2. Major Building PartsMajor Building Parts
SuperstructureSuperstructure
SubstructureSubstructure
FoundationFoundation
3. FootingFooting
Definition
Footings are structural members used to support columns and walls
and to transmit and distribute their loads to the soil in such a way
that the load bearing capacity of the soil is not exceeded, excessive
settlement, differential settlement,or rotation are prevented and
adequate safety against overturning or sliding is maintained.
4. Depth and location of foundation depends on
1. Zone of significant volume changes in soil.
2. Adjacent structures and property lines.
3. Ground water
4. Underground defects
Depth and location of foundationDepth and location of foundation
5. Zone of significant volume changes in soil :
Clays having high plasticity shrink and swell considerably
up on drying and wetting respectively.Volume change is
greatest near ground. Decreases with increasing depth.
Volume changes usually insignificant below a depth from
1.5-3.0 m and does not occur below.
Depth and location of foundationDepth and location of foundation
6. Adjacent structures and property lines.
Depth and location of foundationDepth and location of foundation
Structures may be damaged by the construction of new
foundations, as a result of vibrations, undermining by
excavation or lowering of the water table. After new
foundations have been constructed, the (new) loads
they place on the soil may cause settlement of
previously existing structures as a result of new stress
pattern in the surrounding soil.
In general, deeper the foundations and
closer to the old structure, greater will
be the potential for damage to old
structures.
450 Existing
Footing
New Footing
Limit for bottom of
deeper Footing
Part extending
property line Property line
7. Primary Factors Affecting
Foundation Choice
Subsurface soilSubsurface soil
Ground water conditionsGround water conditions
Structural requirementsStructural requirements
8. Secondary Factors Affecting Foundation Choice
Construction access, methods & site conditionsConstruction access, methods & site conditions
Environmental factorsEnvironmental factors
Building Codes & RegulationsBuilding Codes & Regulations
Impact on surrounding structuresImpact on surrounding structures
Construction scheduleConstruction schedule
Construction risksConstruction risks
9. 9
Types of Foundation
Types of Foundation
Shallows Foundations
Deep Foundations
Shallows Foundations
Shallow foundations are generally where the founding depth (Df) is less than the
width of the footing and less than 3m. Shallow foundations are used when
surface soils are sufficiently strong and stiff to support the imposed loads;
they are generally unsuitable in weak or highly compressible soils, such as
poorly-compacted fill, peat, recent lacustrine and alluvial deposits, etc.
10. FootingFooting
Definition
Footings are structural members used to support
columns and walls and to transmit and distribute their
loads to the soil in such a way that the load bearing
capacity of the soil is not exceeded, excessive
settlement, differential settlement,or rotation are
prevented and adequate safety against overturning or
sliding is maintained.
11. Types of FootingTypes of Footing
Wall footings are used to support
structural walls that carry loads
for other floors or to support
nonstructural walls.
12. 12
Types of Footings
Pad or column footings (Isolated )
Used to support single columns. This is one of the most economical
types of footings and is used when columns are spaced at
relatively long distances
13. 13
Combined Footing
Usually support two columns, or three columns not in a row.
Combined footings are used when tow columns are so close that
single footings cannot be used or when one column is located at or
near a property
Types of Footings
14. 14
Cantilever or strap footings
Consist of two single footings connected with a beam or a strap
and support two single columns. This type replaces a combined
footing and is more economical.
Types of Footings
15. 15
Types of footings
Continuous footings
Support a row of three or more columns. They have limited
width and continue under all columns.
16. 16
Types of footings
Mat (Raft) footings (Thickened slabs)
Consists of one footing usually placed under the entire building
area. They are used, when soil bearing capacity is low, column
loads are heavy single footings cannot be used, piles are not used
and differential settlement must be reduced.
Raft foundations are used to spread the load from a structure over a
large area, normally the entire area of the structure. They are
used when column loads or other structural loads are close
together and individual pad foundations would interact.
17. 17
A raft foundation normally consists of a concrete slab which
extends over the entire loaded area. It may be stiffened by
ribs or beams incorporated into the foundation.
Raft foundations have the advantage of reducing differential
settlements as the concrete slab resists differential movements
between loading positions. They are often needed on soft or
loose soils with low bearing capacity as they can spread the
loads over a larger area.
19. 19
Types of Foundation
Deep Foundations
Deep foundations are those with depths >3 m below finished
ground level. Deep foundations can be used to transfer the loading
to a deeper, more competent strata at depth if unsuitable soils are
present near the surface.
They are used when there are weak (“bad”) soils near the surface or
when loads are very high, such as very large skyscrapers. They
derive their support from deeper soils or bedrock
20. 20
Pile foundations
Are relatively long, slender members that transmit foundation
loads through soil strata of low bearing capacity to deeper
soil or rock strata having a high bearing capacity. They are
used for economic, constructional or soil condition
considerations, when it is desirable to transmit loads to
strata beyond the practical reach of shallow foundations.
In addition to supporting structures, piles are also used to
anchor structures against uplift forces and to assist
structures in resisting lateral and overturning forces.
Common Types of Deep Foundations
22. 22
Types of Foundation
Pile caps are thick slabs used to tie a group of piles together
to support and transmit column loads to the piles.
Typical pile foundation
23. 23
Piers
Are foundations for carrying a heavy structural load which is
constructed in-situ in a deep excavation.
Caissons
Are a form of deep foundation which are constructed above
ground level, then sunk to the required level by excavating
or dredging material from within the caisson.
24. 24
Compensated foundations
Are deep foundations in which the relief of stress
due to excavation is approximately balanced by
the applied stress due to the foundation. The net
stress applied is therefore very small. A
compensated foundation normally comprises a
deep basement.
25. Distribution of Soil PressureDistribution of Soil Pressure
When the column load P is applied on
the centroid of the footing, a uniform
pressure is assumed to develop on the
soil surface below the footing area.
However the actual distribution of the soil is not uniform, but
depends on may factors especially the composition of the soil and
degree of flexibility of the footing.
26. Distribution of Soil Pressure
Soil pressure distribution in cohesionless soil. Soil pressure distribution in cohesive soil.
27. Design ConsiderationsDesign Considerations
Footings must be designed to carry the column loads and
transmit them to the soil safely while satisfying code limitations.
Bearing capacity of columns at their base
Dowel requirements
Development length of bars
Differential settlement
1.
2.
3.
4.
28. Questions
28
1.Different types of foundations
(Advantages and disadvantages)
2.Factor affecting the choice of foundation type
3.What are the factors governing the depth of foundation
4.Differntiate between deep and shallow foundations
