If two individual footings are too close as per design, then they should be converted as
Combined footing
Understanding Footing Design and Placement
Footings are essential components of a building's foundation, transferring the loads from columns and walls to the underlying soil. Proper design ensures that these loads are distributed over a sufficient area to prevent excessive settlement and maintain structural stability. One critical aspect of foundation design is the spacing between individual footings.
The Problem: Individual Footings Too Close
When two individual footings are designed too close to each other, their zones of stress influence in the soil can overlap significantly. This overlap can lead to:
- Increased pressure on the soil below the footings.
- Potential for differential settlement between the footings.
- Complexity in calculating the bearing capacity and settlement accurately for each individual footing.
To avoid these issues and ensure structural integrity, a different foundation type is often required when individual footings are in close proximity according to the design.
The Solution: Combined Footing
When two individual footings are too close, the appropriate solution is typically to replace them with a single, larger footing that supports both columns. This is known as a combined footing.
A combined footing is designed to carry the loads from two or more columns. By creating a single footing area under both columns, the loads are combined, and the resulting pressure on the soil is distributed over a larger, continuous area. This approach helps in:
- Handling the combined load and stress interaction effectively.
- Simplifying the design process compared to analyzing closely spaced individual footings with overlapping stress zones.
- Often centering the resultant load from both columns on the footing, which helps manage eccentric loading and potential overturning moments, although the footing shape (rectangular or trapezoidal) might be adjusted to achieve this ideal load centering.
Comparing Foundation Types
Let's briefly look at why other options might not be the primary choice when simply dealing with two *individual* footings that are too close:
| Footing Type | Description | Typical Application | Suitable for Two Close Individual Footings? |
|---|---|---|---|
| Individual Footing | Supports a single column or isolated load. | Widely spaced columns. | No, if too close due to overlapping stress zones. |
| Combined Footing | Supports two or more columns on a single footing. | Columns too close, or a column is near property line preventing eccentric individual footing. | Yes, designed specifically for this situation. |
| Strap Footing | Two individual footings connected by a structural beam (strap). | Used when one column is near a property line, and an eccentric individual footing is not feasible. The strap connects to an interior footing to counteract eccentricity. | Can be an alternative to combined footing in specific eccentric load/property line situations, but combined footing is the direct solution for simply "too close". |
| Strip Footing (Raft) | A continuous footing supporting a load-bearing wall or a line of closely spaced columns. | Walls or rows of columns. | No, typically for linear loads, not just two isolated columns. |
| Mat/Raft Foundation | A large concrete slab covering the entire footprint of the building, supporting all columns and walls. | When soil bearing capacity is low, or building loads are very heavy, or when multiple individual/combined footings would cover > 50% of the area. | No, overkill for just two close columns; used for entire building support. |
Based on standard foundation design principles, when two individual footings are found to be too close according to design requirements (usually based on factors like soil type, load, and stress distribution analysis), converting them into a combined footing is the most direct and appropriate solution.
Revision Table: Foundation Types for Close Columns
| Condition | Recommended Foundation Type |
|---|---|
| Individual footings too close together (overlapping stress zones). | Combined Footing |
| Column near property line requiring eccentric footing. | Strap Footing (or Combined Footing if multiple columns are close) |
| Soil bearing capacity is low, or building loads are very high. | Mat/Raft Foundation |
| Load-bearing wall or line of closely spaced columns. | Strip Footing |
Additional Information on Combined Footings
Combined footings come in different shapes depending on the column loads and spacing, primarily rectangular or trapezoidal. The shape is often chosen to ensure the centroid of the footing area coincides with the resultant of the column loads, thereby minimizing bending moments and ensuring uniform soil pressure distribution.
- Rectangular Combined Footing: Suitable when column loads are roughly equal or spacing is uniform.
- Trapezoidal Combined Footing: Often used when one column load is significantly larger than the other or when it helps align the centroid with the resultant load, especially if columns are spaced far apart.
Designing a combined footing involves calculating the total load, determining the required footing area based on soil bearing capacity, and analyzing bending moments and shear forces to determine the required concrete dimensions and reinforcement.
Important Questions from Shallow Foundation
According to Terzaghi theory, what is the value of coefficient (Nc) for an angle of shear resistance (ϕ) = 0?
A raft foundation of 6 m × 9 m is placed at a depth of 3 m in a cohesive soil having c = 120 kN/m 2. The net ultimate bearing capacity of the soil using Terzaghi's theory will be.
Piles are usually driven by
The type of footing in which the load bearing structures share the common rectangular or trapezoidal footing is called:
A Grillage foundation is essentially a
