Footings are primarily "measured" by determining their required size to adequately support a structure's load and distribute it safely onto the soil. This calculation ensures the footing prevents settling or structural failure.
The most critical measurement involves calculating the necessary surface area of the footing based on the anticipated structural load and the capacity of the underlying soil.
Determining Footing Size
To figure out how large a footing needs to be, a fundamental engineering principle is applied using a simple formula. This calculation provides the minimum required footprint of the footing.
According to standard practice, a simple formula is used to determine the size (specifically, the required surface area) of the footing:
Structural Load (lbs) / Soil Capacity (psf) = Footing Surface Area Required (sq. ft.)
Understanding the Formula Components
Let's break down the elements of this calculation:
- Structural Load (lbs): This is the total weight the footing will need to support. It includes the weight of the structure itself (dead load) plus any anticipated weight from occupants, furniture, snow, or other factors (live load). This load is typically measured in pounds (lbs).
- Soil Capacity (psf): Also known as the soil bearing capacity, this is a measure of how much weight the soil can safely support per unit of area without compressing excessively. It is usually determined through soil tests performed by a geotechnical engineer and is commonly expressed in pounds per square foot (psf).
- Footing Surface Area Required (sq. ft.): The result of the calculation, this is the minimum area the bottom of the footing must cover to distribute the structural load over the soil safely. The area is measured in square feet (sq. ft.).
Example Calculation
Imagine a column supporting a Structural Load of 8,000 lbs. A soil test indicates the Soil Capacity is 2,000 psf.
Using the formula:
8,000 lbs / 2,000 psf = 4 sq. ft.
This means the footing needs a minimum surface area of 4 square feet. A common way to achieve this is with a square footing measuring 2 feet by 2 feet (2 ft * 2 ft = 4 sq. ft.).
Practical Considerations
While the formula gives the required surface area, actual footing dimensions also include:
- Width/Length: Determined from the calculated surface area (e.g., a 4 sq ft area could be 2'x2', or potentially 1'x4' depending on structural needs).
- Depth/Thickness: This dimension is crucial for the footing's structural integrity and its ability to resist bending forces. Minimum depths are often specified by building codes based on frost lines (to prevent movement caused by freezing water) and structural requirements.
- Reinforcement: Steel rebar is often added within the footing to provide tensile strength, measured by its diameter and spacing.
Therefore, while the calculation of required surface area is the primary "measurement" for sizing, the final footing design includes width, length, and depth based on engineering standards and local building codes.
| Measurement Aspect | How it's Determined | Unit |
|---|---|---|
| Required Surface Area | Calculated using Structural Load / Soil Capacity | Square Feet |
| Width & Length | Based on required surface area and structural layout | Feet / Inches |
| Depth / Thickness | Based on structural requirements and building codes (frost line) | Feet / Inches |
| Reinforcement (Rebar) | Specified by structural engineer | Bar Size (#) |
In summary, measuring footings primarily involves calculating the necessary surface area based on the load and soil, which then dictates the required length and width, while depth is determined by structural needs and code requirements.
