Earthquake-Resistant Design for Civil Engineering Structures, Earth
Earthquake Resistant Building Structures
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Transcript of Earthquake Resistant Building Structures
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Earthquake Resistant Building Structures
Shubhayu Dutta, Smita S. Kamble, Nikhil V. Bandwal
Civil Engineering Department,
Yashwantrao Chavan College Of Engineering, Nagpur.
Abstract--We know that now a days, reinforced concrete
buildings are widely used on a large scale, in almost in every
nook and corner of the world. Further it should be taken into
account that, at some part, ‘EARTHQUAKE’ is responsible
for the failure and dismantilation of structures, apart from
bad workmanship, which results into loss of life to the most
other than financial losses. Hence while building structures;
Earthquake resistant philosophy should be taken into
consideration. As the saying goes “PREVENTION IS
BETTER THAN CURE”. Thus we should try to build
earthquake resistant buildings rather than go for
rehabilitation after the building undergoes failure.
Earthquake shaking generates inertia forces in the
building, which are proportional to building mass.
Earthquake resistant buildings, especially their main
elements, need to be built with ductility in them. This is
because ductile members undergo more deformation before
failing. Further all major components like Foundation,
Beams, Columns, Beam-column joint; Shear walls should be
well designed. If the structure is build in a high a high seismic
zone it is essential that structures should be isolated or even
seismic dampers should be provided for more safety.
Design of beam, a horizontal member in RC building for
seismic performance is very essential. Beams fail due to
flexural and shear failure. Column, a vertical member in a
building, should also be well designed for good seismic
performance. Indian Standard IS13920-1993 prescribes
design for both beam and column. Design of beam-column
joint, and shear wall importance should be taken into
consideration in RC building for good seismic performance.
Vertical plates like RC walls, called shear walls should be
provided in addition to slab, beams and columns. Shear walls
are likely vertically oriented wide beams that carry
earthquake load downwards to foundation. We cannot afford
to build concrete buildings meant to resist several
earthquakes without shear walls. Reinforcement bars in RC
walls, isolation technique and dampers are the mostimportant for earthquake proof building. Earthquake
Resistant Structures therefore depend on capacity of
structures to resist the earthquake inertial force.
I. INTRODUCTION
Earthquake is catastrophic movement of earth’s surface causing
the ground to shake. Primary cause of earthquake is the rapture of
fault in the earth crust and associated rapid slips on the faults.
Large strain energy released during an earthquake and then travel
as seismic wave in all direction through earth’s layer. Seismic
waves then pass through structural components such as
foundation, beams, columns, column-beam-joints, slab, that
generate inertia forces at top of structure due to which structure
may collapse. This leads to loss of human beings and financial
losses too. So, to avoid this, performance of building during
earthquake has to improve. As now a days, reinforced concrete
buildings are mostly used, some design for improving
performance of RCC building during earthquake are given in
paper.
The majority of deaths, injuries and losses from earthquake are
caused by the damage or collapse of buildings and other structural
components. These losses can be reduced through documenting
and understanding how structures respond to earthquakes.
Gaining such knowledge requires a long term commitment
because large devastating earthquakes occur at irregular and often
long intervals. Recording instruments must be in place and
waiting, ready o capture the response to the next temblor
whenever it occurs. The new information acquired by these
instruments can then be used to better design earthquake resistant
structures.
II . WHAT IS EARTHQUAKE ?
Earthquakes are the earth natural means of releasing stress.
When the earth’s plates move against each other, stress is put on
the lithosphere. When this stress is great enough, the lithosphere
breaks or shifts.
Imagine holding a pencil horizontally. If you were to apply a
force to both ends, you would see the pencil bend. After enough
force was applied, the pencil would break in the middle, releasing
the stress you have put on it. The earth’s crust acts in the same
way as the plates move, they put forces on themselves. When the
forces are large enough, the crust is forced to break. When the
break occurs, the stresses are released as energy which moves
through earth in the form of waves, which we feel and call an
Earthquake. Energy is released during the earthquake in several
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forms, including as movement along the fault,
seismic waves that radiate out from the “source” a
ground to shake, sometimes hundreds of kilometres
III. CAUSES
Earthquakes cause from deformation of outer, b
from tectonic plates the earth’s outermost layer of c
mantle. Due to heating and cooling of the rocks
plates resulting convections causes the adjacently oto move, under great stresses. Sometimes tremend
built-up within a single or between neighbouring
cumulated stress exceeds the strength of the rocks,
break suddenly, releasing the stored energy as an ea
IV. BUILDING’S RESPONSE TO EARTHQ
Response of the building to ground m
complicated as the ground motion itself, yet t
different. It also begins to vibrate in a complex
because it is now a vibratory system, it also posses
content. However, the building's vibrations tend to
one particular frequency, which is known as i
fundamental frequency.The building during
experiences displacement and acceleration.
Building does not undergo displacements t
compared to the building size itself. So it is not th
the building moves that causes damage, instead, it i
sudden force that causes the building to shift quick
the building to suffer damage. This is governed by
law of motion. F=ma
It is important to know that F is actually what'
inertial force, that is, the force created by the buildi
to remain at rest, and in its original position, ev
ground beneath it is moving. This is in accordanc
important physical law known as D'Alembert's Princ
V. BEHAVIOR OF RC BUILDING DURING EA
The earthquake shaking generates inertia forces i
which are proportional to the building mass. Thes
downward through slab; and beam to column; and t
foundation from where they are dispersed to the
Therefore, lower storey experiences higher earthq
forces and thus should be designed to be stronger
storey above (Fig:1)
Fig:1 Total earthquake forces
increases from top to bottom.
as heat, and
nd causes the
away.
rittle portions
ust and upper
below these
erlying platesous energy is
plates. If the
the rocks can
thquake.
UAKE
otion is as
pically quite
manner, and
ses frequency
centre around
ts natural or
earthquake
at are large
distance that
is more of the
ly that causes
Newton's 2nd
s known as an
ng's tendency
n though the
with another
iple.
THQUAKE
n the building
forces travel
en column to
round.(Fig:2)
uake induced
than those in
Fig:2
VI. SEISMIC DESIGN
Severity of ground shaking at given
can be minor, moderate or strong
frequently; moderate shaking, occasio
rarely. The structures may be designed
a) Under minor, but frequent shaking
building can carry vertical and horizo
damaged, however building parts tha
sustain repairable damages. B) Unshaking, the main members may sustai
the other parts of the building may b
may even have to be replaced after ea
but rare shaking, main members may
the building should not collapse.
VII. DESIGNS OF
For a building to remain safe d
columns (which receives forces from
than beams; and foundations(whic
columns), should be stronger than col
between beams and columns; and
should not fail so that beams can safelyand columns to foundation.
When this strategy is adopted in d
occur initially in beams. When beams
large amounts. In contrast, if column
suffer severe local damage at the top
storey. This localized damage can lead
although columns at storey level
undamaged.
Beam is a horizontal member in
sustain basically two types of failure:-
failure and shear failure.
Designing a beam involves sel
properties; amount and displacement
beam. These must be determined
calculations as per Indian standards I
CRITERIA
location during earthquake
. Minor shaking occurs
nally and strong shaking
by the following criteria:-
the main members of the
ntal forces should not be
t do not carry load may
er moderate, occasionalrepairable damage, while
e damaged such that they
rthquake. C) Under strong
ustain severe damage but
EAMS
ring earthquake shaking,
beam), should be stronger
h receives forces from
umns. Further, connection
olumns and foundations
transfer forces to columns
sign, damage is likely to
are built properly to have
s are made weaker, they
and bottom of a particular
to collapse of a building,
above remain almost
RC building. Beam can
amely flexural or bending
ection of its materials
f steel; to provide in the
by performing deign
13920-1993.longitudinal
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bars and stirrups should be provided in a beam for better
performance.
F
Fig: Beam Reinforcement as per IS 13920-1993
1. Longitudinal bar:- Longitudinal bar is provided to resist
flexural cracking on the side the beam that stretches. Since both
top and bottom faces stretch during strong earthquake shaking,longitudinal steel bars are required on both faces at the end and on
the bottom face at mid length. Indian standard code IS 13920-
1993 prescribes that.
1. At least two bars go through the full length of the beam at the
top as well as bottom of the beam.
2. At the end of the beams, the amount of steel provided at the
bottom is at least half that at the top.
2.Stirrups:
Stirrups in RC beam helps in three ways:-
a)They carry the vertical shear force and thereby resist diagonal
shears cracks
b)They protect the concrete from bulging outward, due to flexure.
c)They prevents the buckling of compressed longitudinal bars due
to flexure.
VIII. DESIGN OF COLUMNS
Column is the vertical member in building consisting of two
steel reinforcement namely long straight bars called longitudinal
bar and tansverse ties placed at regular intervals. Obviously, the
columns should be straight. Column sustains two types of
damages as axial flexural failure and shear failure.
Designing of column involves selection of material to be used,
choosing shape and size of cross section and calculating amount
steel distribution. Column should be at least 300mm wide.
Column that requires resisting earthquake must be designed to
prevent shear failure by skilful selection of reinforcement.
Closely spaced horizontal closed ties help in three ways:-
1. They carry horizontal shear induced by earthquake and
resist diagonal shear cracks.
2. They hold together the vertical bar and prevent them
from excessively bending outward called buckling.
3. They contain concrete in the column within the loop.
Fig: Column and joint detailing as per IS 13920-1993
The end of the ties must be bent as 135 hook. Such hook end
prevents opening of loops and consequently buckling of concrete
and bulging of vertical bars. Indian standard IS13920-1993
prescribes that for earthquake resist column.
a) Closely spaced ties must be provided at the two ends of
column over a length not less than larger dimension of
the column one sixth of the column height or 450mm.
b) Over the distance specified in paragraph above the
beam-column junction, the vertical spacing of the ties inthe column should not exceed D/4. Where D, is the
smallest dimension of the column. This spacing need
not more than 100mm and less than 75mm.
c) The length of tie beyond the 1356 bend must be at least
10 times diameter of steel bar used to make closed tie;
this extension beyond the bend should not be less than
75mm.
IX.DESIGN OF BEAM-COLUMN JOINT
In RC buildings, portion of column that is common to the beam
at their intersection is called beam-column joint. This joint has
limited force carrying capacity. When forces, larger than these are
applied during earthquake, joints are severely damaged.
Under earthquake shaking, beams adjoining a joint are
subjected to moments(clockwise and anticlockwise direction).
These forces are balanced by bond stresses developed by concrete
and steel in joint region. If the column is not wide enough or if
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strength of concrete in joint region is low, there
grip of concrete on steel bar. In such circumstance
inside the joint region and beams lose their capacity
Further under action of pull-push force at the top an
if column cross sectional size is insufficient, the c
joint develops diagonal cracks.
Problem of diagonal cracking and crushing of co
region can be controlled by providing closely spac
steel ties around column bar in the joint region.
together the concrete in the joint and also resist sh
reducing the cracking and crushing of concrete.
X.SHEAR WALL
Vertical plate like RC wall in RC building c
addition to lab, beams and column. These walls ge
foundation level and continuous throughout the bu
There thickness can be as low as 150mm or as high
high-rise buildings. Shear walls are like vertically
beams that carry earthquake load downwards to fou
Advantages of shear walls:- We cannot afford to
building meant to resist severe earthquake withou
Shear wall are effective both in terms of construc
effectiveness in minimizing earthquake damage in
non-structural elements like glass window and buil
Shear walls in building must be symmetrically locat
reduce ill effect of twist in building. Shear w
effective when located along exterior perimeter of
Such a layout increases resistance of the buildin
Shear walls perform well if designed to be ductile.
XI. BASE ISOLATION TECHNIQUE
The concept of base isolation is to isolate the
the ground in such a way that earthquake m
transmitted up through the building or at least grea
the building is made to rest on flexible pads that
against lateral movement, then some effect of the g
will be transferred to the building above. If the bui
pads are properly chosen the force induced by gr
is insufficient
the bar slips
to carry load.
d bottom end,
oncrete in the
crete in joint
d closed-loop
The ties hold
ear force and
alled shear in
erally start at
ilding height.
as 400mm in
oriented wide
dation.
uild concrete
t shear walls.
tion cost and
structural and
ing contents.
ed in plane to
lls are more
the building.
to twisting.
building from
tion are not
tly reduced. If
offer resistant
ound shaking
ilding flexible
und Shaking
can be a few times smaller than that e
build directly on ground.
The flexible pads are called base isolat
which are protected by means of the
isolated structures. This technology in
structures. Therefore, a robust medium
concrete building becomes extremely
often designed to absorb energy and
system. This helps in reducing the
building. The brand of isolation availab
a rubber pads. Base isolation is not sui
not suitable for high-rise building or bui
XII. SEISMIC DA
We can improve the seismic perfor
installing seismic dampers in place of s
diagonal braces. These damper acts
absorber in car-much of the sudden je
thus damps the motion of the building.
1990’s to protect building against eart
of dampers has proven very effec
Commonly used seismic
1.Viscous damper:- Energy is absorb
passing between piston-cy
2. Friction damper:- Energy is absorb
between them rubbing a
3. Yielding damper:- Energy is absorb
that yield as shown in figure below.
Fig Seismic
XIII. CONC
Earthquake is not wholly
structures, but improper
elements are responsible. TBetter workmanship, best
prescribed by IS and designi
as per IS and use of shear wa
technique & seismic damp
keeping in mind –“Prevention
perienced by the building
ors, whereas the structures
e devices are called base
troduces the flexibility in
rise masonry or reinforced
flexible. The isolation is
thus add damping to the
seismic response of the
le in the market looks like
table for all building. It is
ilding rested on soft soil.
PERS
mance of the building by
tructural elements, such as
like the hydraulic shock
rks absorbs part of it and
Dampers were used since
hquake effects. Hence use
tive during earthquakes.
dampers are:-
ed by silicone based fluid
linder arrangement.
d by surface with friction
ainst each other.
d by metallic components
Dampers
LUSION
responsible for failure of
designing of structural
hus a combined effect of t quality of materials
ng of beams and columns
ll and use of base isolation
rs in structures. Finally
Is Better Than Cure.”
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