Unit 5 Substructure Design Foundations Compressed

7/27/2019 Unit 5 Substructure Design Foundations Compressed

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Unit 3 SUBSTRUCTURE DESIGN - FOUNDATIONS

School of the Built Environment

Construction Technology D19SC

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:





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





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.





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





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





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:






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.






Water requirements for

different types of trees






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






Main types of foundations

Spread foundations Piled foundations






SPREAD FOUNDATIONS






Simplest form of foundation is the strip foundation, used to

support a load bearing wall

Main types ofstripfoundations

Conventional strip Deep strip Wide strip






CONVENTIONAL

STRIPFOUNDATIONS

Spread the load

exerted by the

building over asufficiently wide area

to prevent the

supporting ground

being overstressed






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





A good quality freehand sketch of a

deep strip or trench fill foundation





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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Construction Technology D19SCTypical

replacement

or bored pile



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Pile cap



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C SC


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BASEMENTS


C t ti T h l D19SC


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C t ti T h l D19SC


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C t ti T h l D19SC


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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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Unit 5 Substructure Design Foundations Compressed

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