The correct answer is D. All the above.
A combined footing is a type of foundation that supports two or more columns. It is so proportioned that the center of gravity of the supporting area coincides with the center of gravity of the two column loads. This ensures that the footing will distribute the load evenly and prevent settlement.
A combined footing may be either rectangular or trapezoidal in shape. The shape of the footing is determined by the loading conditions and the type of soil. Rectangular footings are typically used for light loads and uniform soil conditions. Trapezoidal footings are typically used for heavy loads and non-uniform soil conditions.
Trapezoidal shaped footings may be provided under any loading. However, they are most commonly used for heavy loads and non-uniform soil conditions. This is because the trapezoidal shape allows the footing to distribute the load more evenly over the soil.
Here are some additional details about each option:
- Option A: A combined footing is so proportioned that centre of gravity of supporting area coincides with centre of gravity of two column loads. This is done to ensure that the footing will distribute the load evenly and prevent settlement.
- Option B: A combined footing may be either rectangular or trapezoidal in shape. The shape of the footing is determined by the loading conditions and the type of soil. Rectangular footings are typically used for light loads and uniform soil conditions. Trapezoidal footings are typically used for heavy loads and non-uniform soil conditions.
- Option C: Trapezoidal shaped footings may be provided under any loading. However, they are most commonly used for heavy loads and non-uniform soil conditions. This is because the trapezoidal shape allows the footing to distribute the load more evenly over the soil.