Raft Foundation Area Threshold Rule

A raft foundation, also known as a combined footing, is a large concrete slab that covers the entire footprint of a building or a significant portion of it. It's used when individual footings would be too close together or when soil conditions are weak.

Condition for Raft Foundation Use

A raft foundation is typically recommended when the total area required by individual column footings exceeds a certain percentage of the building's total plan area. This indicates that the loads are too widespread for isolated footings to be efficient or practical.

The common rule of thumb used in structural design is that if the sum of the individual footing areas is more than 50% of the building's total plan area, a raft foundation should be considered.

Reasoning

• Individual footings spaced closely indicate significant load distribution.
• Using a single large raft distributes the building's load more evenly over the soil.
• This avoids overlapping individual footings and improves stability, especially on weaker soils.
• The threshold of 50% signifies a point where a raft becomes more economical and structurally sound than numerous individual footings.

Therefore, a raft foundation is provided if its area exceeds the plan area of the building by 50%.