earthquake introduction with seismic zones and types of forces,terminology,causes of damages,architectural features effects building during earthquake,seismic design philosophy for building,earthquake resistance building,building design codes,horizontal layout of building,vertical layout of building
2. Contents
1. INTRODUCTION ............................................................................................... 4
1.1 ZONES............................................................................................................................... 5
1.2 Types of zones .................................................................................................................. 5
Zone 5........................................................................................................ 5
Zone 4........................................................................................................ 5
Zone 3........................................................................................................ 5
Zone 2........................................................................................................ 5
Zone 1........................................................................................................ 5
1.3 TYPE OF FORCE ................................................................................................................. 6
There are three different types of stress occurs on earth crust. ...................... 6
Tension...................................................................................................... 6
Compression.............................................................................................. 6
Shearing..................................................................................................... 6
1.4 TERMINOLOGY.................................................................................................................. 6
1.4.1 Epicenter:................................................................................................ 6
1.4.2 Hypocenter or Focus:.............................................................................. 6
1.4.3 Magnitude: ............................................................................................. 6
1.4.4 Richter scale:........................................................................................... 6
1.4.5 Intensity:................................................................................................. 7
2. CAUSES OF DAMAGE ....................................................................................... 7
2.1 The Effect of Ground Shaking............................................................................................ 7
2.2Ground Displacement ........................................................................................................ 8
2.3 Flooding............................................................................................................................ 9
2.4 Tsunamis........................................................................................................................... 9
2.4.1 Tsunami Initiation ................................................................................... 9
2.5 Fire.................................................................................................................................. 11
2.6 Landslides and Liquefaction............................................................................................ 11
3. How Architectural Features Affect Building During Earthquakes? ................. 12
3. 3.1 Importance of Architectural Features ............................................................................. 13
3.2 Architectural Features .................................................................................................... 13
3.3 Size of Buildings .............................................................................................................. 13
3.4 Horizontal Layout of Buildings ........................................................................................ 14
3.5 Vertical Layout of Buildings............................................................................................. 14
3.6 Adjacency of Buildings .................................................................................................... 15
3.7 Building Design and Codes… ........................................................................................... 16
4.What is the Seismic Design Philosophy for Buildings?....................................... 17
4.1Earthquake-Resistant Buildings........................................................................................ 17
4.2Damage in Buildings: Unavoidable................................................................................... 18
4.3Acceptable Damage: Ductility.......................................................................................... 19
5.How to Make Buildings Ductile for Good Seismic Performance? ...................... 19
5.1Construction Materials .................................................................................................... 19
5.2Capacity Design Concept.................................................................................................. 20
5.3Earthquake-Resistant Design of Buildings........................................................................ 21
5.4Quality Control in Construction ....................................................................................... 22
4. Figure 1Nepal building destroyed due to earthquake................................................................ 4
Figure 2 India earthquake zone.................................................................................................. 5
Figure 3 Forces in earth crust..................................................................................................... 6
Figure 4 THESE MEN BARELY ESCAPED WHEN THE FRONT OF THE ANCHORAGE J.C. PENNY'S
COLLAPSED DURING THE 1964 GOOD FRIDAY EARTHQUAKE..................................................... 7
Figure 5 - ONE SIDE OF THIS ANCHORAGE STREET DROPPED DRASTICALLY............................... 7
Figure 6 THESE BUILDINGS IN JAPAN TOPPLED WHEN THE SOIL UNDERWENT LIQUEFACTION . 8
Figure 7 Tsunamis are initiated by a sudden displacement of the ocean, commonly caused by
vertical deformation of the ocean floor during earthquakes. Other causes such as deformation
by landslides and volcanic processes also generate tsunamis.................................................... 9
Figure 8 In deep water tsunamis are not large and pose no danger. They are very broad with
horizontal wavelengths of hundreds of kilometers and surface heights much much smaller,
about one meter...................................................................................................................... 10
Figure 9 When a tsunami approaches the shore, the water depth decreases, the front of the
wave slows down, the wave grows dramatically, and surges on land. ..................................... 10
`Figure 10 THE SEWARD, ALASKA, RAILROAD YARD WAS A TWISTED MESS AFTER BEING HIT BY
A TSUNAMI IN 1964. THE TSUNAMI WAS TRIGGERED BY THE GOOD FRIDAY EARTHQUAKE. .. 11
Figure 11 SAN FRANCISCO BURNING AFTER THE 1906 EARTHQUAKE...................................... 11
Figure 12 Buildings with one of their overall sizes much larger or much smaller than the other
two, do not perform well during earthquake........................................................................... 13
Figure 13Simple plan shape building do well during earthquakes............................................ 14
Figure 14 Pounding can occur between adjoining buildings due to horizontal vibrations of the
two buildings ........................................................................................................................... 15
Figure 15 Sudden deviations in load transfer [path along the height lead to poor performance
of buildings .............................................................................................................................. 16
Figure 16Performance objectives under different intensities of earthquake shaking – seeking
low repairable damage under minor shaking and strong shaking ............................................ 17
Figure 17 Masonry is strong in compression but...................................................................... 19
Figure 18 Tension Test on Materials – ductile.......................................................................... 20
Figure 19 Ductile chain design ................................................................................................ 21
Figure 20 Reinforced Concrete Building Design: ...................................................................... 22
5. 1.INTRODUCTION
An earthquake is a trembling or a shaking movement of the ground, caused by the
slippage or rupture of a fault within the Earth's crust. A sudden slippage or rupture
along a Fault line results in an abrupt release of elastic energy stored in rocks that
are subjected to great strain. This energy can be built up and stored over a long
time and then released in seconds or minutes. Strain on the rocks results in more
elastic energy being stored which leads to far greater possibility of an earthquake
event. The sudden release of energy during an earthquake causes low-frequency
sound waves called seismic waves to propagate through the Earth's crust or along
its surface.
Every year more than 3 million earthquakes take place, most of these unnoticed by
humans. In contrast, a severe earthquake is the most frightening and catastrophic
event of nature which can occur anywhere on the surface of our planet! Although
usually lasting only seconds, a severe earthquake in a densely populated area may
have catastrophic effects causing the death of hundreds of thousands of people,
injuries, destruction and enormous damage to the economies of the affected area.
Figure 1Nepal building destroyed due to earthquake
6. Hundreds of thousands of people have been killed by earthquakes despite
scientists being able to predict and forewarn in advance and engineers construct
earthquake-safe buildings. Unfortunately earthquakes occur often in countries
which are unable to afford earthquake-safe construction.
Besides the immediate, obvious threat presented by an earthquake, it can also set
off several other natural hazards. The energy release resulting from earthquakes
can easily trigger slope failures. A tsunami may be formed which causes flood on
coastal areas. These events occur along with volcanic activity, resulting in even
more potential danger. (htt1)
1.1 ZONES
The Indian subcontinent has a history of devastating earthquakes. The major
reason for the high frequency and intensity of the earthquakes is that the Indian
plate is driving into Asia at a rate of approximately 47 mm/year. Geographical
statistics of India show that almost 54% of the land is vulnerable to earthquakes. A
World Bank and United Nations report shows estimates that around 200 million
city dwellers in India will be exposed to storms and earthquakes by 2050. The
latest version of seismic zoning map of India given in the earthquake resistant
design code of India [IS 1893 (Part 1) 2002] assigns four levels of seismicity for
India in terms of zone factors. In other words, the earthquake zoning map of India
divides India into 4 seismic zones (Zone 2, 3, 4 and 5) unlike its previous version,
which consisted of five or six zones for the country. According to the present
zoning map, Zone 5 expects the highest
level of seismicity whereas Zone 2 is
associated with the lowest level of
seismicity.
1.2 Types of zones
Zone 5
Zone 4
Zone 3
Zone 2
Zone 1
Figure 2 India earthquake zone
7. 1.3 TYPE OF FORCE
There are three different types of
stress occurs on earth crust.
Tension
Compression
Shearing
1.4 TERMINOLOGY
1.4.1 Epicenter:
It is the point on the (free) surface of the earth vertically above the place of origin
(hypocenter) of an earthquake. This point is expressed by its geographical latitude
and longitude.
1.4.2 Hypocenter or Focus:
It is the point within the earth from where seismic waves originate. Focal depth is
the vertical distance between the hypocenter and epicenter.
1.4.3 Magnitude:
It is the quantity to measure the size of an earthquake in terms of its energy and is
independent of the place of the observation.
1.4.4 Richter scale:
Magnitude is measured on the basis of ground motion recorded by an instrument
and applying standard correction for the epicenter distance from recording
station. It is linearly related to the logarithm of amount of energy released by an
earthquake and expressed in Richter scale.
Figure 3 Forces in earth crust
8. 1.4.5 Intensity:
It is the rating of the effects of an earthquake at a particular place based on the
observations of the affected areas, using a descriptive scale like Modified Scale.
(htt2)
1.CAUSES OF DAMAGE
Earthquakes really pose little direct danger to a person.can't be shaken to death by
an earthquake. Some movies show scenes with the ground suddenly opening up
and people falling into fiery pits, but this just doesn't happen in real life.
2.1 The Effect of Ground Shaking
The first main earthquake hazard (danger) is the effect of ground shaking. Buildings can be
damaged by the shaking itself or by the ground beneath them settling to a different level than
it was before the earthquake (subsidence).
Figure 4 THESE MEN BARELY ESCAPED WHEN THE FRONT OF THE
ANCHORAGE J.C. PENNY'S COLLAPSED DURING THE 1964 GOOD FRIDAY
EARTHQUAKE.
Figure 5 - ONE SIDE OF THIS ANCHORAGE STREET DROPPED
DRASTICALLY
9. Buildings can even sink into the
ground if soil liquefaction
occurs. Liquefaction is the mixing of
sand or soil and groundwater (water
underground) during the shaking of a
moderate or strong earthquake.
When the water and soil are mixed,
the ground becomes very soft and
acts similar to quicksand. If
liquefaction occurs under a building,
it may start to lean, tip over, or sink
several feet. The ground firms up
again after the earthquake has past
and the water has settled back down
to its usual place deeper in the
ground. Liquefaction is a hazard in
areas that have groundwater near
the surface and sandy soil.
Buildings can also be damaged by strong surface waves making the ground heave and lurch.
Any buildings in the path of these surface waves can lean or tip over from all the movement.
The ground shaking may also cause landslides, mudslides, and avalanches on steeper hills or
mountains, all of which can damage buildings and hurt people.
2.2Ground Displacement
The second main earthquake hazard is ground displacement (ground
movement) along a fault. If a structure (a building, road, etc.) is built across a
fault, the ground displacement during an earthquake could seriously damage or
rip apart that structure.
From Figure 4 you can tell that the San Andreas Fault is a right-lateral transverse
(strike-slip) fault because the other side of the road (on the opposite side of the
fault) has moved to the right, relative to the photographer's position.
Figure 6 THESE BUILDINGS IN JAPAN TOPPLED WHEN THE
SOIL UNDERWENT LIQUEFACTION
10. 2.3 Flooding
The third main hazard is flooding. An earthquake can rupture (break) dams or levees along a
river. The water from the river or the reservoir would then flood the area, damaging buildings
and maybe sweeping away or drowning people.
2.4 Tsunamis
A sometimes dramatic byproduct of certain types of earthquakes are tsunamis. Tsunami is a
Japanese term that means "harbor wave". Tsunamis are frequently confused with tidal waves,
but they have nothing to do with the tides, they are the result of a sudden vertical offset in the
ocean floor caused by earthquakes, submarine landslides, and volcanic deformation. In 1883
the volcanic eruption of Krakatoa resulted in the collapse of a caldera that initiated a tsunami
which killed 36,000 people on nearby islands. On June 25, 1896 an earthquake off the
Japanese coast generated a tsunami that hit the shore with wave heights ranging from 10 to
100 feet. As the fishing fleets returned to shore following an overnight trip they found their
villages destroyed and 22,000 people dead. In the last century more than 50,000 people have
died as a result of tsunamis.
2.4.1 Tsunami Initiation
A sudden offset changes the elevation of the ocean and initiates a water wave that travels
outward from the region of sea-floor disruption. Tsunamis can travel all the way across the
ocean and large earthquakes in Alaska and Chile have generated waves that caused damage
and deaths in regions as far away as California, Hawaii and Japan.
Figure 7 Tsunamis are initiated by a sudden displacement of the ocean, commonly caused
by vertical deformation of the ocean floor during earthquakes. Other causes such as
deformation by landslides and volcanic processes also generate tsunamis.
The speed of this wave depends on the ocean depth and is typically about as fast as a
commercial passenger jet (about 0.2 km/s or 712 km/hr). This is relatively slow compared to
seismic waves, so we are often alerted to the dangers of the tsunami by the shaking before the
wave arrives. The trouble is that the time to react is not very long in regions close to the
earthquake that caused the tsunami.
11. Figure 8 In deep water tsunamis are not large and pose no danger. They are very broad
with horizontal wavelengths of hundreds of kilometers and surface heights much much
smaller, about one meter.
Tsunamis pose no threat in the deep ocean because they are only a meter or so high in deep
water. But as the wave approaches the shore and the water shallows, all the energy that was
distributed throughout the ocean depth becomes concentrated in the shallow water and the
wave height increases.
Figure 9 When a tsunami approaches the shore, the water depth decreases, the front of the
wave slows down, the wave grows dramatically, and surges on land.
Typical heights for large tsunamis are on the order of 10s of meters and a few have
approached 90 meters (about 300 feet). These waves are typically more devastating to the
coastal region than the shaking of the earthquake that caused the tsunami. Even the more
common tsunamis of about 10-20 meters can "wipe clean" coastal communities.
Deadly tsunamis occur about every one to two years and they have at times killed thousands
of people. In 1992-93 three large tsunamis occurred: one in Japan, Indonesia, and Nicaragua.
All struck at night and devastated the local communities.
Tsunamis and seiches can also cause a great deal of damage. A tsunami is what most people
call a tidal wave, but it has nothing to do with the tides on the ocean. It is a huge wave caused
by an earthquake under the ocean. Tsunamis can be tens of feet high
when they hit the shore and can do enormous damage to the coastline. Seiches are like small
tsunamis. They occur on lakes that are shaken by the earthquake and are usually only a few
feet high, but they can still flood or knock down houses, and tip over trees.
12. 2.5 Fire
The fourth main earthquake hazard is fire. These fires can
be started by broken gas lines and power lines, or tipped
over wood or coal stoves. They can be a serious problem,
especially if the water lines that feed the fire hydrants are
broken, too. For example, after the Great San Francisco
Earthquake in 1906, the city burned for three days. Most
of the city was destroyed and 250,000 people were left
homeless.
Figure 11 SAN FRANCISCO BURNING AFTER THE 1906 EARTHQUAK
Most of the hazards to people come from man-made structures themselves and the shaking
they receive from the earthquake. The real dangers to people are being crushed in a collapsing
building, drowning in a flood caused by a broken dam or levee, getting buried under a
landslide, or being burned in a fire.
2.6 Landslides and Liquefaction
Buildings aren't the only thing to fail under the stresses of seismic waves. Often unstable
regions of hillsides or mountains fail. In addition to the obvious hazard posed by large
landslides, even nonlethal slides can cause problems when they block highways they can be
inconvenient or cause problems for emergency and rescue operations.
`Figure 10 THE SEWARD, ALASKA, RAILROAD YARD WAS A
TWISTED MESS AFTER BEING HIT BY A TSUNAMI IN 1964. THE
TSUNAMI WAS TRIGGERED BY THE GOOD FRIDAY
EARTHQUAKE.
13. Occasionally large landslides can be triggered by earthquakes. In 1970 an earthquake off the
coast of Peru produced a landslide than began 80 miles away from the earthquake. The slide
was large (witnesses estimated its height at about 30 meters or 100 feet), traveled at more
than one-hundred miles per hour and plowed through part of one village and annihilated
another, killing more than 18,000 people.
In some cases, when the surface is underlain by a saturated, sand rich layer of soil, prolonged
shaking can cause the expulsion of fluid from the sand layer resulting in large "sand blows"
that erupt through the overlying strata.
In the 1811-12 earthquakes the sand blows were enormous and covered large regions of the
Missouri bootheel. Liquefaction can cause other problems as the soil loses it ability to resist
shear and flows much like quick sand. Anything relying on the substrata for support can shift,
tilt, rupture, or collapse. (htt3)
2.How Architectural Features Affect Building During
Earthquakes?
14. 3.1 Importance of Architectural Features
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.”
3.2 Architectural Features
A desire to create an aesthetic and functionally efficient structure drives architects to conceive
wonderful and imaginative structures. Sometimes the shape of the building catches the eye of
the visitor, sometimes the structural system appeals, and in other occasions both shape and
structural system work together to make the structure a marvel. However, each of these
choices of shapes and structure has significant bearing on the performance of the building
during strong earthquakes.
The wide range of structural damages observed during past earthquakes across the world is
very educative in identifying structural configurations that are desirable versus those which
must be avoided.
3.3 Size of Buildings
In tall buildings with large height-to-base size ratio (Figure
1a), the horizontal movement of the floors during ground
shaking is large. In short but very long buildings (Figure
1b), the damaging effects during earthquake shaking are
many. And, in buildings with large plan area like
warehouses (Figure 1c), the horizontal seismic forces can
be excessive to be carried by columns and walls.
Figure 12 Buildings with one of their overall sizes
much larger or much smaller than the other two, do
not perform well during earthquake
(a) too tall (b) too long
15. 3.4 Horizontal Layout of Buildings
In general, buildings with simple geometry in plan (Figure 2a) have performed well during
strong earthquakes. Buildings with re-entrant corners, like those U, V, Hand + shaped in plan
(Figure 2b), have
(a) Simple Plan
:: good (b)
Corners
Figure 13Simple plan shape building
do well during earthquakes
(and Curves : :
poor
(c) Separation joints
make complex plans
into simple plans
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
(Figure 2c). 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.
3.5 Vertical Layout of Buildings
The earthquake forces developed at different floor levels in a building need to be brought
down along the height to the ground by the shortest path; any deviation or discontinuity in
this load transfer path results in poor performance of the building. Buildings with vertical
setbacks (like the hotel buildings with a few storeys wider than the rest) cause a sudden jump
in earthquake forces at the level of discontinuity (Figure3a). Buildings that have fewer columns
16. or walls in a particular storey or with unusually tall storey (Figure3b), tend to damage or
collapse which is initiated in that storey. Many buildings with an open ground storey intended
for parking collapsed or were severely damaged in Gujarat during the 2001 Bhuj earthquake.
Buildings on a sloping ground have unequal height columns along the slope, which causes ill
effects like twisting and damage in shorter columns (Figure 3c). Buildings with columns that
hang or float on beams at an intermediate storey and do not go all the way to the foundation,
have discontinuities in the load transfer path (Figure 3d). Some buildings have reinforced
concrete walls to carry the earthquake loads to the foundation. Buildings, in which these walls
do not go all the way to the ground but stop at an upper level ,are liable to get severely
damaged during earthquakes.
3.6 Adjacency of Buildings
When two buildings are too close to each other, they may pound on each other during strong
shaking.
Figure 14 Pounding can occur between adjoining buildings due to
horizontal vibrations of the two buildings
(a) Setbacks
17. (b) Weak or Flexible Storey (c) Slopy Ground
(d) Handing or
Floating
Columns
Figure 15 Sudden
deviations in load transfer
[path along the height
lead to poor performance
of buildings
(e) Discontinuing Structural Members
With increase in building height, this collision can be a greater problem. When building heights
do not match (Figure 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.
3.7 Building Design and Codes…
Looking ahead, of course, one will continue to make buildings interesting rather than
monotonous. However, this need not be done at the cost of poor behaviour and earthquake
safety of buildings. Architectural features that are detrimental to earthquake response of
buildings should be avoided. If not, they must be minimised. When irregular features are
included in buildings, a considerably higher level of engineering effort is required in the
structural design and yet the building may not be as good as one with simple architectural
features. Decisions made at the planning stage on building configuration are more important,
or are known to have made greater difference, than accurate determination of code specified
design forces. (Murty)
18. 4.What is the Seismic Design Philosophy for Buildings?
1. Under minor but frequent shaking, the main members of the building that
carry vertical and horizontal forces should not be damaged, however
building parts that do not carry load may sustain repairable damage.
2. Under moderate but occasional shaking, the main members may sustain
repairable damage, while the other parts of the building may be damaged
such that they may even have to be replaced after the earthquake.
3. Under strong but are shaking, the main members may sustain severe (even
irreparable) damage, but the building should not collapse
Figure 16Performance objectives under different intensities of earthquake shaking – seeking low repairable damage under minor shaking and strong
shaking
Severity of ground shaking at a given location
during an earthquake can be minor, moderate and
strong. Relatively speaking, minor shaking occurs
frequently, moderate shaking occasionally and strong
shaking rarely.
4.1Earthquake-Resistant Buildings
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 resistantsuch buildings resist the effects of ground
19. 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 thereby a disaster is avoided. This is a major
objective of seismic design codes throughout the world.
Thus, after minor shaking, the building will be fully operational within a short time
and the repair costs will be small. And, after moderate shaking, the building will be
operational once the repair and strengthening of the damaged main members is
completed. But, after a strong earthquake, the building may become dysfunctional
for further use, but will stand so that people can be evacuated and property
recovered.
The consequences of damage have to be kept in view in the design philosophy. For
example, important buildings, like hospitals and fire stations, play a critical role in
post-earthquake activities and must remain functional immediately after the
earthquake. These structures must sustain very little damage and should
be designed for a higher level of earthquake protection. Collapse of dams during
earthquakes can cause flooding in the downstream reaches, which itself
can be a secondary disaster. Therefore, dams (and similarly, nuclear power plants)
should be designed for still higher level of earthquake motion.
4.2Damage in Buildings: Unavoidable
Design of buildings to resist earthquakes involves controlling the damage to
acceptable levels at a reasonable cost. Contrary to the common thinking that any
crack in the building after an earthquake means the building is unsafe for
habitation, engineers designing earthquake-resistant buildings recognize that
some damage is unavoidable. Different types of damage (mainly visualized though
cracks; especially so in concrete and masonry buildings) occur in buildings during
earthquakes. Earthquake-resistant design is therefore concerned about ensuring
that the damages in buildings during earthquakes are of the acceptable variety,
and also that they occur at the right places and in right amounts. This approach of
earthquake-resistant design is much like the use of electrical fuses in houses: to
protect the entire electrical wiring and appliances in the house, you sacrifice some
small parts of the electrical circuit, called fuses; these fuses are easily replaced
after the electrical overcurrent. Likewise, to save the building from collapsing,
20. you need to allow some pre-determined parts to undergo the acceptable type and
level of damage.
4.3Acceptable Damage: Ductility
So, the task now is to identify acceptable forms of damage and desirable building
behaviour during earthquakes. To do this, let us first understand how different
materials behave. Consider white chalk used to write on blackboards and steelpins
with solid heads used to hold sheets of paper together. Yes... a chalk breaks
easily'.'. On the contrary, a steel pin allows it to be bent back-and-forth. Engineers
define the property that allows steel pins to bend back-and-forth by large
amounts, as ductility; chalk is a brittle material. Earthquake-resistant buildings,
particularly their main elements, need to be built with ductility in them.
Such buildings have the ability to sway back-and-forth during an earthquake, and
to withstand earthquake effects with some damage, but without collapse
. Ductility is one of the most important factors affecting the building performance.
Thus, earthquake-resistant design strives to predetermine the locations where
damage takes place and then to provide good detailing at these locations to
ensure ductile behaviour of the building.
5.How to Make Buildings Ductile for Good Seismic Performance?
5.1Construction Materials
In India, most non-urban buildings are made in masonry. In the plains, masonry is
generally made of burnt clay bricks and cement mortar. However, in hilly areas,
stone masonry with mud mortar is more prevalent; but, in recent times, it is being
replaced with cement mortar. Masonry can carry loads that cause compression
(i.e., pressing together), but can hardly take load that causes tension
Figure 17 Masonry is strong in compression but
21. Concrete is another material that has been popularly used in building construction
particularly over the last four decades. Cement concrete is made of crushed stone
pieces (called aggregate), sand, cement and water mixed in appropriate
proportions. Concrete is much stronger than masonry under compressive loads,
but again its behaviour in tension is poor. The properties of concrete critically
depend on the amount of water used in making concrete; too much and too
little water, both can cause havoc. In general, both masonry and concrete are
brittle, and fail suddenly. Steel is used in masonry and concrete buildings as
reinforcement bars of diameter ranging from 6mm to 40mm. Reinforcing steel can
carry both tensile and compressive loads. Moreover, steel is a ductile material.
This important property of ductility enables steel bars to undergo large elongation
before breaking. Concrete is used in buildings along with steel reinforcement bars.
This composite material is called reinforced cement concrete or simply reinforced
concrete (RC). The amount and location of steel in a member should be such that
the failure of the member is by steel reaching its strength in tension before
concrete reaches its strength in compression. This type of failure is ductile failure,
and hence is preferred over a failure where concrete fails first in compression.
Therefore, contrary to common thinking, providing too much steel in RC buildings
can be harmful even!!
5.2Capacity Design Concept
Let us take two bars of same length and cross sectional area - one made of a
ductile material and another of a brittle material. Now, pull these two bars until
they break!! You will notice that the ductile bar elongates by a large amount
before it breaks, while the brittle bar breaks suddenly on reaching its maximum
strength at a relatively small elongation. Amongst the materials used in building
construction, steel is ductile, while masonry and concrete are brittle.
Figure 18 Tension Test on Materials – ductile
22. Now, let us make a chain with links made of brittle and ductile materials (Figure 3).
Each of these links will fail just like the bars shown in Figure 2. Now, hold the last
link at either end of the chain and apply a force F. Since the same force F is being
transferred through all the links, the force in each link is the same, i.e., F. As more
and more force is applied, eventually the chain will break when the weakest link in
it breaks. If the ductile link is the weak one (i.e., its capacity to take load is less),
then the chain will show large final elongation. Instead, if the brittle link is the
weak one, then the chain will fail suddenly and show small final elongation.
Therefore, if we want to have such a ductile chain, we have to make the ductile
link to be the weakest link.
.
5.3Earthquake-Resistant Design of Buildings
Buildings should be designed like the ductile chain. For example, consider the
common urban residential apartment construction - the multi-storey
building made of reinforced concrete. It consists of horizontal and vertical
members, namely beams and columns. The seismic inertia forces generated at its
floor levels are transferred through the various beams and columns to the ground.
The correct building components need to be made ductile. The failure of a
column can affect the stability of the whole building, but the failure of a beam
causes localized effect. Therefore, it is better to make beams to be the ductile
weak links than columns. This method of designing RC buildings is called the
strong-column weak-beam design method . By using the routine design codes
(meant for design against non-earthquake effects), designers may
not be able to achieve a ductile structure. Special design provisions are required to
help designers improve the ductility of the structure. Such provisions are usually
put together in the form of a special seismic design code, e.g., IS:13920-1993 for
Figure 19 Ductile chain design
23. RC structures.These codes also ensure that adequate ductility isprovided in the
members where damage is expected.
Figure 20 Reinforced Concrete Building Design:
5.4Quality Control in Construction
The capacity design concept in earthquake resistant design of buildings will fail if
the strengths of the brittle links fall below their minimum assured values. The
strength of brittle construction materials, like masonry and concrete, is highly
sensitive to the quality of construction materials, workmanship, supervision, and
construction methods. Similarly, special care is needed in construction to ensure
that the elements meant to be ductile are indeed provided with features that give
adequate ductility. Thus, strict adherence to prescribed standards of construction
material and construction processes is essential in assuring an earthquake-
resistant building. Regular testing of construction materials at qualified
laboratories (at site or away), periodic training of workmen at professional training
houses, and on-site evaluation of the technical work are elements of good quality
control. (Murty)
24. Bibliography
(n.d.). Retrieved from http://www.sms-tsunami-warning.com/pages/earthquakes-
introduction#.Wh5l5VWWbDc
(n.d.). Retrieved from http://amssdelhi.gov.in/earthquake_terminology.htm
(n.d.). Retrieved from http://www.geo.mtu.edu/UPSeis/hazards.html
http://www.sms-tsunami-warning.com/pages/earthquakes-introduction#.Wh5l5VWWbDc.
(n.d.).
Murty, C. (n.d.). Indian Institute of Technology Kanpur, India. Building Materials and
Technology Promotion Council, New Delhi. Retrieved from
http://www.devalt.org/newsletter/sep03/of_2.htm
