How to calculate the Dimensions of Foundation

 

In construction engineering, the foundation acts as the main pillar upon which the whole building rest. Its dimensions are crucial for stability of structure. This article discuss about the definition, functions, and guidelines for fixing the dimensions of foundations, elucidating different types and their respective considerations.

 

 Functions of Foundation

Foundations are structural elements that transmit loads from buildings to the underlying soil or rock. They serve several pivotal functions:

1. Load Distribution: Foundations distribute the structural load over a larger area to prevent overloading of the soil.

2. Stability: They anchor the structure, ensuring stability against external forces such as wind, seismic activity, and settlement.

3. Settlement Control: Foundations mitigate uneven settlement by providing uniform support to the structure.

4. Protection: They shield the structure from detrimental factors like moisture, frost, and soil movement.

 

 Dimensions of Foundation

Foundation dimensions are determined based on various factors, including soil properties, anticipated loads, and structural requirements. The following guidelines dictate minimum dimensions:

 

Depth of Foundation

The depth of a foundation is crucial for ensuring stability and preventing excessive settlement. While Rankine’s Formula offers a theoretical framework for calculating minimum depth, practical considerations often supersede theoretical calculations. The formula is expressed as:

 

H =  P/w{1-sinO/1+sinO}2

Where:

H = Depth of foundation

p = Safe bearing capacity of the soil

w = Unit weight of soil

θ = Angle of repose of soil

 

However, a minimum depth of 0.9 meters is universally recommended to ensure adequate stability. Determining the safe bearing capacity requires expert analysis through field tests or laboratory experiments.

 

Width of Foundation

The width of a foundation, whether for walls or columns, is contingent upon the anticipated load and soil bearing capacity. It is calculated by dividing the load by the safe bearing capacity. Specifically:

Width of wall foundation is determined by the load per unit length of the wall divided by the safe bearing capacity of the soil.

Area of column footing is calculated by dividing the load carried by the column by the safe bearing capacity of the soil.

 

 Types of Foundations

Foundations vary in design and construction based on structural requirements, soil conditions, and budget constraints. Common types include:

 

1. Shallow Foundations

Shallow foundations, also known as spread footings, are employed when the load-bearing stratum is located near the surface. Types include:

- Strip Footing: Utilized for load-bearing walls, distributing the load over a wide area.

- Pad Footing: Suited for individual columns or isolated loads, providing concentrated support.

- Mat or Raft Foundation: Ideal for distributing heavy loads over a large area, mitigating differential settlement.

 

2. Deep Foundations

Deep foundations are employed when the soil near the surface is inadequate to support the structure's load. Types include:

- Pile Foundation: Consisting of slender columns driven or cast into the ground, transferring loads through skin friction and end bearing.

- Caisson Foundation: Large-diameter cylindrical structures sunk into the ground, providing support through skin friction, bearing, or a combination.

 

 Factors Influencing Foundation Dimensions

Several factors influence the dimensions of a foundation, including:

1. Soil Properties: Soil type, bearing capacity, cohesion, and angle of internal friction influence foundation design.

2. Structural Loads: The magnitude and distribution of loads exerted by the structure impact foundation dimensions.

3. Environmental Conditions: Factors such as frost depth, seismic activity, and water table elevation necessitate specific foundation depths and dimensions.

4. Building Codes and Regulations: Local building codes and regulations prescribe minimum standards for foundation dimensions to ensure structural safety and compliance.

 

Foundation dimensions are pivotal considerations in structural engineering, directly influencing the stability, longevity, and safety of buildings. By following the guidelines and considering various factors such as soil properties, structural loads, and environmental conditions, engineers can design foundations that withstand the test of time and ensure the structural integrity of buildings.