When it comes to building a strong structure, the foundation is the most important aspect. One such important type of foundation used in construction is the combined footing. A combined footing becomes the ideal solution, especially in areas with space constraints or where two or more columns are closely placed. But what is a combined footing, when is it used, and what are its types? Let’s break it down in simple terms.
What is Combined Footing?
Simply explained, Combined footing is a type of foundation that supports two or more columns at the same time. In contrast to isolated footing, a combined footing foundation is built as one large concrete slab on which multiple columns are constructed. This ensures that the structural load is evenly distributed and balanced, especially when the soil’s load-bearing capacity is uneven or individual footings are so close to each other that their footings overlap.
Why Use a Combined Footing Foundation?
Combined footing has certain advantages in specific conditions. These are used to prevent overlapping of footings- one footing supports two or more columns preventing imbalance. Another circumstance where combined footings are used is when the column is intended to carry a heavy load and a bigger footing is required to distribute loads evenly.
Sometimes, this type of foundation is useful when the columns are close to the boundary wall and there isn’t space enough for individual footings. In this case, the combined footing foundation helps in centring the load evenly and efficient use of space.
Types of Combined Footing
Based on the design of the structure and the distribution of its load, there are five types of combined footing–
1. Rectangular Combined Footing
As the name suggests, this type of combined footing foundation is rectangular in shape and is used to support two or more columns intended to carry an equal amount of load. This can be used in both residential and commercial construction.
2. Trapezoidal Combined Footing
This type of footing is used to support columns that are intended to carry an unequal amount of load. The trapezoidal shape helps in distributing the load and the pressure on the soil uniformly, maintaining the solid foundation of the structure.
3. Slab Type Combined Footing
This type of combined footing is built as one large flat single slab under the columns in areas where the soil is weak or when the structure is heavy. This slab provides an even base and prevents any soil displacement from affecting the stability of the structure. As they offer more strength, they are ideal for use in industrial and infrastructural construction.
4. Strap Combined Footing
Strap footing is a kind of combined footing built as a beam or a strap that connects the footings of two or more columns. This helps in situations where one column footing is carrying more load than the others or is placed too close to the property boundary. The beam of the strap footing is connected to the footing of the next column. This helps in the load of the column being shared and keeping the load distribution even.
5. Raft Combined Footing
Raft footing is built as one large concrete base to support all elements of the structure- the walls, beams, columns- the full area of construction. This eliminates all risks of uneven foundation, unequal load distribution and any effects of soil displacement on the structure.
Combined Footing Design
As mentioned above, the different types of combined footing designs offer different advantages. The combined footing design depends on the load-bearing capacity of the soil, the placement of columns, the load the columns are intended to support and the size of the construction area.
One thing to keep in mind is that the balance between the centre of gravity of the footing and the centre of the load must be maintained to prevent uneven settlement.
In construction, one size certainly doesn’t fit all. When it comes to foundations, their design depends entirely upon the load, type of construction and the soil. A combined footing foundation offers smart solutions in challenging construction conditions and designs. The types of combined footing foundations give you the flexibility to design your project the way you want and keep your structure strong and stable.
FAQs
1. “Why would someone go for a combined footing? Isn’t isolated footing enough?”
Honestly, we all start with isolated footing in mind. But sometimes the space just doesn’t allow it. If two columns are too close and their footings overlap — or worse, one of them is stuck near a property line — that’s when we bring in combined footing. One big base to handle both loads. It’s not fancy; it’s just common sense.
2. “So when exactly does it become necessary?”
There are usually three situations:
- Columns are too close to each other.
- One column is awkwardly near the boundary, and we can’t center its footing.
- Or the soil’s not strong enough in spots, and we need to spread the load.
We’ve seen this a lot in tier-2 town plots, especially where every inch of land is tight — think Kanpur ka purana mohalla ya Jaipur ke inner colonies.
3. “Are all combined footings the same?”
Not quite. There’s the plain rectangular one — works when both columns carry more or less equal load.
Then there’s trapezoidal — needed when one column is way heavier than the other.
And then there’s strap footing — two footings joined with a strap beam. That one’s used when one column is too close to the edge but we still want to balance the load centrally.
4. “Does this affect cost much?”
It can, yes. More steel, more shuttering, more concrete. But not by a crazy amount. And frankly, it’s better to spend a bit more now than deal with cracks or tilting walls later. We’ve seen people cut corners and regret it — especially on boundary walls or shared walls with neighbours.
5. “What about soil type — does that play a role?”
Definitely. If you’re working on loose sandy soil — like we sometimes find near rivers or coastal zones — or on that stubborn black cotton soil that expands in the monsoon, isolated footings can’t always hold up. Combined footing spreads the pressure better.
Honestly, we all start with isolated footing in mind. But sometimes the space just doesn’t allow it. If two columns are too close and their footings overlap — or worse, one of them is stuck near a property line — that’s when we bring in combined footing. One big base to handle both loads. It’s not fancy; it’s just common sense.
There are usually three situations:
Columns are too close to each other.
One column is awkwardly near the boundary, and we can’t center its footing.
Or the soil’s not strong enough in spots, and we need to spread the load.
We’ve seen this a lot in tier-2 town plots,
Not quite. There’s the plain rectangular one — works when both columns carry more or less equal load.
Then there’s trapezoidal — needed when one column is way heavier than the other.
And then there’s strap footing — two footings joined with a strap beam. That one’s used when one column is too close to the edge but we still want to balance the load centrally.
It can, yes. More steel, more shuttering, more concrete. But not by a crazy amount. And frankly, it’s better to spend a bit more now than deal with cracks or tilting walls later. We’ve seen people cut corners and regret it — especially on boundary walls or shared walls with neighbours.
Definitely. If you’re working on loose sandy soil — like we sometimes find near rivers or coastal zones — or on that stubborn black cotton soil that expands in the monsoon, isolated footings can’t always hold up. Combined footing spreads the pressure better.
