Earthquake Resistant Building Structures

8/6/2019 Earthquake Resistant Building Structures

http://slidepdf.com/reader/full/earthquake-resistant-building-structures 1/6

Earthquake Resistant Building Structures

Shubhayu Dutta, Smita S. Kamble, Nikhil V. Bandwal

Civil Engineering Department,

Yashwantrao Chavan College Of Engineering, Nagpur.

[email protected]

[email protected]

[email protected]

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



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



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



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.”





Earthquake Resistant Building Structures

Documents