Safety Symposium & Exposition · possible seismic response of buildings ,which may be ......

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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