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Unit 3 SUBSTRUCTURE DESIGN - FOUNDATIONS
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Unit 3
SUBSTRUCTURE DESIGN -
FOUNDATIONS
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Unit 3 SUBSTRUCTURE DESIGN - FOUNDATIONS
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In any building
the superstructure
the substructure (foundations)
the supporting soil
act together to give the building
structural stability
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Foundations are the vital link between the superstructure and the ground.
The criteria for a successful foundation are:
it should be at a minimum depth and size, without exceeding the allowable
bearing capacity of any soil layer below the foundation.
it should have settlement consistent with the supporting structure
it should be able to withstand natural ground movements from frost, moisture
and heat
regard is given to buildability
it should be economical
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The principles of foundations.
The basic function of a foundation is to intercept the load exerted by abuilding structure and transfer this load to the supporting soil in such
a way that the building will not sink into the ground (subside)
Structural stability is normally achieved in either of two ways, or
indeed a combination of both.
Spread the load exerted by the building over a sufficiently wide
area to prevent the supporting ground being overstressed
Divert or transfer the load to a strata, deep in the ground, which iscapable of supporting the imposed load without failure
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When a building is placed on the ground
it exerts a force on the soil.
Safe foundations place that load such
that the soil is not overloaded.
The ability of the ground or soil to bear
a load varies with types of soil and
ground depth
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Type of subsoil Bearing Capacity
(kN/M2)
Rocks, granites and chalk 600 10000
Non-cohesive soils
Compact sands
Loose uniform sands
100 - 600
Cohesive soils
Hard clays
Soft clays and silts
0 600
Peats and made-up ground To be determined by investigation
Typical subsoil bearing capacities
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Be constructed of materials that will not be degraded by chemicals found in the
soil around the foundation. Normally foundations are composed of concrete and
when conditions demand, the specification of the concrete will need to be altered
to avoid corrosive elements in the soil.
Able to withstand the effect of frost (also applies to services buried in ground)
To achieve this basic function the foundation must be:
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TYPES OF SOIL
Rock
The hardest rock is igneous e.g. granite and basalt. Normally they have a highsafe bearing capacity, 2-3 times that of sedimentary rocks and 25-30 times that
of clays and sands. Generally bedrock is an excellent base to build on but
unfortunately the cost of levelling and the cost of excavating service trenches
outweighs the initial advantage of a good natural base.
Course grained non-cohesive soilsGravels and sands come under this heading. When loaded they shear if
unconfined. The particles slide over each other at an angle known as the angle
of internal friction .
Fine grained cohesive soils
These include clays and silts. The major problem with these types of soil is thattheir nature changes with the level of moisture in the soil. When the soil drys out
they shrink, but when the moisture content is increased the soil swells. When
water trapped in the soil freezes it can cause vertical heave
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Types of soil
Organic soils
These include peat, loam and mud. Generally unsuitable for building on. Normally150-200 mm thick. Such soil (top soil) is usually removed before building begins.
Made up soil
As the stock of quality building land diminishes, poorer ground is often used. Today
made up ground is being utilised. Extreme care should be taken to ensure that such
land is properly investigated.
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Working out imposed loads
C1 C4 C4 C4 C4 C1
C2 C5 C5 C5 C5 C2
C6 C3 C3 C3 C3 C6
B1 B1 B1 B1 B1
B1 B1 B1 B1 B1
B1 B1 B1 B1 B1
B1
B1
B1
B1
B1
B1
B2
B2
B2
B2
B2
B2
A B C D E F
1
2
3
6.000 6.000 6.000 6.000 6.000
6.000
9.000
B3
B3
B3
B3
B3
B3
B3
B3
B3
B3
B3
B3
B3
B3
B3
B3
B3
B3
B3
B3
B3
B3
B3
B3
B3
B3
B3
B3
B3
B3
SD1
SD1
SD1
SD1
SD1
SD1
SD1
SD1
SD1
SD1
B1 = 406 x 130 x 39 Universal Beam C1 = 254 x 254 x 71 Universal Column Note that this building has 3 storeys
B2 = 457 x 152 x 74 Universal Beam C2 = 305 x 305 x 88 Universal Column above ground level + a concrete roof
B3 = 305 x 127 x 37 Universal Beam C3 = 305 x 305 x 149 Universal Column having the same construction as the
C4 = 305 x 305 x 79 Universal Column floors. The ground floor slab is ground
SD1 = Structural Concrete Composite Floor C5 = 305 x 305 x 186 Universal Column supported and is to be disregarded in
using Corus ComFlor 80 Composite Floor C6 = 254 x 254 x 85 Universal Column foundation assessments.
Decking- depth of slab = 150mm.
Load imposed by ComFloor Deck = 0.75kN/m 2
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QUESTION FOR DISCUSSION IN CLASS
The loads exerted by the building vary according to the size, use and form
of construction used.
What loads are exerted by a building on to the ground below the building?
As discussed in unit 2, the building is
exposed to both dead and live loads
Permanent or dead loads: the weight
of the structure, cladding and fixed
equipment
Temporary or live loads :
imposed loads people furniture, non-
fixed equipment. environmental ordynamic loads - snow or wind.
thermal loads temperature changes
causing load on structure
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S h l f th B ilt E i t
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Capable of withstanding changing
conditions in the ground if they occur, e.g.
movement caused by shrinking and
swelling, water pressure, etc.
Accommodate initial settlement of the structure. It is especially important that
uneven settlement does not occur.
That the installation of foundations does not overstress the ground such that
adjacent existing foundations and services are damaged. It should be noted that
the installation of new ground based services can undermine existing foundations.It should also be noted that where services pass under or adjacent to foundations
the load exerted on them by the foundations may cause failure. In such situations,
such as a sewer collapse this may in turn undermine the foundation.
To achieve this basic function the foundation must be:
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BUILDING NEAR TREES
The combination of shrinkable soils and trees, hedgerows or shrubs represents ahazard to structures that requires special consideration. Trees, hedgerows and
shrubs take moisture from the ground and, in cohesive soils such as
clay, this can cause significant volume changes resulting in ground movement. This
has the potential to affect foundations and damage the supported structure. In
order to minimise this risk, foundations should be designed to accommodate the
movement or be taken to a depth where the likelihood of damaging movement islow.
S h l f th B ilt E i t
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Water requirements for
different types of trees
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Near Surface Foundations - spread foundations
Strip Foundations Pad Foundations
Continuous Column Founds
Balanced Footings
Rafts:
Plain Slabs
Stiffened Edge
Downstand Raft
Upstand Raft
Cellular Raft
Buoyancy tanks
Deep Foundations
Piled Foundations
Bored Piles
Driven Piles
Main types of foundations
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Main types of foundations
Spread foundations Piled foundations
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SPREAD FOUNDATIONS
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Simplest form of foundation is the strip foundation, used to
support a load bearing wall
Main types ofstripfoundations
Conventional strip Deep strip Wide strip
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CONVENTIONAL
STRIPFOUNDATIONS
Spread the load
exerted by the
building over asufficiently wide area
to prevent the
supporting ground
being overstressed
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A good quality freehand sketch of a
simple strip foundation
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Deep strip
foundations
Tend to be used at
depths greater than
1.2 m deep
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A good quality freehand sketch of a
deep strip or trench fill foundation
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Pad foundation
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GROUND BEAMS
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RAFT FOUNDATIONS
Spread the load over a wider
area
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RAFT FOUNDATIONS
Spread the load over a wider
area
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Raft foundation
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PILE FOUNDATIONS
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Divert or transfer the load
to a strata, deep in the
ground, which is capable
of supporting the imposed
load without failure
Pile Foundations
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Pile foundationsTwo main types displacement & replacement
Typicaldisplacement or
driven pile
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Typical
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replacement
or bored pile
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Pile cap
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C SC
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BASEMENTS
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What type of foundation would you use in the assignment?
Where would you locate the foundations?
Is there a role for a basement?
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