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Earthquake :: Vertical
Bands
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Earthquake Design and Construction
by C.V.R.Murty of IIT Kanpur and
Sponsored by BMTPC (Building Materials and Technology Promotion Council) New Delhi.
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Why
is vertical reinforcement required in
masonry buildings?
Response of Masonry Walls
Horizontal bands are provided in masonry
buildings to improve their earthquake
performance. These bands include plinth
band, lintel band and roof band. Even if
horizontal bands are provided, masonry
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buildings are weakened by the openings in
their walls (Figure 1). During earthquake
shaking, the masonry walls get grouped into
three sub-units, namely spandrel masonry,
wall pier masonry and sill masonry. |
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Consider a hipped roof building with two
window openings and one door opening in a
wall (Figure 2a). It has lintel and plinth
bands. Since the roof is a hipped one, a
roof band is also provided. When the ground
shakes, the inertia force causes the
small-sized masonry wall piers to disconnect
from the masonry above and below. These
masonry sub-units rock back and forth,
developing contact only at the opposite
diagonals (Figure 2b). The rocking of a
masonry pier can crush the masonry at the
corners. Rocking is possible when masonry
piers are slender, and when weight of the
structure above is small. Otherwise, the
piers are more likely to develop diagonal
(X-type) shear cracking (Figure 2c); this is
the most common failure type in masonry
buildings.
In
un-reinforced masonry buildings (Figure 3),
the cross-section area of the masonry wall
reduces at the opening. During strong
earthquake shaking, the building may slide
just under the roof, below the lintel band
or at the sill level. Sometimes, the
building may also slide at the plinth level.
The exact location of sliding depends on
numerous factors including building weight,
the earthquake-induced inertia force, the
area of openings, and type of doorframes
used.
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Why
is vertical reinforcement required in
masonry buildings?
How Vertical Reinforcement Helps
Embedding vertical reinforcement bars in the
edges of the wall piers and anchoring them
in the foundation at the bottom and in the
roof band at the top (Figure 4), forces the
slender masonry piers to
undergo bending instead of rocking. In wider
wall piers, the vertical bars enhance their
capability to resist horizontal earthquake
forces and delay the X-cracking. Adequate
cross-sectional area of these vertical bars
prevents the bar from yielding in tension.
Further, the vertical bars also help protect
the wall from sliding as well as from
collapsing in the weak direction.
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Protection
of Openings in Walls
Sliding
failure mentioned above is rare, even in
unconfined masonry buildings. However, the
most
common damage, observed after an earthquake,
is diagonal X-cracking of wall piers, and
also inclined
cracks at the corners of door and window
openings. When a wall with an opening
deforms during
earthquake shaking, the shape of the opening
distorts and becomes more like a rhombus -
two opposite corners move away and the other
two come closer. Under this type of
deformation, the corners that come closer
develop cracks (Figure 5a). The cracks are
bigger when the opening sizes are larger.
Steel bars provided in the wall masonry all
around the openings restrict these cracks at
the corners (Figure 5b). In summary, lintel
and sill bands above and below openings, and
vertical reinforcement adjacent to vertical
edges, provide protection against this type
of damage.
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Related IITK - BMTPC Earthquake Tip
Tip
5: What are the seismic effects on
structures?
Tip12: How brick masonry houses behave
during earthquakes?
Tip13: Why masonry buildings should have
simple structural configuration?
Tip14: Why horizontal bands are required in masonry buildings?
Resource
Material
- Amrose,J., (1991), Simplified Design of Masonry Structures, John Wiley & Sons, Inc., New York, USA.
- BMTPC, (2000), Guidelines: Improving Earthquake Resistance of Housing, Building Materials and Technology Promotion Council, New Delhi.
- IS 4326, (1993), Indian Standard Code of Practice for Earthquake Resistant Design and Construction of Buildings, Bureau of Indian Standards, New Delhi.
- IS 13828, (1993), Indian Standard Guidelines for Improving Earthquake Resistance of Low-strength Masonry Buildings, Bureau of Indian Standards, New Delhi.
Authored
by: C.V.R.Murty
Indian Institute of Technology
KanpurKanpur, India
Sponsored by: Building Materials and
Technology Promotion
Council, New Delhi, India
This release is a property of IIT Kanpur
and BMTPC New Delhi. It may be reproduced
without changing its contents and with due
acknowledgement.
Suggestions/comments may be sent to: eqtips@iitk.ac.in.
Visit www.nicee.org or www.bmtpc.org, to see
previous IITK-BMTPC Earthquake Tips.
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