Walls I
3A9 Construction Technology
S Pavía
Dept of Civil Engineering
Trinity College Dublin
contents
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Types of walls
Strength of walls
Strength requirements
crushing, buckling, settling, overturning
Types according to construction
• Solid
– Masonry- building units and mortar built in horizontal
layers (courses).
• Solid brick-single leaf
• Solid block
• Stone
– Monolithic- one single material initially requiring
support ie concrete
• Cavity- double leaf and cavity
• Frame- frame of members with facing or sheeting e.g.
timber.
• Membrane- sandwich of 2 skins of reinforced plastic, metal
or plywood attached to plastic core or ribs. Light weight,
high strength.
• Curtain wall - outer covering, non-structural but merely to
protect from the weather.
Solid Walls
Brick Wall
Stone Wall
Concrete Block Wall
Solid Walls
Clay walls
Solid Wall – Concrete
Solid Wall -Innovative Materials
Hemp and lime
Straw bale
Cavity Walls
Wall ties built into cavity walls are intended to share lateral forces and
deflections between the 2 leaves.
Frame
Frames consist of
horizontal elements
(beams, plates) and vertical
elements (columns, studs)
connected by joints.
Cladding application of one material over
another to provide a wall around a frame or
for aesthetic purposes
Advantages and Disadvantages
• Solid
– Stronger (The strength of a 255 mm cavity wall with loads spread over
both leaves is 20% less than that of a one-brick wall).
– Higher thermal mass
• Cavity
– Prevents damp penetration
– Insulation can be added to cavity
• Frame
– Fast construction
– Low labour costs
Functions and requirements - external walls
• To enclose space
• To support upper floors and roofs together with the loads
that will be imposed upon them- domestic / industrial
ADEQUATE STRENGTH AND STABILITY
• To resist damp penetration and biological colonisation
DAMP PROOF MEMBRANES / UNDER-FLOOR
VENTILATION.
• To provide an adequate insulation to reduce heat loss
INSULATORS / THERMAL BRIDGING
• To offer adequate resistance to fire and the environmentrain, wind
DURABLE , FIRE RESISTANT
MATERIALS / BOND STRENGTH
• To look attractive and accommodate windows and doors
Types according to function
• Loadbearing- support loads from floors and roof in
addition to their own weight, resist side pressures from
wind - sometimes, from stored objects.
• Non-loadbearing- carry no loads.
• Partition- Internal -either loadbearing or not-, divide the
space within a building.
• Compartment- separate internal space for fire
protection.
• Internal- separate adjoining buildings e.g terrace
houses.
• Separating- separate single occupancies within the
same building or different buildings.
• Retaining- support and resist the horizontal forces
caused by retained earth or subsoil water.
Parapet Wall
• Parapet- upper part of external wall carried
above the level of a roof plane or a roof
gutter.
Retaining Wall
• provide lateral support to
vertical slopes of soil
built in order to hold back earth
which would otherwise move
downwards.
• stability depends on:
maximum lateral earth
(horizontal) pressure.
lateral pressures depend on
the moisture content of the soil
overturning, base sliding, soil
bearing capacity failure
Gravity, piling,
cantilevered, butressed,
anchored
Gravity Walls
– Depend on their own weight
and any soil resting on the
concrete in resisting lateral
earth forces.
– Usually they are sufficiently
massive to be unreinforced.
– The mass of the structure must
be sufficient to develop enough
frictional resistance to sliding
– The base or footing of the
structure must be wide enough
to develop sufficient moment to
resist overturning earth forces
Cantilever Retaining Wall
• Constructed of reinforced
concrete: relatively thin stem
and a base slab.
• Use less concrete than
gravity walls but require
more design and careful
construction.
• They gain a larger effective
mass due to the soil placed
on the horizontal
cantilevered section of the
wall.
Buttressed/Counterfort Retaining Walls
• similar to cantilever walls
except for the buttresses
along the back of the wall to
improve its strength resisting
high loads.
Piling Wall
•used in soft soils and tight
spaces.
•typically made of reinforced
concrete or steel driven into
the ground.
•usually driven 1/3 above
and 2/3 below ground,
•tall sheet pile walls will need
a tie-back or anchor behind
the face of the wall, that is
tied to the wall, usually by a
cable or a rod.
Anchored Wall
An anchored retaining wall can
be constructed in any of the
aforementioned styles
Additional strength is provided
by cables or other stays
anchored in the rock or soil
behind it.
Technically complex, very useful
where high loads are expected,
or where the wall itself has to be
slender and would otherwise be
too weak.
Diaphragm Wall
• generally a reinforced concrete wall
constructed in the ground in areas of soft
earth close to open water or with a high
ground water table i.e. surrounding
tunnels, car parks and open cuts.
– Built up areas
– Suited for deep basements
Diaphragm
Walls
Fix alignment with guide walls
Trench excavated in discontinuous sections
Fill trench with slurry (bentonite) which provides
hydraulic pressure to the trench walls and prevents
trench collapsing
Insert reinforcement cage into trench
Fill with concrete, displacing slurry
Strength requirements of walls
• Strength is determined by design:
– strength of material,
– wall thickness,
– height/thickness,
– lateral support.
• Under a vertical load a wall may crush, buckle or
settle
• A horizontal load may cause overturning
• The design of loadbearing members must ensure
that the design strength is greater than the design
load
Crushing
• The wall must be of sufficient thickness to
keep the stresses within the safe
compressive stress limits of the materials to
avoid crushing by overloading.
• In small scale buildings of masonry construction thickness is rarely
determined by strength alone as the load of (e.g.) a two storey building
is quite small, well within the bearing capacity of a normal half-brick
wall.
• A material crushes when the compressive strength load exceeds the
strength of the material.
Buckling
• Buckling is the failure of a
structural member by applied
axial load causing excessive
lateral deflection at a stress
lower than the crushing stress
• Imperfections in the material,
off-centre application of loads
Buckling usually occurs in
etc… can induce secondary
slender members with a
bending and buckling in a
high height/thickness ratio
material
the greater the ratio the
highest the tendency to
buckle.
Buckling
• Provide lateral support to walls by
adequately bracing floors and roof.
• Connections to floors and roofs by means
of tension straps and joist hangers are
needed to provide horizontal lateral
restraint and be capable of resisting
lateral loads
Must have a
base wide
enough so that
the wall load is
distributed over
a sufficiently
large area of
soil - for not to
exceed the
stress limit of
the soil and
avoid failure by
settlement.
Settlement
Settlement
• Limit the width of
openings in order
to provide
sufficient wall
bearing area to
ensure strength
and stability.
• Provide lateral
support to walls
by adequately
bracing floors and
roof.
Provision of an
extended foundation
strip
HORIZONTAL
LOADS - SLIDE /
OVERTURN
• Provide the weight
necessary to
provide stability
and avoid slidingincrease height or
thickness.
• friction and the
passive pressure
of the soil on
which the wall
rests prevent
sliding.
weight
sliding
Passive
pressure
of soil
friction
OVERTURN
• Increase wall weight
or width of base
(trapezoidal walls) to
avoid overturning by
rotation induced by an
overturning force.
• Adequate lateral
support to resist
overturningbuttresses, fins, struts.
Overturning by rotation
Overturning by settlement
Overturning by settlement
Overturning
Adequate lateral support to resist overturningbuttresses, fins, struts.
Flying buttresses
Bonding of masonry units will
restrict buckling, differential
settlement and overturning.
Strong masonry bond to withstand
tensile stresses induced by unequal
loading-eccentric loading.
Openings
• Load must be transferred
from over an opening to
the surrounding wall ie
lintels in the case of
windows
• Limit the width of openings
in order to provide
sufficient wall bearing area
to ensure strength and
stability.
Removing internal partition walls
• Determine if wall is load bearing
• If load bearing wall, the load must be
transferred to another load bearing
element with enough strength capacity
to carry the additional load
Openings
No Lintel
Lintel
Arch
Wall Openings-Terms
• Jamb-wall immediately
adjacent to the side of the
opening
– Reveal-return face to the
jamb
– Square jamb- the reveal is
flat
– Rebated jamb-the reveal
is recessed.
• Head-supports the wall
above opening-lintel or arch.
– Soffit-return face to the
head.
• Cill-bottom of a window
opening.
• Threshold-bottom of doorway
(J S Foster 1994)
Internal
walls
• Primary function
to act as a
vertical divider
of floor space
forming a storey
height enclosing
element.
• Other functions
Basic design concepts for internal walls
• Load bearing
-those which
accept and
transmit
structural
loads to the
foundations.
• Partitionssupport only
their own self
weight and do
not accept
any structural
loads.
Chudley and Greeno 1988
Ceiling
/floor
joists
transmit
loads to
wall
Roof struts
transmit loads
to wall
Strength of walls
• It depends on the strength of its materials
and the strength of the bond between them.
• Materials must be durable- testing: thermal,
freeze /thaw and salt crystallization cycling .
• Physical properties of different materials in
the wall must be compatible-combinations
permeable/impermeable; strong/weak
unsuitable for long-term durability.
• Chemical composition of different materials
in the wall to also be compatible.
Code of Practice for the use of masonry- part 1Structural use of unreinforced masonry
BS 5628-1:2005 Design of walls: objectives & recommendations.
The design of loadbearing masonry members must ensure that the design
strength of a member is greater than or equal to the design load.
The factor γm makes allowance for the variation in the quality of the
materials and for the possible difference between the strength of
masonry constructed under site conditions and that of specimens built in
the laboratory.
Partial safety factors for loads (γf) are introduced to take account of:
a) possible unusual increases in load beyond those considered;
b) inaccurate assessment of effects of loading,
unforeseen stress redistribution within the structure;
c) the variations in dimensional accuracy achieved in construction.
The design recommendations assume that all the lateral forces acting on
the whole structure are resisted by walls, or by suitable bracing.
a) buildings should be designed to be capable of resisting a uniformly
distributed horizontal load equal to 1.5 % of the total characteristic dead
load
b) Connections to floors and roofs by means of tension straps and joist
hangers are needed to provide horizontal lateral restraint and be
capable of resisting lateral loads –appendix detail drawings.
Strength of walls
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Loads: the following should be used as characteristic loads.
– a) Characteristic dead load. The characteristic dead load Gk is the weight of
the structure complete with
– finishes, fixtures and partitions and should be taken as equal to the dead
load as defined in and
– calculated in accordance with BS 6399-1.
– b) Characteristic imposed load. The characteristic imposed load Qk should
be taken as the imposed load
– as defined in and calculated in accordance with BS 6399-1 and BS 6399-3.
– c) Characteristic wind load. The characteristic wind load Wk should be
taken as the wind load calculated
– in accordance with BS 6399-2.
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The strength of a 255 mm cavity wall with loads spread over both leaves is
20% less than that of a one-brick wall.
– Wall ties built into cavity walls are intended to share lateral forces and
deflections between the 2 leaves.
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EN 8103 design of low-rise buildings. It includes permissible stresses for
load-bearing walls and prescribes their minimum thickness (determined in
relation to the load to be carried).
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Thickness not only determined by strength but also by weather and fire
resistance and thermal insulation.
Strength requirements
Walls transfer applied loads to the ground
• Self-weight loads (EN 1991-1-1:2002) the total self-weight
of structural and non-structural members including finishes,
fixtures and partitions
• Imposed loads (EN 1991-1-1:2002) loads on buildings
arising from occupancy including
– normal use by persons;
– furniture and moveable objects (e.g. moveable partitions, storage,
the contents of containers); - vehicles;
– anticipating rare events, such as concentrations of persons or of
furniture, or the moving or stacking
• Wind load (EN 1991-1-4:2005) Load applied to a building
due to wind
• Other loads include snow, fire, accidental such as impacts and
explosions.
Eurocodes
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Eurocode BS EN 1990:2002 Basis of structural design
Eurocode 1 BS EN 1991-1-1:2002 Actions on structures.
Eurocode 2 BS EN 1992-1-1:2004 Design of concrete structures.
Eurocode 3 BS EN 1993-1-1:2005 Design of steel structures.
Eurocode 4 BS EN 1994-1-1:2004 Design of composite steel and
concrete structures.
Eurocode 5 BS EN 1995-1-1:2004 Design of timber structures.
Eurocode 6 BS EN 1996-1-1:2005 Design of masonry structures.
Eurocode 7 BS EN 1997-1:2004 Geotechnical design.
Eurocode 8 BS EN 1998-1:2004 Design of structures for earthquake
resistance.
Eurocode 9 BS EN 1999-1-1:2007 Design of aluminium structures.