Footing Design Excel (Isolated Footing, Combined Footing) Sheet Download
1.
Isolated Footing
Definition:
An isolated footing is a type of foundation used for individual columns. It is
typically designed in a square, rectangular, or circular shape to support the
load from a single column and distribute it to the ground.
When to Use:
Isolated footings are suitable when columns are spaced far apart and the soil
beneath the structure can bear the loads efficiently. These footings are
commonly used in buildings with widely spaced columns and strong soil
conditions.
Design
Considerations:
When designing an isolated footing, engineers need to consider the following:
- The axial load
from the column
- The soil’s
bearing capacity
- The size and
shape of the footing
- The required
depth of the footing to ensure 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:
- 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 - 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 combined footing is used when two or more columns are closely spaced, and
individual footings would overlap or be impractical. It is designed to
distribute the load from multiple columns to the soil, ensuring that the
columns share the load efficiently.
When to Use:
Combined footings are appropriate when columns are placed near each other, and
the soil’s bearing capacity is weak or uneven. This type of footing is often
used in buildings with closely spaced columns or where the loads need to be
balanced across the footing.
Design
Considerations:
Key factors for designing a combined footing include:
- The load from
multiple columns
- The spacing
between the columns
- The bearing
capacity of the soil
- The shape and
geometry 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
Definition:
A bi-axial footing is a foundation used when two columns are subjected to loads
in different directions (both along the x and y axes). This type of footing
must be designed to resist both axial loads and moments in two directions.
When to Use:
Bi-axial footings are typically used when columns experience both vertical and
lateral loads or when they are misaligned. This type of footing is particularly
useful in buildings with irregular layouts or where moments in both directions
need to be accounted for in the design.
Design
Considerations:
For bi-axial footings, the design must account for:
- Axial loads
from each column in both x and y directions
- Bending
moments due to eccentric loads
- The bearing
capacity of the soil
- The shape and
dimensions of the footing (usually rectangular or trapezoidal)
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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