Safety Symposium & Safety Symposium &
ExpositionExposition
September 11, 2015September 11, 2015
“Construction Safety in the Seismically “Construction Safety in the Seismically
active Zone”
Presented by:
Dr. K. Bandyopadhyay
Prof. & Head,
Deptt. Of Construction Engg.,
Jadavpur University
Goal of seismic zoning
To delineate regions
Providing a regions
of similar probable intensity of ground motion in a country
Providing a guideline for provision of an adequate earthquake resistance in constructed facilities
Providing step to disaster mitigation
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In terms of pure economic In terms of pure economic
theory, earthquake causes two theory, earthquake causes two
types of losses :types of losses :
� primary losses
�secondary losses
Primary an
results in the
Primary loss
an irrecoverable loss
in the loss of human life
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All other secondary All other losses
secondary losses
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A seismic zoning
map having
seven zones
adopted in IS:
1893-1962.
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A four zone
seismic zoning
map adopted in
IS:1893-2002.
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� The behaviour of soil column during strong motions generated by large, strong earthquakes � differs substantially from that during small earhthquakes (weak motions).earhthquakes (weak motions).
� The large strain level associated with the strong motions during large earthquakes forces the soil � to respond non-linearly as against an essentially linear response during small strain weak motions.
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Types of Earthquakes
(according to its depth of focus)
Shallow focus earthquakes
(depth of focus <70km.Nearly
Intermediate Deep focus
(depth of focus <70km.Nearly 80% of total earthquakes)
Intermediate focus
earthquakes
(depth ε [70,30] km)
Deep focus earthquakes
(focal depth > 300km)
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IntensityIntensity�Intensity is a qualitative measure of the strength of
an earthquake.
�An earthquake has many intensities, the highest �An earthquake has many intensities, the highest
near the maximum fault displacement and
progressively to lower grade at further away.
�The popular intensity scale is the Modified Mercalli
(MMI) scale with twelve gradation denoted by
Roman numerals from I to XII.
MagnitudeMagnitude
� The magnitude is a quantitative or absolute measure of the size of an earthquake.
� It can be correlated to the amount of wave energy released at the source of an earthquake.
� The elastic wave energy is that portion of total strain energy stored in lithospheric rock that is not consumed as mechanical work( e.g through consumed as mechanical work( e.g through faulting) during an earthquake.
� Local (Richter) magnitude (ML) is logarithmic to the base 10 of the maximum seismic wave amplitude in microns (10 -3) recorded on Wood-Andersonseismograph (having period 0.8s, nearly critical damping and magnification 2800)at a distance ( Δ) 100km from the epicentre of earthquake.
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Earthquake & Vibration Effect Earthquake & Vibration Effect
on Structureson Structures� Structures of the earth generally subjected
to two types of load:
�Static
�Dynamic
• Static loads are constant with time• Static loads are constant with time
• Dynamic loads are time-varying
• In general, the majority of Civil
Engineering structures designed with
the assumption that all applied loads
are static11/9/2015 12
••The dynamic force may be an earthquake The dynamic force may be an earthquake
force resulting from rapid movement along the force resulting from rapid movement along the
plain of faults within the earth’s crust.plain of faults within the earth’s crust.
---- This sudden movement of faults releases This sudden movement of faults releases
great energy in the form of seismic waves, great energy in the form of seismic waves,
which are transmitted to the structure through which are transmitted to the structure through
their foundations, and causes motion in the their foundations, and causes motion in the
structure.structure.structure.structure.
----These These motions are complex in nature and motions are complex in nature and
induce abrupt horizontal and vertical induce abrupt horizontal and vertical
oscillations in structures , which result oscillations in structures , which result
accelerations, velocities, and displacement in accelerations, velocities, and displacement in
the structure.the structure.
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••The induced accelerations generate inertial The induced accelerations generate inertial forces in the structure, which are proportional forces in the structure, which are proportional to acceleration of the mass and acting to acceleration of the mass and acting opposite to the ground motion as in fig.opposite to the ground motion as in fig.
Structure subjected to earthquake excitation
(Syrmakwzis & Sophocleous, 2001)
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The energy produced in the structure by the
ground
dissipated through internal friction within
the structural and non-ground
motion the structural and non-structural members
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This dissipation of energy
Dampingenergy
called
g
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Geological context:Geological context:•Indo-Gangetic alluvial plains of Bengal, Bihar,
Uttar Pradesh and the Punjab lie in between the
Peninsula and the Extra-Peninsula as per the
different stratigraphic groups of the vast Indian different stratigraphic groups of the vast Indian
subcontinent.
•This Indo-Gangetic alluvial plain has been
formed during the Quaternary era.
•They are basically made up of sand, clay and
peat beds.
•The thickness of this structurally simple
sedimentary column is of the order of about 1.5 to
6 kilometers.
SubSub--Soil of Soil of KokataKokata
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Normal Calcutta deposit
Sub-soil of Kolkata
��In some locations, superposition In some locations, superposition
of these two deposits observedof these two deposits observed
Alluvial deposit
Kolkata
Alluvial deposits
Main concern
Because of its susceptibility to liquefaction
s nto liquefaction
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Zone Depth
below
ground
level (m)
N values Remarks
III, IV & V Upto 5 15 For values of
depth
between 5 to
Desirable field values of N (as per IS 1893: 1984)
between 5 to
10m linear
interpolation
recommende
d
10 25
I & II (for
important
structures
only)
Upto 5 10
10 20
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Submerged loose sands and Submerged loose sands and
soils (classification SP) with soils (classification SP) with
standard penetration values (N) standard penetration values (N)
less than values specified aboveless than values specified above
vibrations caused by earthquake vibrations caused by earthquake vibrations caused by earthquake vibrations caused by earthquake
may cause liquefaction or may cause liquefaction or
excessive total and differential excessive total and differential
settlements.settlements.
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�In important projects, this aspect
of the problem need be
investigated.
�Appropriate methods of
compaction or stabilization adopted
to achieve suitable N.to achieve suitable N.
�Alternatively, deep pile foundation
chosen.
�Termination depths selected on
layers which are not likely to liquefy.
Seismic Evaluation and Seismic Evaluation and
RetrofittingRetrofitting
•Many of the existing buildings lacking in adequate earthquake resistance because these are not designed according to modern codes and prevalent earthquake resistant design practice.
• Also many buildings damaged in earthquakes may • Also many buildings damaged in earthquakes may need repair and upgradation of their strength in order to make them seismically resistant.
•The aim of seismic evaluation is to assess the possible seismic response of buildings ,which may be seismically deficient or earthquake damaged for its possible future use.
•The evaluation is also helpful for adopting the retrofitting of structure.
••The means of retrofitting is to upgrade The means of retrofitting is to upgrade
the strength and structural capacity of the strength and structural capacity of
an existing structure to enable it to an existing structure to enable it to
safely withstand the effect of strong safely withstand the effect of strong
earthquakes in future.earthquakes in future.
The methods available for seismic The methods available for seismic The methods available for seismic The methods available for seismic
evaluation of existing buildings can be evaluation of existing buildings can be
broadly divided into two categoriesbroadly divided into two categories
ii) qualitative methods) qualitative methods
ii) analytical methodsii) analytical methods
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Reinforced concrete multiReinforced concrete multi--
storeyedstoreyed buildings in India for buildings in India for
the first time, have been the first time, have been
subjected to a strong ground subjected to a strong ground
motion shaking in motion shaking in BhujBhujmotion shaking in motion shaking in BhujBhuj
earthquake (January 26, 2001).earthquake (January 26, 2001).
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Source of
failure
floating columns
mass irregularities
poor quality of
construction material
soil and foundation effect
pounding of adjacent structures
failure
soft storeys
poor quality of
construction material
faulty construction practices
inconsistent earthquake response
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Reinforced Concrete Building Reinforced Concrete Building
Construction PracticesConstruction Practices
Reinforced concrete construction Reinforced concrete construction ----�� the the
most common type of construction in the most common type of construction in the
major cities.major cities.
The building framing system The building framing system --�� moment moment
resisting, consisting of reinforced concrete resisting, consisting of reinforced concrete
slabs, beams and columns on shallow slabs, beams and columns on shallow
isolated footing.isolated footing.
The upper floors The upper floors --�� constructed with infill constructed with infill
walls made of unreinforced bricks, cut stones walls made of unreinforced bricks, cut stones
or cement concrete blocks.or cement concrete blocks.
The ground floor /basement The ground floor /basement --�� used for used for
commercial and parking purposes, commercial and parking purposes,
Infill walls Infill walls --��omitted, resulting in soft or weak omitted, resulting in soft or weak 11/9/2015 31
The infill walls present in upper The infill walls present in upper
floors floors ----�� absent in the ground floor absent in the ground floor
,create a floating box type situation.,create a floating box type situation.
The dynamic analysis of a G+4 The dynamic analysis of a G+4
storey RC building on floating storey RC building on floating storey RC building on floating storey RC building on floating
columns columns --��show that these show that these
buildings vibrate in buildings vibrate in torsionaltorsional mode, mode,
which is undesirable.which is undesirable.
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a) Floating box construction,
b) 3-D mathematical model of a floating type RC building,
c) First mode shape of the building in plan- a torsional mode.
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Soft Storey Failure
•The first storey has lesser strengthened
stiffness compared to upper storeys, which
are stiffened by masonry infill walls.
•This characteristic of building construction
creates ‘weak’ or ‘soft’ storey problems in
multi-storey buildings.multi-storey buildings.
•Increased flexibility of first storey results in
extreme deflections, which in turn, leads to
concentration of forces at the second storey
connections accompanied by large plastic
deformations.
•In addition, most of the energy developed
during the earthquake is dissipated by the
columns of the soft storeys.11/9/2015 34
•It has been observed that the damage
� due to collapse and buckling of
columns especially where parking
spaces not covered appropriately.
•On the contrary, the damage � reduced
considerably where the parking spaces
covered adequately.
•This type of failure � results from the•This type of failure � results from the
combination of several other
unfavourable reasons, e.g., torsion,
excessive mass on upper floors, P-∆
effects and lack of ductility in the
bottom storey.
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Fig. Soft Storey failures in reinforced concrete buildings
a) Apollo Apartment at Ahmedabad, groundfloor was
completely collapsed, b) G+6 RC framed building at Bhuj ,
intermediate weak-storey failure.
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Failure of reinforced concrete buildings with floating
columns a) 15th August Apartment,Ahmedabad, collapse
of building on floating columns, b) Nilima Park
Apartment, Ahmedabad, large scale damage in the upper
floors.
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Plan and Mass Irregularity
� The plans of both the buildings are irregular.
� The Mansi Complex has C shaped plan while
the Shikhar apartment has U shaped plan with
no expansion or separation joint as reported.
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Poor quality of construction material and
corrosion of reinforcement
Fig. Damage of RC buildings due to poor quality of construction
a) Old construction corroded reinforcement prior to earthquake,
Mehta Chambers, Ahmedabad,
b) Poor quality of material, corrosion of reinforcement.
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Damage to vertical circulation systems
� Staircases and lifts � only means of vertical
movement in building and
� the staircases also serve � as escape routes
during an earthquake.
Damage to staircase
� Isolation of stairs from the primary structural
system � minimises damage to the stair system.
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Damage to elevator
� Fig shows the undamaged lift core of a building
during the earthquake at Gandhidham.
Fig. Undamaged lift core of a reinforced
concrete building.11/9/2015 41
Reduction of
damages
Design of buildings based on seismic codes IS 1893 (Part-I):2002 and IS:13920:1993 The multi-
storeyedreinforced
The multi-storeyedreinforced concrete
buildings with vertical and mass
irregularities ,to be designed on the basis of dynamic
greater emphasis on quality of construction.
damages to RC
buildingsanalysis and inelastic design
dynamic analysis and inelastic design
Ductility provisions to be incorporated
Shear walls should be employed for increasing stiffness and uniformly
distributed in both principal directions
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Longitudinal reinforcementLongitudinal reinforcement
a) the top as well as bottom reinforcement shall consist of at least two bars throughout the member length.
b) the positive steel at a joint face must be at least equal to half of the negative steel at that face.face.
c) In an external joint, both the top and the bottom bars of the beam � provided with anchorage length, beyond the inner face of the column, equal to the development length in tension plus 10 times the bar diameter minus the allowance for 90 degrees bend(s).
d) In an internal joint, both face bars of the beam � taken continuously throughout the column.
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� The longitudinal bars shall be spliced, only if hoops are
provided over the entire splice length, at spacing not
exceeding 150mm.
� Lap splices shall not be provided a) within a joint, b) within a
quarter length of the member where flexural yielding may
generally occur.
� Not more than 50 per cent of the bars shall be spliced at one
section.
� Use of welded spliced and mechanical connections may also
be made ,as per IS 456:1978.However, not more than half
the reinforcement shall be spliced at a section where flexural
yielding may take place.
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Web reinforcement
� Web reinforcement shall consist of vertical hoops.
� A vertical hoop is a closed stirrup having a
135°hook with a 10-diamter extension(but
not<75mm) at each end that is embedded in the
confined core.
� The minimum diameter of the bars forming a hoop � The minimum diameter of the bars forming a hoop
shall be 6mm.
� However, in beams with clear span exceeding 5m,
the minimum bar diameter shall be 8m.
� The contribution of bent up bars and inclined hoops
to shear resistance of the section shall not be
considered.
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Transverse ReinforcementTransverse Reinforcement
� Transverse Reinforcement for circular
columns shall consist of spiral or
circular hoops.
� In rectangular columns, rectangular � In rectangular columns, rectangular
hoops may be used.
� A rectangular hoop is a closed stirrup,
having a 135°hook with 10-diameter
extension (but not<75mm) at each
end that is embedded in the confined
core.11/9/2015 46
Shear wallsShear walls
� The thickness of any part of the wall shall
preferably be not less than 150mm.
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RecommendationsRecommendations
� The weight and rigidity of a structure must be uniformly and symmetrically distributed to the plane of symmetry passing through the centre of gravity.
� The proportionality requirements must be met by the building dimensions i.e the length and height of the building should not be too great.
� The structure must be light, as practicable and have its centre of gravity as low as possible.
� Desirable tough, light and elastic material be used which have uniform properties.
� In the vertical and horizontal planes, the load carrying elements must be coupled to form closed contours.
� The foundations of earthquake resistant buildings must be firm and have enough depth.
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