introduction to earthquake resistant structures

8/13/2019 Introduction to Earthquake Resistant Structures

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SEISMORESISTANT

BUILDING ARCHITECTURE

BY:

Ekta Tripathi -801222004

Divya Chopra-801222003

Priyanka kumari-801222012


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INTRODUCTION TO

EARTHQUAKE RESISTANT

STRUCTURES Earthquake-resistant structures are safe

structures designed to withstand earthquake.

While no structure can be entirely immune to damage from

earthquakes, the goal of earthquake-resistant construction isto erect structures that fair better during seismic activity than

their conventional counterparts.

Earthquake-resistant structures are intended to withstand the

largest earthquake of a certain probability that is likely tooccur at their location.

This means the loss of life should be minimized by preventing

collapse of the buildings for rare earthquakes while the loss of

functionality should be limited for more frequent ones.


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SEISMORESISTANT BUILDING

ARCHITECTURE Rational studies along with the knowledge regarding the performance of

building in earthquakes show that the building architecture design would create

maladjustment in building elements that would decrease the seismoresistant

capacity of building and also become the cause of collapse of building.

It is believed that structural analysis in itself is not sufficient to ensure the

seismoresistant stability of the buildings.

There is a need to design an integral seismoresistant system in which all

components of the building can positively interact during the seismic action.

Real compatibility between ARCHITECTURAL andSTRUCTURAL DESIGN avoids a stepping of seismoresistant capacity of that building and also providespositive , efficient and integral SEISMIC RESISTANT

SYSTEM


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Several studies and recommendations have been carried out to

avoid situations affecting negatively the buildings earthquake

resistant behavior.

These studies enable architects to develop a systematic study and a

methodology to be applied to the architectural design of building to

optimize earthquake resistant capacity . This study is called

SEISMO RESISTANT BUILDING ARCHITECTURE.

The SRAS deals with the interaction of each subsystem of the

building during seismic shaking , in order that the architectural

project does not originate structural maladjustment which would

decrease the seismoresistant capacity of the building.

MAIN OBJECTIVES OF SRAS

1. To prevent seismo resistance stepping

2. To optimize the seismo resistance


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MAJOR ASPECTS INVOLVED IN

SEISMORESISTANT BUILDING

CONSTRUCTION

1. SELECTION OF LOAD RESISTING SYSTEM

2. BUILDING CONFIGURATION

3. BASIC DYNAMIC CHARACTERISTICS

4.

QUALITY OF CONSTRUCTION ANDMATERIAL


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LATERAL LOAD RESISTING

SYSTEMS The load resisting system must be of CLOSED LOOPS so that it is

able to transfer all the forces acting vertically or horizontally to the

ground .

BIS has approved 3 major types of lateral load resisting system in

code IS 1893 (Part 1) :2002

MOMENT RESISTING FRAME

BUILDING WITH SHEARWALL OR BEARING WALL

SYSTEM BUILDING WITH DUAL

SYSTEM

LATERALLOAD

RESISTINGSYSTEMS


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1. MOMENT RESISTING FRAMES

Moment-resisting frames can be

constructed of steel, concrete, ormasonry.

Moment frames consist of beamsand columns in which bending ofthese members provides the

resistance to lateral forces.

This system is generallypreferred by architects becausethey are relatively unobtrusivecompared to shear walls or

braced frames , but there may bepoor economic risk unlessspecial damage controlmeasures are taken


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BUILDING WITH SHEAR WALL

OR BEARING WALL SYSTEM Bearing wall systems consist of

vertical load carrying wallslocated along exterior wall linesand at interior locations asnecessary.

Many of these bearing walls arealso used to resist lateral forcesand are then called shear walls

In general , bearing wall systemhas comparable low R valuesince the system lacks

redundancy and has a poorinelastic response capacity.

This system is not muchpreferred by the architects.


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BUILDING CONFIGURATION

Second step in seismo resistant construction is configuration of load

resisting systems of buildings.

IS 1893 ( part 1):2002 has recommended building configuration system in

section 7 for the better performance of building during earthquake.

Most important feature in building configuration is its REGULARITY and

SYMMETRY in horizontal and vertical plane.

Seismic behavior of REGULAR PLANS and IRREGULAR SHAPED

PLANS differ

IRREGULAR SHAPED PLAN is subjected to their asymmetry and can

present local deformation due to presence of reentrant corners or

excessive openings. Both effects give origin to undesired stress

concentrations in some resisting members of buildings..


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REGULAR SHAPED PLAN i.e. ideal rectangular or square

plan , structurally symmetric, with enough in plan stiffness inits diaphragm , presents an ideal behavior because it has

same displacement at every point in the slab.

Therefore building shaped like a box , such as rectangular ,

both in plan and elevation is inherently stronger than the one

that is L-shaped or U-shaped, that is building with wings.


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IRREGULAR SHAPED PLANS


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ARCHITECTURAL

PROBLEMS

STRUCTURAL PROBLEMS REMEDIAL MEASURES

1. Extreme

heights/depthratio

High overturning forces , large

drift causing non structuraldamages , foundation stability

Revise proportion or

special structuralsystem

2. Extreme large

length/depth ratio

Built up large lateral forces in

perimeter , large difference in

resistance of two axes

Experience greater variationsin ground movement and soil

conditions

Sub divide building by

seismic joints

3.Re-entrant

corners

Torsion , stress concentration

at the notches

Separate walls , uniform

box , centre box ,architectural relief ,

diagonal reinforcement

4. Soft story frame Causes abrupt changes of

stiffness at piont of

discontinuity

Add bracings , add

columns braced


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ARCHITECTURAL

PROBLEMS

STRUCTURAL PROBLEMS REMEDIAL MEASURES

5. Variation incolumn stiffness Causes abrupt changes instiffness , much higher

forces in stiffer columns.

Redesign structuralsystem to balance

stiffness

6. Discontinuousshear wall Results in discontinuitiesin load path and stress

conc. For most heavily

loaded elements

Primary concern overthe strength of lower

level column and

connecting beams that

support the load of

discontinuous frame

7. Weak column-

strong beam

Column failure occures

before beam , short column

must try and accommodate

storey height displacement

Add full walls to reduce

column forces , use

light weight curtain

walls with frames


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BUILDING CHARACTERISTICS The seismic force exerted on a building are not externally

developed forces like wind instead they are the response ofcyclic motions at the base of building causing accelerations

and hence inertia force.

The response is therefore dynamic in nature.

The dynamic properties of the structure such asnatural period

,damping and mode shape play a crucial role in determining

the response of buildings.

Other properties such as ductility, building foundation,

response of non-structural elements etc.


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MODE SHAPES AND

FUNDAMENTAL PERIOD The vibration of building consists of fundamental mode of vibration and

additional contribution of various modes which vibrates at higher

frequencies.

In low rise building less than 5 storey high the seismic response dependsprimarily on the fundamental mode of vibration accordingly the period of

vibration of this mode expressed in seconds is one of the most

representative characteristics of the dynamic response of a building.

On the basis of time period building may be classified as- T


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Building with higher natural frequencies and short natural

period tend to suffer higher acceleration but smaller

displacement.

Building with lower natural frequencies and long natural

period tend to suffer lower acceleration but larger

displacement.


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BUILDING FREQUENCY AND

GROUND PERIOD Inertial force generated in the building depend upon the frequencies

of ground on which the building is standing and the building natural

frequency.

When these are near or equal to one another the buildingsresponse reaches a peak level.

This dynamic amplification effect can increase the building

acceleration to a certain value which may double or more than that

of ground acceleration at the base of building.

Past studies shows that predominant period at afirm ground site is

typically in the range .2-.4 sec while the period can reach2 sec or

more on soft ground.


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It is concluded that if the foundation soil is firm, rigid

structure will have more unfavorable seismic response than

the flexible structures, whereas the seismic response of

flexible structure on soft foundation site will less favorable

than the rigid structure.

A spectacular example was in Mexico city during 1985

earthquake, which saw enormous damage in medium height

buildings of 10-20 storey's ,which have period matching the 2

sec period of earthquake motions in the city centre, while

adjacent low rise buildings with much shorter periods wereproportionally far less damaged.


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DAMPING

Damping is the ability of the structural system to dissipate theenergy of the earthquake ground shaking.

Since theearthquake ground shaking is inversely proportional to

damping. The more damping a building possesses ,the sooner itwill stop vibrating which of course highly desirable from the

standpoint of earthquake performances.

Now-a-days some more advanced techniques of earthquake

resistant design and construction employ added damping deviceslike shock absorbers increase damping of the building and so

improve its earthquake performance.


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DUCTILITY

It is defined as the capacity of the building materials, systems orstructures to absorb energy by deforming its elastic range.

The primary task of an engineer designing a building to be

earthquake resistant is to ensure that building will possess

enough ductility.

It is possible to build ductile structures with RC if care is taken

during designing to provide thejoints with sufficient abutments

that can adequately confine the concrete thus permitting it to

deform plastically without breaking.

It is also important for this purpose to ensure that the tension

edges of the structure are adequately reinforced and that there

are sufficient stirrups to ensure that concrete is properly

confined along the compression edge.


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FOUNDATION

Major recommendation on structural design must be taken in

mind.

Foundation should be preferably be designed as continuous

(mat or raft) in order to avoid relative horizontal

displacement.

In case of isolated footing ,they should be joined to each

other by means of foundation beams or ties. These ties

should be designed such that it will bear tension and

compression forces.

If different parts of the building are to be structurally

independentbecause of the shape of their ground plan, their

foundations should also be independent.


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QUALITY OF CONSTRUCTION AND

MATERIALS

The industrially produced materials used in construction such as

cement , reinforcement , brick etc should satisfy minimum

standards of quality and resistance.

a) Quality of concrete-

Grade of concrete specified in design documents may not be

developed during construction mainly due to

Incorrect proportioning.

Insufficient mixing which causes segregation.

Aggregate with excessive impurities or improper grading.

Excessive high w/c ratio.


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b)Construction joint-

A defective concrete joint, which contributed significantly to

causing of failure of many building in past earthquake is due

to

Poor execution of the construction joint/discontinuity.

Not located at the points specified by designer

Accumulation of sawdust, dust and loose materials at the

surface of joint.

c) General detailing requirements-

Proper placing of reinforcement during casting.

Improper confinement and large tie spacing especially in

plastic hinge region.


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Insufficient confinement and anchorage length at joint.

Insufficient concrete cover to reinforcement results resting in

reinforcing bar and crack in surface concrete.


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introduction to earthquake resistant structures

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