 A typical RC building is made of horizontal members (beams and slabs)

and vertical members (columns and walls) , an supported by foundations

that rest on ground.
 The system comprising of RC columns and connecting beams is called
an RC frame which participates in resisting the earthquake forces.
 Earthquake induced inertia forces develop at the floor levels since most
of the building mass is present at the floor levels.
• RC buildings often have vertical
plate-like RC walls called shear
walls, in addition to slabs, beams
and columns. They are like
vertically oriented wide beams
that carry earthquake loads
downwards to the foundation.
• Shear walls start at the
foundation level and are
continuous throughout the height
of the building.
• Thickness- As low as 150 mm, or
as high as 400 mm.
• Provided along both length and
width of buildings.
 Good performance even in strong earthquakes.
 Shear walls in high seismic regions require special detailing (enough well distributed
reinforcement also saves from collapse)
 Used in earthquake prone countries like Chile, New Zealand and USA
 Easy to construct- reinforcement in walls is relatively straight forward, hence easily
implemented in site.
 Efficient- both in terms of construction cost and effectiveness in minimizing earthquake
damage in structural and non structural elements.
• Most RC buildings with shear walls have
columns.
• These columns primarily carry gravity
loads (self weight and contents of
building)
• Shear walls provide large strength and
stiffness to buildings in the direction of
its orientation, which significantly
reduces lateral sway of the building,
hence reduces damage.
• Large overturning effects act on them
since they carry large horizontal load.
• Hence design of their foundation
requires special attention
• Located symmetrically to
reduce ill effects and
twists.
• Can be located in
interior or exterior
• More effective when
located in the exterior of
the building
 Shear walls are provided preferably along both length and width. However, if they are
provided along only one direction, a proper grid of beams and columns in the vertical
plane( called a moment- resistant frame) must be provided along the other direction to
resist strong earthquake effects.
OPENINGS
 Openings can be provided, but size must be small to ensure least interruption to force
flow through walls.
 Openings must be symmetrically located.
 Special design checks are required to ensure that the net cross sectional area of a wall
at an opening is sufficient to carry the horizontal earthquake force
 Walls with staggered openings
are more rigid.
 With the same amount of
reinforcement ductile failure
observed for staggered opening
walls and brittle failure for regular
opening walls
 Staggered opening walls failed at
higher seismic forces and
horizontal displacements.
 RC shear walls perform better if designed to
be ductile.
 Overall geometric proportions of the wall,
types and amount of reinforcement, and
connection with remaining elements in the
building help in improving the ductility of
walls.
 Design guidelines- the Indian standard
ductile detailing code for RC
members(IS:13920-1993)
 Oblong in cross section; rectangular, L and U
shaped sections used.
 Generally shear walls are either plane or
flanged section, while core walls consist of
channel sections.
Commercial buildings Hotel and dormitory buildings

Deflected profiles for shear wall and a

rigid frame

In some cases the wall is pierced by

openings.
These are called coupled shear walls
because they behave as individual
continuous wall sections coupled by
the connecting beams or slabs.
 Thin walled hollow shafts around the elevator core of
buildings also act as shear walls.
REINFORCEM ENT BARS IN RC WALLS
 Steel reinforcement in vertical and horizontal grids
 Placed in 1 or 2 parallel layers called curtains.
 Horizontal reinforcement- anchored at ends of wall
 Minimum area of reinforcing steel is 0.0025 times the cross
sectional area along each of the horizontal and vertical
directions.
 Vertical reinforcement should be distributed uniformly
across the wall cross sections.
 Under large overturning effects caused by horizontal earthquake forces, edges of
shear walls experience high compressive and tensile stresses.
 End regions reinforced in a special manner to sustain load reversals.
 End regions of wall with increased confinement- boundary elements
 RC walls with boundary elements have substantially higher bending strength and
horizontal shear force carrying capacity, and are less susceptible to earthquake
damage than walls without boundary elements.
 Flexural crack observed Diagonal cracks observed in squat
in slender shear wall shear wall
 THE INERTIA FORCES TRAVEL DOWNWARDS
THROUGH SLAB AND BEAMS TO COLUMNS AND
WALLS , AND THEN TO THE FOUNDATIONS
FROM WHERE THEY ARE DISPERSED TO THE
GROUND.

 AS INERTIA FORCES ACCUMULATE

DOWNWARDS FROM THE TOP OF THE BUILDING
, THE COLUMNS AND WALLS AT LOWER
STOREYS EXPERIENCE HIGHER EARTHQUAKE
INDUCED FORCES

 HENCE THEY ARE DESIGNED TO BE STRONGER

THAN THOSE IN THE STOREYS ABOVE.
 Horizontal plate like elements
 Thickness slabs generally 110-150mm
 When beams bend in the vertical
direction during earthquakes, the thin
slabs bend along with them.
 When beams move with columns in
horizontal direction, the slab forces the
beam to move with it.
 The geometric distortion of slab is
negligible in horizontal plane; known as
rigid diaphragm action.
 Also called infill walls, are not
connected to RC columns and beams.

 Due to their heavy weight and

thicknes, these walls attract large
horizontal forces.

 They develop cracks under severe

ground shaking but help share the
load of beams and columns until
cracking.
Using mortars of good strength.
Making proper masonry courses.
Proper packing og gaps between RC frame and masonry
infill walls .
Earthquake loading causes tension on
beam and column faces .
The level of bending moment due to
earthquake loading depends on severity
of shaking and can exceed that due to
gravity loading.
Thus, under strong earthquake shaking ,
the beam ends can develop tension on
either of the top and bottom faces.
Steel bars are required on both faces of
beams to resist reversals of bending
moment.
 For a building to remain safe during earthquake,
columns should be stronger than beams.
 Foundations which recieve forces from column should be
stronger than columns.
 Connections between beams and columns and columns
and foundations should not fail so that beams can safely
transfer forces to columns and columns to foundations.
 Damage is likely to occur first in beams
 If columns are made weaker they suffer severe local
damage, at the top and bottom of a particular storey
 This leads to collapse of a building , although columns at
storeys above remain almost undamaged.
 Beams in RC
Vertical Stirrup
buildings have two
Smaller diameter steel
sets of steel bars that are made
reinforcement: into closed loops and
are placed at regular
a) Long straight bars intervals along the full
length of the beam
placed along its
length
Longitudinal Bar
b) Stirrups placed
Larger diameter steel
vertically at regular bars that go through the
intervals along its full length of the beam
full length
a) FLEXURAL FAILURE: if more steel is b)SHEAR FAILURE:
present on tension face, concrete  A shear crack is inclined at 45
crushes in compression(brittle failure) degree to the horizontal
 Develops at mid-depth near
if less steel on tension face, the steel the support and grows towards
yields first and redistibution occurs the top and bottom faces
in beams until concrete crushes in  Close loop stirrups are
compression(ductile failure) which is provided to avoid such
desirable. shearing action.
 Columns in RC buildings contains
two types of reinforcements:
 Longitudinal bars along its length
 Transvere ties placed horizontally at
regular intervals

 Columns can sustain two tyes of

damage:
 Axial-flexural(combined
compression and bending)
 Shear failure which should be
avoided by providing transverse
ties at close spacing.
(a) Flexural failure (b) Shear failure

Bottom face stretches in tension

and vertical cracks develop
EARTHQUAKE RESISTANT
STRUCTURES

REINFORCED CONCRETE
 HOW DO COLUMN JOINTS IN RC BUILDINGS RESIST EARTHQUAKE?

In RC buildings, portions of columns that are

common to beams at their intersections are called
beam- column joints. Since their constituent
materials have limited strengths, the joints have
limited force carrying capacity. When forces larger
than these are applied during earthquakes, joints
are severely damaged. Repairing damaged joints
is difficult, and so damage must be avoided.
Thus, beam-column joints must be designed to
resist earthquake effects.
 EARTHQUAKE BEHAVIOUR OF JOINTS

Under earthquake shaking, the beams adjoining a joint are subjected to moments in
the same (clockwise or counter-clockwise) direction. Under these moments, the top
bars in the beam-column joint are pulled in one direction and the bottom ones in the
opposite direction. These forces are balanced by bond stress developed between
concrete and steel in the joint region. If the column is not wide enough or if the
strength of concrete in the joint is low, there is insufficient grip of concrete on the steel
bars. In such circumstances, the bar slips inside the joint region, and beams loose their
capacity to carry load. If the column cross-sectional

size is insufficient, the

concrete in the joint

develops diagonal cracks.

 REINFORCING THE BEAM COLUMN JOINT

Diagonal cracking & crushing of concrete in

joint region should be prevented to ensure
good earthquake performance of RC frame
buildings. Using large column sizes is the
most effective way of achieving this. In
addition, closely spaced closed-loop steel
ties are required around column bars to
hold together concrete in joint region and to
resist shear forces. Intermediate column
bars also are effective in confining the joint
concrete and resisting horizontal shear
forces.
 WHY ARE OPEN GROUND STOREY BUILDINGS VULNERABLE IN EARTHQUAKES?

An open ground storey building, having only columns in

the ground storey and both partition walls and columns in
the upper storeys, have two distinct characteristics,
namely:
(a) It is relatively flexible in the ground storey, i.e.,
the relative horizontal displacement it undergoes
in the ground storey is much larger than what
each of the storeys above it does. This flexible ground
storey is also called soft storey.
(b) It is relatively weak in ground storey, i.e., the total
horizontal earthquake force it can carry in the ground
storey is significantly smaller than what each of the storeys
above it can carry. Thus, the open ground storey may also
be a weak storey.
 EARTHQUAKE BEHAVIOUR

The presence of walls in upper storeys makes

them much stiffer than the open ground storey.
Thus, the upper storeys move almost together as
a single block, and most of the horizontal
displacement of the building occurs in the soft
ground storey itself. In common language, this
type of buildings can be explained as a building
on chopsticks. Thus, such buildings swing back-
and-forth like inverted pendulums during
earthquake shaking, and the columns in the
open ground storey are severely stressed. If the
columns are weak (do not have the required
strength to resist these high stresses) or if they
do not have adequate ductility, they may be
severely damaged which may even lead to
collapse of the building .
 IMPROVED DESIGN STRATEGIES

The Code suggests that the forces in the

columns, beams and shear walls (if any) under the action
of seismic loads specified in the code, may be obtained by
considering the bare frame building (without any infills)
(Figure 4b). However, beams and columns in the open
ground storey are required to be designed for 2.5 times
the forces obtained from this bare frame analysis.

The existing open ground

storey buildings need to be
strengthened suitably so as to
prevent them from collapsing
during strong earthquake
shaking.
 Why are Short Columns more Damaged During Earthquakes?

Poor behaviour of short columns is

due to the fact that in an earthquake,
a tall column and a short column of
same cross-section move horizontally
by same amount A (Figure 2).
However, the short column is stiffer
as compared to the tall column, and it
attracts larger earthquake force.

The larger is the stiffness, larger is the force

required to deform it. If a short column is not
adequately designed for such a large force, it can
suffer significant damage during an earthquake.
This behaviour is called Short Column Effect.
The damage in these short columns is often in the
form of X-shaped cracking(due to shear failure.
 The Short Column Behaviour

When a building is rested on sloped

ground, during earthquake shaking all
columns move horizontally by the same
amount along with the floor slab at a
particular level (this is called rigidfloor
diaphragm action.
The short column effect also occurs in
columns that support mezzanine
floors or loft slabs that are added in
between two regular floors

Consider a wall (masonry or RC) of partial height built to fit a window over
the remaining height. The adjacent columns behave as short columns due
to presence of these walls. In many cases, other columns in the same storey
are of regular height, as there are no walls adjoining them. When the floor
slab moves horizontally during an earthquake, the upper ends of these
columns undergo the same displacement
 What is a Shear Wall Building

Reinforced concrete (RC) buildings often have

vertical-like RC walls called Shear Walls in addition to slabs, beams
and columns. These walls generally start at foundation level and
are continuous throughout the building height. Their thickness can
be as low as 150mm, or as high as 400mm in high rise buildings.
Shear walls are usually provided along both length and width of
buildings. Shear walls are like vertically-oriented wide beams that
carry earthquake loads downwards to the foundation.
 Architectural Aspects of Shear Walls

Most RC buildings with shear walls also

have columns; these columns primarily
carry gravity loads (i.e., those due to self-
weight and contents of building). Shear
walls provide large strength and stiffness
to buildings in the direction of their
orientation, which significantly reduces
lateral sway of the building and thereby
reduces damage to structure and its
contents.

Shear walls should be provided along preferably both length and width.

Door or window openings can be provided in shear walls, but their size
must be small to ensure least interruption to force flow through walls.

Shear walls in buildings must be symmetrically located in plan to reduce

ill-effects of twist in buildings
 The structural system consists of moment frames with specific bays provided with
braces throughout the height of the building
 Braces are provided in both plan directions such that no twisting is induced in the
building owing to unsymmetrical stiffness in plan.
 Braces helps :
 in reducing overall lateral displacement of buildings
 in reducing bending moment and shear force demands on beams and columns in
buildings.
 Load transfer: The earthquake force is transferred as axial tensile and compressive
force in the brace members.
• Consider the five-storey benchmark building with three types of local bracing systems
namely, X-, Chevron and K-bracing systems . X- and Chevron braces effectively reduce
bending moment, shear force and axial force demands on the beams and columns of the
original frame and are commonly used.
 K-braces increases shear demand on columns and can cause brittle shear failure .
Thus, some design codes prohibit use of K-braces in earthquake resistant design.

Location of bracing along the

building periphery
  Global braces are effective in
reducing the force demands on
main frame members, sometimes
even more than structural wall.
 The reduction in force demands on
frame members (i.e., beams and
columns) in the building with global
bracing is significant.

BENDING MOMENTS in buildings with no braces, global braces and structural wall: Maximum
reduction in bending moment demand on columns is achieved by global braces