time.
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1.Taipei 101, Taiwan, China
Taipei 101 formerly known as the Taipei World Financial center,
is a landmark skyscraper
located in Taipei, Taiwan. The building ranked officially as the
worlds tallest from 2004 until
the opening of the burj khalifa in dubai 2010.
In july 2011, the building was awarded LEED platinum
certification, the highest award in the
Leadership in Energy and Environmental design (LEED).
Taipei 101 is designed to withstand the typoon winds and
earthquake tremors common in its
area of the Asia-Pacific. Planner aimed for a structure that
could withstand gale winds of
60m/sec (197 ft/s, 216 km/hr) and the strongest earthquake
likely to occur in a 2,500 year
cycle.
Figure 33: Typical Floor Plan
The form of the building is simple geometric shape and
symmetrical itself.
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2.Sky city, Hunan, China
Sky City, an 838-meter (2,750-ft) building to be built by
Chinese construction company Broad
Sustainable Building (BSB), of Broad Group, will not just be the
tallest skyscraper on the planet,
but also it will be most sustainable building on the planet and
most earthquake proof structure.
According to Gizmag: If the target is met, the 838-meter
(2,750-ft) Sky City One will take
only a twentieth of the time that the Burj Khalifa, the worlds
current tallest building, took to
construct, and will stand 10 meters (33 feet) taller still upon
completion.
Sky City One advertises itself as an earthquake-resistant,
carless city which will accommodate
approximately 100,000 people and provide retail and leisure
facilities.
The structural engineers of the sky city confidents about the
buildings stability and assured that
the building can bear earthquake of 9.0 m at a time.
Figure 34: Sky city, Hunan
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The National Disaster Management Authority, in its recently
released guidelines have made it
mandatory for all new constructions in Delhi and Mumbai to have
earthquake-resistant
structures. Delhi falls in seismic zone IV, which makes it
highly vulnerable to earthquakes.
While rajasthan comes under zone II and zone III.
India's increasing population and extensive unscientific
constructions mushrooming all over,
including multistoried luxury apartments, huge factory
buildings, gigantic malls, supermarkets as
well as warehouses and masonry buildings keep - India at high
risk. During the last 15 years, the
country has experienced 10 major earthquakes that have resulted
in over 20,000 deaths. As per
the current seismic zone map of the country (IS 1893: 2002),
over 59 per cent of Indias land area is under threat of moderate to
severe seismic hazard.
The North-Eastern part of the country continues to experience
moderate to large earthquakes at
frequent intervals including the two great earthquakes. Since
1950, the region has experienced
several moderate earthquakes. On an average, the region
experiences an earthquake with a
magnitude greater than 6.0 every year.
Source The National Disaster Management Authority of India
The Bureau of Indian Standards (BIS), updated the seismic hazard
map of India in 2006. Apart
from the merging of Zones I and II, there are no major changes
in the new hazard map with
respect to the state of Rajasthan, as compared with the previous
1984 BIS map.
Western parts of the districts of Barmer and Sirohi as well as
northern sections of Alwar district
lie in Zone IV, where the maximum intensity could reach 8.0M.
The remaining areas of Barmer
and Sirohi districts, as well as the districts of Bikaner,
Jaiselmer and Sirohi lie in Zone III. The
north-eastern districts of Jhunjhunu, Sikar, Bharatpur and the
rest of Alwar also lie in Zone III.
The maximum intensity expected in these areas would be around
7.0 M. The rest of the state,
including the capital, Jaipur, lie in Zone II, where the maximum
intensity expected would be
around 6.0M .It must be noted that BIS estimates the hazard,
based in part, on previous known
earthquakes. Since the earthquake database in India is still
incomplete, especially with regards to
earthquakes prior to the historical period (before 1800 A.D.),
these zones offer a rough guide of
the earthquake hazard in any particular region.
Source http://asc-india.org/seismi/seis-rajasthan.htm
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According to the Indian Standard CRITERIA FOR EARTHQUAKE
RESISTANT
DESIGN OF STRUCTURES, The building should have a simple
rectangular plan and be symmetrical both with respect to mass and
rigidity so that the centres of mass and rigidity of the building
coincide with each other in which case no separation sections other
than expansion joints are necessary. If symmetry of the structure
is not possible in plan, elevation or mass, provision shall be made
for torsional and other effects due to earthquake forces in the
structural design or the parts of different rigidities may be
separated through crumple sections. The length of such building
between separation sections shall not preferably exceed three times
the width. Buildings having plans with shapes like, L, T, E and Y
shall preferably be separated into rectangular parts by providing
separation sections at appropriate places.
Figure 35: TYPICAL SHAPES OF BUILDING WITH SEPARATION
SECTIONS
Source: Indian Standard CRITERIA FOR EARTHQUAKE RESISTANT DESIGN
OF STRUCTURES
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Separation of adjoining structures or parts of the same
structure is required for structures having different total heights
or storey heights and different dynamic characteristics. This is to
avoid collision during an earthquake.
Source: Indian Standard CRITERIA FOR EARTHQUAKE RESISTANT DESIGN
OF STRUCTURES
Figure 36: An irregular shape faces more torsion on the vertical
section.
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Figure 37: Plan with vertical irregularities
Source: Indian Standard CRITERIA FOR EARTHQUAKE RESISTANT DESIGN
OF STRUCTURES
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Inferences
In the previous chapters we have seen that how earthquake
attacks the buildings/structure
and also, we had studied about the buildings that resists these
earthquake not completely but
partially.
Now we will study about the building forms, affects of
earthquake forces and various properties
of building forms based on the earthquake building codes.
With the above study, it is concluded that earthquake behaves
with some forces i.e.,
1. Elastic behavior, and 2. Non-elastic behavior
On buildings.
1. ELASTIC BEHAVIOUR:
Elastic earthquake behavior of buildings is primarily controlled
by configuration and
Stiffness, out of the four virtues of configuration, stiffness,
strength and ductility. All buildings
discussed in this Chapter are designed for full gravity load and
lateral load equal to 10% of the
total building weight to illustrate various concepts of elastic
behavior of buildings; the actual
design lateral force of similar buildings will depend on many
factors, like seismic zone, and type
of framing system, as specified by the design codes. The total
lateral force is distributed over the
building height and plan using provisions given in the Indian
Seismic Code IS:1893 (Part 1) 2007.
Buildings oscillate during earthquake shaking and inertia forces
are mobilized in them.
Then, these forces travel along different paths, called load
paths, through different structural
elements, until they are finally transferred to the soil through
the foundation. The generation of
forces based on basic oscillatory motion and final transfer of
force through the foundation are
Significantly influenced by overall geometry of the building,
which includes:
(a) plan shape, and
(b) Plan Aspect ratio.
Plan Shape: The influence of plan geometry of the building on
its seismic performance is best
understood from the basic geometries of convex- and concave-type
lenses (Figure 38). Buildings
with former plan shape have direct load paths for transferring
seismic inertia forces to its base,
while those with latter plan shape necessitate indirect load
paths that result in stress
concentrations at points where load paths bend. Buildings with
convex and simple plan
geometries are preferred, because they demonstrate superior
seismic performance than those with
concave and complex plan geometries.
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Figure 38: convex- and concave-type building plans
To illustrate the above concept, five-storey moment frame
buildings with seven plan shapes
are considered; six of them have complex plan geometries and one
has the simple rectangular
Geometry (Figure 3.1). Each building has a basic frame grid with
columns spaced at 4m, i.e.,
each unit is of 16m2 area. The rectangular having plan
dimensions of 12m16m, with 3 and 4
bays in the two perpendicular plan directions (Figure 3.2).
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Figure 39: Plan shapes of buildings
Buildings with (a) simple shapes undergo simple acceptable
structural seismic behaviour, while
(b) those with complex shapes undergo complex unacceptable
structural seismic behavior.
Figure 40: Rectangular building plan
Each building with complex shape is composed of the basic 3 bay
by 4 bay rectangular modules with column spacing of 4m in each plan
direction.
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Rectangular (or square) columns are good in resisting shear and
bending moment about
axes parallel to their sides. Thus, it is important to have
buildings oscillating primarily along
their sides translation along diagonals or torsional motions are
NOT good for seismic performance of columns, and hence, of
buildings (Figure 3.3). Further, in regular buildings, the
overall motion is controlled by the first few modes of
oscillation; the fundamental mode
(corresponding to largest natural period) usually contributes
maximum, followed by the 2nd
mode, 3rd mode, etc. Thus, it is desirable to have pure
translation modes as the lower modes of
oscillation and push torsional and diagonal translational modes
to the higher ranks. Primarily,
these undesirable (diagonal translation and torsional) modes
arise when there is lack of
symmetry in the plan shape of buildings along the sides. It is
important to have regular plan
shape of buildings.
Figure 41: Diagonal translational and torsional oscillatory
motions
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Six buildings, without any irregularity in mass or stiffness,
but with complex shapes are
chosen to compare the effect of plan shape on elastic behavior
of buildings 42. These
buildings have approximately the same plan area of about
2496m2.
Figure 42: Buildings of different plan shapes
Buildings with complex shapes, particularly with projections or
re-entrant corners, exhibit
special modes of oscillation, in addition to translatory (pure
or diagonal) or torsional modes.
These include an opening-closing mode, and the unique
local-high-frequency oscillatory mode
like, that of the wagging of a dogs tail. Dog tail wagging mode
of oscillation is interesting
because in this mode, only a slender or long projection
oscillates and the remaining part of the
building almost remains still, just like the dogs body remains
still when its tail wags. The effect of these special modes of
oscillation is to induce high stress concentration at the
re-entrant
corners that may cause minimum structural damage.
Another common discontinuity in load path in moment frames
arises with set-back columns,
i.e., when a column coming from top of the building is moved
away from its original line, again
usually at the ground storey. In such cases, loads from the over
hanging portions take detour and
cause severe stress concentration at the re-entrant corners
while traveling to the nearest set-back
column. In addition, the set-back divides the span of beams into
smaller segments, and thereby,
pushes these beams into shear action (as against flexural
action; Figure 3.36). These beams then
draw large amount of shear force, and can fail in brittle shear
mode. As a consequence, set-back
columns subjected to large axial force, become vulnerable to
combined axial-moment-shear
failure.
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Figure 43: Building with lack of grid planning
Non-uniform distribution of forces can cause localized failures
in members thereby affecting the
structural integrity of the building.
(A)
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(B)
(C)
(D)
Figure 44: Buildings with lack of grid, showing BM
distribution
Source: Earthquake behavior of buildings, Govt. of Gujarat
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2. INELASTIC BEHAVIOUR
Some structural damage is allowed during strong earthquake
shaking in normal buildings,
even though no collapse must be ensured. This implies that
nonlinearity will arise in the overall
response of buildings, which originates from the material
response being nonlinear. This
nonlinearity arising from the material stress-strain curve is
called material nonlinearity. But,
sometimes, the stress-strain curve may be nonlinear and also
elastic, whereby on unloading, the
material retraces the loading path. Structural steel has
definite yield behavior and does not
retrace its loading path when unloaded after yielding. Such a
response is more commonly
referred to as inelastic response. When an inelastic material is
subjected to reversed cyclic
loading (of displacement type) which takes the material beyond
yield, hysteresis takes place, i.e.,
the material under the applied loading absorbs/dissipates
energy. Reinforced concrete and
structural steel are candidate materials for inelastic behavior.
Under strong earthquake shaking,
normal reinforced concrete and steel buildings experience
inelastic behavior.
Hence, with the help of above data and analysis it is noted that
to design an earthquake resistant
structure, both the building form and structural details are to
be considered in designing.
The four important properties of earthquake are to be
considered:
1. Stiffness
2. Strength 3. deformation 4. energy based
Of the four methods of design, the deformation-based design
method is the most advanced, and
is expected to give best earthquake performance. It requires
more engineering experience and
judgment, but the results build more confidence in designers to
arrive at a building that is more
likely to perform as intended. Therefore, this method is best
suited for special buildings, where
earthquake performance of the building should be guaranteed,
e.g., critical and lifeline buildings
that are required to remain functional after the earthquake.
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EARTHQUAKE RESISTING BUILDING FORMS
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