Question

Self-weight considered for the design of footing is

• A. 20% of the column load
• B. 15% of the column load
• C. 10% of the column load
• D. 5% of the column load

Hint:

For the preliminary design of isolated footings, a standard first step is to estimate the self-weight.
A good starting assumption is 10% of the column load.
This allows you to calculate the required footing area and begin the detailed design.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks for a common rule-of-thumb percentage used to estimate the self-weight of a footing during the preliminary design stage.

Step 2: Detailed Explanation:
When designing a footing, the first step is to determine the required area. The area is calculated as:
\[ \text{Area of Footing} = \frac{\text{Total Load}}{\text{Safe Bearing Capacity of Soil}} \] The "Total Load" includes the service load coming from the column ($P$) plus the self-weight of the footing itself ($W_f$).
However, at the beginning of the design, the size and weight of the footing are unknown. To start the calculation, a reasonable assumption must be made for the footing's self-weight.
A widely accepted and commonly used rule of thumb in practice and for academic problems is to assume the self-weight of the footing is approximately

10% of the axial load transferred from the column.
So, the total load used for finding the area is estimated as $P_{total} = P + 0.10 P = 1.1 P$.
After the footing is fully designed, its actual self-weight is calculated and compared to the initial assumption to ensure the design is safe.

Step 3: Final Answer:
A common assumption for the self-weight of a footing for preliminary design is 10% of the column load.