What is the difference between isolated and combined footings?

Short Answer:

The main difference between isolated and combined footings lies in the number of columns they support and their application conditions. An isolated footing supports a single column and is used when columns are placed far apart. A combined footing supports two or more columns and is used when the columns are close together or near property boundaries.

Isolated footings are simple, economical, and commonly used in regular buildings. Combined footings are required when space is limited or when load distribution demands a shared foundation to avoid overlap and ensure balanced support.

Detailed Explanation:

Difference between isolated and combined footings

In RCC construction, footings are the lowest part of a structure, designed to distribute the building’s load to the soil. The selection between isolated and combined footings depends on the position of columns, spacing, soil conditions, and load distribution. Both serve the same basic function but differ in design, shape, and usage.

Let’s understand how these two types of footings differ and when each is used.

Isolated Footing

An isolated footing is a separate footing provided under each individual column. It is the most common type of shallow foundation and is typically used when the loads are light to moderate and columns are not closely spaced.

Characteristics:

  • Supports only one column.
  • Shape may be square, rectangular, or circular.
  • Simple in design and economical in normal soil conditions.
  • Soil pressure is generally uniform under the footing.
  • Reinforcement is provided in both directions to resist bending.

When used:

  • Columns are placed at a reasonable distance from each other.
  • The load is small to moderate.
  • There is sufficient open space around each column.
  • Suitable for residential and small commercial buildings.

Combined Footing

A combined footing is a single footing that supports two or more columns. It is provided when columns are close together, or when one column is near the property line, making it impossible to place a symmetrical isolated footing.

Characteristics:

  • Supports two or more columns.
  • Shape is generally rectangular or trapezoidal, depending on load balance.
  • Designed to ensure uniform soil pressure under the entire footing area.
  • Reinforcement is placed considering the entire combined load and bending effects.

When used:

  • Columns are close, and isolated footings would overlap.
  • One column is near boundary, and isolated footing cannot be centered.
  • When load distribution is unequal, and a shared base is needed.
  • Often used in heavy-loaded structures or industrial buildings.

Key Differences

  1. Support: Isolated footing supports one column; combined footing supports two or more.
  2. Spacing: Isolated for widely spaced columns; combined for closely spaced columns.
  3. Usage: Isolated is simpler and used in normal cases; combined is used in special situations.
  4. Shape: Isolated footings can be square or round; combined footings are usually rectangular or trapezoidal.
  5. Design: Combined footing design is more complex as it must handle load from multiple points.
Conclusion:

The difference between isolated and combined footings lies in the number of columns supported and the space available. Isolated footings are used for single columns placed apart, while combined footings support multiple columns, especially when spacing is tight or near boundaries. Choosing the right type ensures balanced load transfer and foundation safety for the structure.