Foundations: Conventional Spread Footings and RCC Footings
Definition
A foundation is the lowest structural element that transmits loads from the superstructure to the ground. A spread footing is a type of shallow foundation that distributes the load of a wall or column over a wider soil area to keep the soil pressure within safe limits. A conventional spread footing is generally constructed using mass concrete, brickwork, stone masonry, or plain cement concrete, whereas an RCC footing is constructed using reinforced concrete with steel bars to resist bending, shear, and tensile stresses.
In simple terms:
Spread footing
- = a broad base foundation that spreads load.
Conventional footing
- = usually non-reinforced or minimally reinforced base.
RCC footing
- = reinforced concrete base designed for strength and durability.
Main Content
1. Spread Footing as a Shallow Foundation
- Spread footings are called shallow foundations because their depth is generally small compared to their width, and they transfer load to soil close to the ground surface.
- The main purpose of a spread footing is to reduce the pressure on the soil by increasing the contact area, thereby preventing shear failure and excessive settlement.
Spread footings are suitable when the soil near the surface has adequate bearing capacity. They are commonly used under walls and columns in low-rise and medium-rise buildings. If the load from a structure is applied directly on a small area, the soil may fail or settle unevenly. By providing a footing wider than the wall or column, the load is spread over a larger area, making the foundation safer.
Example:
If a wall carries a load of 120 kN and the safe bearing capacity of soil is 100 kN/m², the footing area must be at least 1.2 m² to transfer the load safely. If the footing is made 1.2 m × 1.0 m, the soil pressure becomes acceptable.
Spread footings may be:
Wall footing
Isolated footing
Combined footing
Strip footing
For basic academic understanding, wall footings and isolated column footings are the most important types.
2. Conventional Spread Footings
- Conventional spread footings are simple foundations constructed using materials like stone masonry, brick masonry, or plain cement concrete, without steel reinforcement or with very little reinforcement.
- They are economical, easy to construct, and suitable for structures with light to moderate loads and good soil conditions.
These footings are often used in older construction practices and in low-cost buildings. In wall load-bearing structures, a conventional footing may consist of stepped brick masonry or stone masonry below the wall. In some cases, a layer of plain concrete is provided at the bottom to act as a leveling course and to distribute load better.
Common characteristics:
- Suitable for light structures
- Constructed with locally available materials
- Simple design and execution
- Lower cost than RCC footings
- Limited use where tensile or bending stresses are high
Typical forms:
Stepped brick footing
Stone masonry footing
Plain cement concrete footing
Advantages of conventional footings in practice:
- Easy to build with ordinary labor
- Economical for small buildings
- Does not require steel reinforcement
- Useful where load is small and soil is strong
Limitations:
- Poor resistance to tension and bending
- Not suitable for heavy concentrated loads
- Less durable under differential settlement
- Larger size may be required compared to RCC footings
3. RCC Footings
- RCC footings are foundations made of concrete reinforced with steel bars to resist tensile, shear, and bending stresses.
- They are preferred for column loads, heavy structures, weak soil conditions, and situations where higher structural safety and durability are required.
Concrete is strong in compression but weak in tension. When a column load acts on a footing, the soil pressure creates bending in the footing slab. Reinforcement is added at the bottom or top depending on the design so that the footing can resist these stresses safely. RCC footings may be designed as:
Isolated footing
Strip footing
Combined footing
Sloped footing
Stepped footing
Raft foundation
- in special cases
Main features of RCC footing:
- High load-carrying capacity
- Better crack resistance
- Suitable for variable soil conditions
- Strong against bending and shear
- Long service life when properly designed and constructed
Example:
A single RCC isolated footing may support one reinforced concrete column in a residential or office building. The footing spreads the column load over a larger area and is reinforced with steel mesh at the bottom to resist tensile stress.
Limitations:
- Higher cost than conventional footing
- Requires careful design and skilled construction
- Needs proper curing, shuttering, and reinforcement placement
- More material and technical supervision required
Working / Process
1. Load transfer from structure to footing
The wall or column transfers the superstructure load to the footing. In a wall footing, the load is distributed along the wall length. In a column footing, the load is concentrated at one point and must be spread uniformly.
2. Load distribution to soil
The footing increases the base area so that the pressure on the soil remains within the safe bearing capacity. In conventional footings, the load is distributed mainly by the mass and shape of the footing. In RCC footings, concrete and reinforcement work together to distribute load and resist structural stresses.
3. Soil reaction and stability
The soil provides an upward reaction that balances the applied load. A properly designed footing prevents excessive settlement, tilting, sliding, or overturning. In RCC footings, the steel reinforcement helps control cracking and improves stability under non-uniform loading.
Construction sequence commonly followed:
- Site marking and excavation to required depth
- Levelling and compacting the base
- Laying PCC or footing base course
- Placing masonry or RCC reinforcement and formwork
- Concreting and compaction
- Curing and backfilling after sufficient strength is gained
Advantages / Applications
- Spread footings are economical and effective for structures where the soil near the surface has adequate bearing capacity, making them suitable for many common buildings.
- Conventional spread footings are simple, low-cost, and easy to construct, so they are widely used in small houses, boundary walls, and load-bearing wall structures.
- RCC footings are highly durable and strong, making them ideal for columns, multi-storey buildings, commercial structures, and places where soil strength is moderate or where loads are heavy.
Applications include:
- Residential buildings
- Small and medium commercial structures
- Load-bearing walls
- RCC column supports
- Boundary walls and compound walls
- Industrial sheds where shallow foundation is suitable
Summary
- Foundations transfer structural loads safely to the soil.
- Conventional spread footings are simple, economical shallow foundations used mainly for light loads.
- RCC footings are reinforced foundations used for heavier loads and better structural performance.
- The choice of footing depends on soil condition, load, safety, and economy.