Transcript Unit 3 SUBSTRUCTURE DESIGN - FOUNDATIONS

School of the Built Environment
Construction Technology D19SC
Unit 3
SUBSTRUCTURE DESIGN FOUNDATIONS
Unit 3 SUBSTRUCTURE DESIGN - FOUNDATIONS
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School of the Built Environment
Construction Technology D19SC
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 a
building 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 is
capable 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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Typical subsoil bearing capacities
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
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To achieve this basic function the foundation must be:
• 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)
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TYPES OF SOIL
Rock
The hardest rock is igneous e.g. granite and basalt. Normally they have a high
safe 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 soils
Gravels 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 that
their 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. Normally
150-200 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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Unit 3 SUBSTRUCTURE DESIGN - FOUNDATIONS
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Working out imposed loads
B3
C5
B1
B3
B3
B1
B3
B3
SD1
B3
C6
SD1
B3
C5
C5
B1
B3
SD1
B3
B3
C5
C3
B1
B3
SD1
B3
2
B3
SD1
B3
B3
B1
6.000
C2
B1
SD1
B3
B3
B1
SD1
B3
B3
B3
C3
B3
1
B1
B1
SD1
B3
B3
C3
B1
C1
B1
B3
C4
B1
SD1
B1
B2
B2
B3
B3
B1
SD1
B1
F
6.000
B3
B2
C2
B3
C4
B1
B1
B1
B1
C4
E
6.000
B2
C4
D
6.000
B1
C1
C
6.000
B2
B
6.000
B2
A
9.000
B3
C3
B3
B1
C6
3
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/m2
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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, nonfixed equipment. environmental or
dynamic loads - snow or wind.
thermal loads – temperature changes
causing load on structure
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To achieve this basic function the foundation must be:
Strong enough to prevent downward vertical loads shearing
through the foundation
Capable of withstanding the opposing forces,
the weight of the building and the resistance of
the soil, such that the foundation will not bend
Stable so that it will not overturn .Whenever
possible loads on foundations should be placed
centrically.
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To achieve this basic function the foundation must be:
• 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.
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BUILDING NEAR TREES
The combination of shrinkable soils and trees, hedgerows or shrubs represents a
hazard 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 is
low.
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Water requirements for
different types of trees
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Main types of foundations
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
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Main types of foundations
Spread foundations
Unit 3 SUBSTRUCTURE DESIGN - 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 of strip foundations
Conventional strip
Deep strip
Unit 3 SUBSTRUCTURE DESIGN - FOUNDATIONS
Wide strip
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CONVENTIONAL
STRIP
FOUNDATIONS
Spread the load
exerted by the
building over a
sufficiently 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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Pile Foundations
Divert or transfer the load
to a strata, deep in the
ground, which is capable
of supporting the imposed
load without failure
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Pile foundations
Two main types displacement & replacement
Typical
displacement or
driven pile
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Displacement or percussive piles being installed
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Unit 3 SUBSTRUCTURE DESIGN - FOUNDATIONS
Typical
replacement
or bored pile
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Pile cap
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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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GROUND FLOOR PLAN
Stairs &
Lifts
Grid 3 m
Entrance
Reception
Toilets
Toilets
Area A
Area B
Stairs
& Lifts
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UPPER FLOOR PLAN
Stairs &
Lifts
Reception
Area C
Toilets
Grid 3 m
Toilets
ATRIA
Area E
Area D
Stairs &
Lifts
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