Types of Footing:
1. Isolated Footing:
An isolated footing is a type of foundation
used for individual columns. They are generally square, rectangular, or round
in shape to bear the load from a single column and spread this to the earth.
An isolated footing is a type of foundation
used for individual columns. It is usually rectangular, square or circular in
shape to take the load from one column and put it down on the soil.
When to Use: Isolated footings are suitable
when there is a long way between columns, which are all loads able to be borne
effectively by the soil beneath a building.
These footings are common in structures with
widely spaced columns and strong soil conditions.
Design Considerations
During the design of an isolated footing, the
following points need to be taken into consideration: The axial load carried by
the column.
· The bearing capacity of the soil.
· The size and shape of the footing.
· The depth necessary for stability
Example:
For a column with an axial load of 400 kN and a soil bearing capacity of 150 kN/m², the size of the footing can be calculated as follows:1.
Determine the area of the
footing:
The area of the footing can be found by dividing the load on the column by the
bearing capacity of the soil.
Area of footing = 400 kN / 150 kN/m² = 2.67 m²
2.
Determine the side length (if
footing is square):
To determine the side length of a square footing, take the square root of the
area.
Side length = √2.67 = 1.63 m
Thus, the dimensions of the isolated footing are approximately
1.63 m x 1.63 m.
2.
Combined
Footing
Definition: a kind of footing that been used for two or more
columns are very close to each other and it is not practical to have individual
footings. Their purpose is to have multiple columns sharing a load, which is
distributed equally among them all.
Combined footings are appropriate when
columns are located close to each other and the bearing capacities of the soil
are low or varying. This kind of footing is often used in buildings where
columns are close together because they want equal loads on all four corners
(or sides) of the slab.
Design Considerations
Key factors: for designing a combined footing
are:
· The load from multiple columns.
· The spacing between columns.
· The soil's bearing capacity.
· The shape and design of the footing (usually
rectangular or trapezoidal)
Example:
Consider two columns with loads of 400 kN and 600 kN, spaced 3 meters apart,
and a soil bearing capacity of 150 kN/m². The total load on the combined
footing is:
·
Determine the total load on
the footing:
Total load = 400 kN + 600 kN = 1000 kN
·
Determine the area of the
footing:
Area of footing = Total load / Bearing capacity of soil
Area of footing = 1000 kN / 150 kN/m² = 6.67 m²
·
Determine the dimensions of
the footing:
Assuming the footing is rectangular, let’s assume the length of the footing is
4 meters.
Width of footing = Area of footing / Length of footing
Width of footing = 6.67 m² / 4 m = 1.67 m
Thus, the dimensions of the combined footing are approximately 4
meters by 1.67 meters.
3. Bi-axial footing
When
two columns are subjected to loads in different directions (e.g. both along the
x and y axes), a foundation can be called bi-axial footing. This type of
footing must be designed to resist both axial loads and moments in two
directions.
When to Use:
Bi-axial
footings are commonly used when a column experiences both vertical and lateral
loads. This type of footing is particularly useful in buildings with irregular
layouts or where moments going in both directions (rather than only one of
them) need to be accounted for during design.
Design Considerations:
For
bi-axial foundations include: Axial loads from each column in both x and y
directions Eccentric loads lead to bending moments The bearing capacity of
soils The shape and the dimensions of the footing (that is, usually rectangular
or trapezoidal, depending on each case)
Example:
For two columns with loads of 400 kN and 600 kN, spaced 3 meters apart, and a
soil bearing capacity of 150 kN/m², the total load on the footing is 1000 kN.
However, because of the bi-axial moments, the footing dimensions
will be adjusted to accommodate bending in both the x and y directions.
The calculation for moment resistance and load distribution will
guide the final dimensions of the footing, ensuring that both axial and moment
loads are adequately supported.
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