Transcript File - Civil Engineering Notes

Masonry Structures
SIDDHARTH SHANKAR
Department of Civil(structure)
Engineering
Pulchowk Campus
Advantage of Masonry Structure
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Strength: Strong in compression
Heat Absorber: Resists temp. fluctuation
Maintenance free: Most of wall not require painting .
Fire resistant: Fire protection due to formation of noncombustible material.
Environmental-friendly: Leading contributor to green
building that has low impact on nature
Great sound proofing: Blocks out noise more efficiently
than more traditional building material such as timber
Economic: Use of locally available materials and
availability of labor.
Disadvantage
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Moisture absorber: Absorbs moisture when raining.
Color deterioration: Extreme weather causes
masonry to degrade, materials such as wall surface
decolorize due to frost damage.
Strength: Masonry structure has low tensile strength.
Opening: Problem in large opening.
Education: Lack of education in masonry.
Needs heavy foundation: Due to heavy weight,
large foundation is required. Also cracking and
settlement may occur.
Basic Terms in Masonry
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Course: Horizontal layer of bricks or stones.
Bed: Lower surface of bricks/stones in each course which is
perpendicular to the line of pressure.
Backing: Unexposed wall
Facing: Exposed to weather
Hearting: Between the facing and backing
Joint: Bed and perpend.
Header: Length is perpendicular to the face of the wall
Stretcher: Longest side parallel to the face of the wall.
Bond: Individual brick units are tied together with mortar is
called as bond.
Basic Terms in Masonry
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Closer: Portion of brick cut in such a manner that its one long
face remains uncut.
Queen closer: Length-wise cutting of brick.
King closer: Cutting off the triangular piece between the
center of one end and the center of the other (long) side.
Bat: Brick cut across the width
Soldier: Laid vertically with the long narrow side of the brick
exposed.
Sailor: Laid vertically with the broad face of the brick
exposed.
Rowlock:Laid on the long narrow side with the short end of
the brick exposed.
Basic Terms in Masonry
Construction Technology
Construction Technology
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Lap should be minimum 1/4th brick along the length of
wall and 1/2nd brick across the thickness of wall.
Bricks bats should be discouraged, except in special
locations.
The vertical joint in the alternate courses should be along
the same perpend.
All the finished masonry walls should be cured for at least
7 days.
Thickness of mortar joint should be uniform and not more
than 13mm in any case.
Height of brick masonry in one day should not exceed
1.5m
Types of Bond
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Stretcher bond
Header bond
English bond
Flemish bond
Facing bond
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Brick on edge bond
Raking bond
Dutch bond
Zigzag bond
Garden wall bond
Rat-trap bond
Types of Bond
Fig: Flemish Bond
Fig: English Bond
S.N.
English bond
S.N.
Flemish bond
1
Headers and stretchers are laid in
alternate courses.
1
Headers and stretchers are laid
alternately in each course.
2
Strongest of the types of bonds.
2
Comparatively less strong for walls
more than 30cm thick.
3
Provides rough appearance.
3
Provide good appearance.
4
Absence of vertical joints in the
structure.
4
Partly continuous vertical joints
appear in the structure.
5
Special attention is not required for
this bond.
5
Special attention is required for this
bond.
6
Progress of work is more.
6
Progress of work is less.
7
Costly, no brick bats are used.
7
Economical, as brick bats are used.
8
Skilled labor is not required for its
construction
8
Skilled labor required for its
construction.
9
Less mortar is used.
9
More mortar is used due to use of
bats.
Rat-trap bond
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Shiner and rowlock are visible on the face of
masonry; this gives the wall with an internal cavity
bridged by the Rowlock.
Economic use of brick.
The cavity provides thermal comfort inside the wall.
Used in load bearing as well as thick partition wall.
The structural strength increased by inserting steel.
Due to cavity, the weight of the building is reduced.
Aesthetically pleasing wall.
Rat-trap bond
Hollow concrete blocks
Advantage of Hollow concrete blocks
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Hollow block needs low maintenance.
High Durable.
Construction speed is high when compare to conventional construction
systems.
It has high tensile strength.
Economical friendly and cheap rates.
It reduces the building costs.
The acoustic system of the room is good with hollow cement construction.
Maintains the thermal balance system of any hollow block construction.
It reduces the labor cost and total construction cost.
High heat and water resistant.
It’s no needs of plastering
Low water absorption than brick.
Main advantage, block voids can be filled by other filling materials.
Compressed Earth Block
Advantage of Compressed Earth Block
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Shipping cost: Suitable soils are often available at or near the
construction site.
Uniformity: Manufactured to a predictable size and has true flat
sides and 90-degree angle edges.
Non-toxic: Materials are completely natural, non-toxic,
chemical-free, and do not out-gas.
Sound resistant: An important feature in high-density
neighborhoods, residential areas adjacent to industrial zones.
Fire resistant: Earthen walls do not burn.
Insect resistant: Insects are discouraged since the walls are solid
and very dense, and have no food value.
Reduction in transportation cost due to locally available material
used.
Masonry as infill walls
Masonry as infill walls
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Masonry in-fills cause undesirable effects under seismic loading:
short-column effect, soft-storey effect, torsion, and out-of-plane
collapse.
In-fills are separated from the RC frame and do not interfere with
the frame under lateral deformations. All lateral force on the
building is carried by the bare RC frame alone.
In-fills are built integral with the RC frame as non-structural
elements. The entire lateral force on the building is carried by the
bare RC frame alone. Most common design practice in the
developing countries.
In-fills are built integral with the RC frame, and considered as
structural elements. In-plane stiffness offered by the infill walls is
considered in analysis. The forces from this analysis are used in
the design of RC frame members and joints.
Reinforced masonry
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A
construction
system
where
steel
reinforcement is embedded in the mortar joints
of masonry or placed in holes and after filled
with concrete or grout is called Reinforced
masonry. Reinforced masonry can be classified
into three types:
Reinforced hollow unit masonry
Reinforced grouted cavity masonry
Reinforced pocket type walls
Reinforced grouted cavity masonry
Reinforced pocket cavity masonry construction
Typical anchorages of reinforcing bars
Unreinforced masonry
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Likely to be damaged during earthquake.
Mortar holding the masonry together is generally
not strong enough to resist earthquake forces.
Anchorage of walls to the floor and the roof is
critical.
These houses are weak (brittle) and can break
apart.
Walls may fall away or buckle, resulting in
damage.
Unreinforced masonry
Confined masonry
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1.
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The masonry walls carry the seismic loads and the concrete is
used to confine the walls.
Construction system where masonry structural walls are
surrounded on all four sides with reinforced concrete.
Confinement should be in vertical and horizontal both
direction to resists lateral and gravity loads.
The confining members are effective for:
Enhancing the stability and integrity of masonry wall for inplane and out-of-plane earthquake loads.
Enhancing the strength of masonry wall under EQ.
Reducing brittleness of masonry improving EQ performance.
Confined masonry
The structural component of confined masonry are:
 Masonry wall
 Confining elements (tie-beams and tie columns)
 Floor and roof slab
 Plinth band
 Foundation
Confined masonry
Chapter-08
Design of masonry
walls for gravity
loads
IS1905-1987
Introduction to Codal Provisions
Scope
 Structural design of unreinforced load
bearing and non-load bearing walls
constructed with solid or perforated
burnt clay brick, concrete block.
 Do not apply to walls constructed in
mud mortars.
Walls
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Most essential component of building.
Enclose or divide space of building.
Provide privacy, afford security and protection
against heat, rain and cold.
Designed wall should have
Strength and stability
Weather resistance and thermal insulation
Durability and fire resistance
Sound insulation
Walls
Vertical load-bearing member, the width of
which (length) exceeds 4 times the thickness.
 Isolated vertical load bearing member, the
width of which does not exceed 4 times the
thickness is called column.
Types of walls
 Load-bearing wall- Designed to carry superimposed load and self weight.
 Non-load bearing wall- Designed to carry self
weigh only.
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Walls
Partition wall: Thin internal wall used to divide the space within
building.
Internal wall: Load bearing partition wall.
Panel wall: External non-load bearing wall (commonly related to
frame structure)
Party wall: Wall separating the adjoining buildings. May or may
not be load bearing.
Separating wall: Wall separation different occupancies within the
same building.
Curtain wall: Self-supporting wall carrying no other vertical load
but subject to lateral loads. E:\File\code\codes\1905.PDF
Cross-walls: Load bearing walls construction in which all the
loads are carried by internal walls, running at right angles to the
length of building.
Load bearing Walls
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Solid masonry walls: Most commonly used but may have opening.
Cavity walls: Wall comprising two leaves, each leave being build
of masonry units and separated by a cavity and tied together with
metal ties or bonding units to ensure that two leaves act as one
structural units. Space between two leaves being left continuous or
filled with other non-load bearing insulating material.
Solid Wall
Cavity
Wall
Load bearing Walls
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Faced walls: Wall in which the facing and backing are of two
different materials are bonded together to ensure common
action.(IS1905 Figure 3, pageno.4)
E:\File\code\codes\1905.PDF
Veneered walls: Wall in which facing is attached to the backing
but not so bonded as to result in a common action under load.
Design Consideration
General
 Masonry structure gain stability from the
supports offered by cross walls, floors,
roofs and other elements.
 Structure is so planned that eccentricity
of loading on the members is small as
possible.
 Mix proportions should follow Table 1
(IS1905-1987) page no. 6
Design Consideration cont…
Selection of mortar
 Requirement of mortar for masonry structure workability,
strength, water retentively and low drying shrinkage.
 Mortar strength in general should not be greater than that of
masonry unit.
Letter H, M and
L indicates the
high strength,
medium
strength and
low strength
Masonry unit strength
(N/mm2)
Mortar type
Below 5
M2
5 to 14.9
M1
15-24.9
H2
25 or above
H1
Design Consideration cont….
Stability requirement
Lateral support: Wall can be laterally supported either at vertical intervals by
floor roof transmitting horizontal forces to cross-walls and then to the
foundation or at horizontal interval by cross-wall, piers transmitting
horizontal forces to foundations. Lateral support has to perform two
important functions as:
 To resist horizontal components of the forces so as to ensure the structure
against over-turning.
 To limit the slenderness ratio of masonry elements in order to prevent
failure by buckling.
Adequate lateral support: If supports is capable of resisting the sum of
following lateral forces than wall considered as adequate lateral support.
 Simple static reactions at the point of lateral support to all the lateral
loads.
 2.5% of the total vertical load that the wall or column is designed to carry
at the point of lateral support.
Design Consideration cont….
Effective Height of wall
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The effective height of a load bearing wall is assessed based on the relative
stiffness of the elements of the structure connected to the wall together with
the efficiency of the connection.
A wall is stiffened by floors , or roofs , suitable cross walls or any similar
construction element.
Refer: Effective
height of wallTable 4
(IS1905-1987)
page no
.11E:\File\code\
codes\1905.PD
F
Design Consideration cont….
Effective length of wall
Refer: Effective length of wall- Table 5 (IS1905-1987) Page no. 12
Effective thickness of wall
Refer: Effective thickness of wall (Clause 4.5 of IS1905-1987) Page no: 13
Slenderness ratio (SR)
The ratio of effective height or effective length to the effective thickness of
wall whichever is less is the design value of slenderness ratio.
The slenderness ratio for a load bearing column shall not exceed 12.
Maximum SR for a load bearing wall
No of Storey
Maximum Slenderness Ratio
Using Portland cement in mortar
Using mortar lime
Not exceeding 2
27
20
Exceeding 2
27
13
Eccentricity: Clause 4.7 IS1905
E:\File\code\codes\1905.PDF
Refer: Table 7 (IS 1905-1987)
Structural Design
General
 Analyzed by accepted principle of mechanics to ensure safe and
proper functioning in service of its components.
 All components shall be capable of sustaining the most adverse
combinations of loads, which the structure reasonably expected
to be subjected to during and after construction.
Design Loads
 Dead load of walls, columns, floors and roofs.
 Live load on floor and roof. (Calculated by using IS875 II)
 Wind load on walls and sloping roof.
 Seismic forces (In zone I and II, not necessary and zone III, IV
and IV should adopted)
Structural Design cont…
Load dispersion (Clause 5.3 IS1905)
General: The angle of dispersion of vertical load on walls shall
be taken as not more than 30° from the vertical.
Arching action: The arching action of well-bonded walls
supported on lintels and beams should be account.
Lintels: See detail on code (Clause 5.5.3 IS1905)
Structural Design cont….
Permissible stresses (Clause 5.4 IS1905)
Permissible compressive stress (clause 5.4.1)
Permissible compressive stress = Fb*Ks*Ka*Ks, where
 The basic compressive stress based on type and strength of the
masonry units and mix of mortar, Fb (Table 8)
 Stress reduction factor (Ks) depends on SR and eccentricity of
loading (Table 9)
 Area reduction factor (Ka), Ka = 0.7 +1.5A, A= area in m2
 Shape reduction factor (Ks), (Table 10)
Permissible tensile stress (Clause 5.4.2)
Permissible shear stress (clause 5.4.3)
Fa =0.1+Fd/6 ≤0.5N/mm2 , Fa =permissible shear stress,
Fd =compressive stress due to dead load
Wall Opening
Effect of openings
 Reduce
lateral strength of URM
walls and should be small &
centrally located.
 Total length of openings should
be half for one-storey and onethird for two- storey Buildings.
Refer:
NBC 109