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Unit 3 SUBSTRUCTURE DESIGN - FOUNDATIONS. In any building the superstructure the substructure ( foundations ) the supporting soil act together to give the building structural stability. Foundations are the vital link between the superstructure and the ground.
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Unit 3 SUBSTRUCTURE DESIGN - FOUNDATIONS
In any building • the superstructure • the substructure (foundations) • the supporting soil • act together to give the building structural stability
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
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
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
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)
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
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.
B3 B3 B3 B3 B3 B3 B3 B3 B3 B3 B3 B3 B3 B3 B3 SD1 SD1 SD1 SD1 SD1 SD1 SD1 SD1 B3 B3 B3 B3 B3 B3 B3 B3 B3 B3 B3 B3 B3 B3 B3 A 1 B 3 C F D E 2 Working out imposed loads 6.000 6.000 6.000 6.000 6.000 C1 C4 C4 C4 C4 C1 B1 B1 B1 B1 B1 B1 B1 B1 B1 B1 B1 6.000 SD1 SD1 C2 C5 C5 C5 C5 C2 B1 B1 B1 B1 B1 9.000 B2 B2 B2 B2 B2 B2 C6 C3 C3 C3 C3 C6 B1 B1 B1 B1 B1 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
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 equipmentTemporary or live loads : imposed loads – people furniture, non-fixed equipment. environmental or dynamic loads - snow or wind.thermal loads – temperature changes causing load on structure
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. To achieve this basic function the foundation must be:
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.
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.
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
Main types of foundations Spread foundations Piled foundations
Simplest form of foundation is the strip foundation, used to support a load bearing wall Main types of stripfoundations Deep strip Wide strip Conventional strip
CONVENTIONAL STRIP FOUNDATIONS • Spread the load exerted by the building over a sufficiently wide area to prevent the supporting ground being overstressed
A good quality ‘freehand’ sketch of a simple strip foundation
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
RAFT FOUNDATIONS Spread the load over a wider area
RAFT FOUNDATIONS Spread the load over a wider area
Pile Foundations Divert or transfer the load to a strata, deep in the ground, which is capable of supporting the imposed load without failure
Pile foundations Two main types displacement & replacement Typical displacement or driven pile
