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FBE03: Building Construction & Science. Lecture 2 Foundations. Introduction. In this chapter it is intended to consider foundations suitable for a lightly loaded structure
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FBE03: Building Construction & Science Lecture 2 Foundations
Introduction • In this chapter it is intended to consider foundations suitable for a lightly loaded structure • interaction between the foundations and the loadbearing walls as well as the foundations and the soil beneath the building
Primary function of a foundation: • transmit all dead load; • live load • superimposed loads from the building to the surrounding ground
Foundations should be constructed: • avoid damage by swelling; • shrinkage or freezing of the subsoil • capable of resisting attack by sulphate salts
Choice of foundation • Choice of foundation based on three factor: • the total load of the building; • the nature and bearing capacity of the sub-soil; • the amount of settlement produced by the loading.
Choice of foundation • The total load of the building • three main types of loading on a building: • dead load; • live load; • superimposed load • Uplift • Earthquake
Choice of foundation • The nature and bearing capacity of the sub-soil • type of ground, • the nature and bearing capacity of the sub-soil (i.e. the soil beneath the topsoil) varies with the type of soil, its degree of compressibility and the amount of moisture in the soil
Choice of foundation • The amount of settlement produced by the loading: • Soil is compressible to varying degrees. As a load is applied to a foundation then the soil beneath the foundation will be compressed, the water and air in the voids between the particles will be squeezed out and the foundation will settle. • This process of consolidation will continue until the forces between the particles are equal to the applied load. The speed of this consolidation or settlement is determined by the speed of the migration of the water and air from between the soil particles. • Foundations built on sands therefore settle relatively rapidly, whilst the settlement of foundations built on clay soils is much slower and can last for a number of years. • Shallow foundations should not be used on sites consisting of made up ground
SETTLEMENTS OF FOUNDATIONS NO SETTLEMENT * TOTAL SETTLEMENT * DIFFERENTIAL SETTLEMENT Uniform settlement is usually of little consequence in a building, but differential settlement can cause severe structural damage
Requirement of good foundation • It should be sustain dead load and live load and safely transmit to sub soil without settlement. • Provide stability & safety to structure or building. • It should be rigid for different settlement can be minimized specially for super impose load • it should be provide deep guard to building for damage & distress by swelling or shrinkage of sub-soil. • It should be perform for at least as its design life period.
Types of foundation • Shallow foundations • Deep foundations
Shallow foundations • Types of shallow foundations: • strip • pad or • raft foundations
Shallow foundations • transfer the load of the building to the sub-soil at a level close to the surface • cheapest to install and are generally used where sufficient depth of strong subsoil • pressure on the sub-soil beneath is equal at all points in order to avoid unequal settlement
Traditional strip foundations • consist of a strip of concrete under a continuous wall that carries a uniformly distributed load • imposed load is therefore considered as a load per metre run of loadbearing wall • Factor of Safety of 1.5 should be sufficient
failure of the foundation due to shear stresses may result • foundation should be constructed of an adequate thickness such that the line of shear failure
Wide strip foundations • lower part of the concrete foundation must be reinforced with high tensile steel reinforcement rods to combat the tensile stresses
Raft Foundation • Raft slab foundations • used as a ground floor slab and is lightly reinforced with fabric mesh reinforcement to counteract stresses formed within the foundation by the transfer of loads to the sub-soil and the soil’s • Distribution of load evenly • Faster and chepest
Raft Foundation • Raft foundations (sometimes referred to as raft footings or mat foundations) are formed by reinforced concrete slabs of uniform thickness that cover a wide area, often the entire footprint of a building. • They spread the load imposed by a number of columns or walls over the area of foundation, and can be considered to ‘float’ on the ground as a raft floats on water.
Raft Foundation • Floor areas are small and structural loadings are low, such as in one or two-storeybuilding. • Ground conditions are poor and strip or pad foundations would require significant excavation, for example on soft clay, alluvial fill, and so on. • Settlement, or differential settlement is likely. • Where it may be impractical to create individual strip or pad foundations for a large number of individual loads.
Raft Foundation • Raft foundations can be fast and inexpensive to construct, as they tend not to require deep excavations compared to strip or pad foundations and they may use less material as they combine the foundation with the ground slab. • they tend to be less effective where structural loads are focussed on in a few concentrated areas, and they can be prone to erosion at their edges.
Deep foundations • firm soils do not exist at a level close to the surface of the ground, • but do exist at a much lower depth • poor loadbearing strata down to the higher loadbearing strata beneath. • bored pile foundation • A reinforced concrete beam supports the loadbearing wall over the piles and links all the piles together
