What is a Combined Footing?

A combined footing is a type of foundation that supports two or more columns in a single structural unit. This engineering solution is typically employed when columns are spaced closely together, or when one column is located near a property line where a spread footing cannot expand symmetrically. By combining the base, the structural integrity is improved, and the pressure on the soil is distributed more evenly.

When to use this Combined Footing Calculator?

This online tool is essential for civil engineers and students to quickly estimate the geometric requirements for a rectangular combined footing. You should use this calculator when:

• Two columns are so close that their individual footings would overlap.
• The soil bearing capacity is low, requiring a larger area than a single footing can provide.
• One column is a "boundary column," necessitating the shifting of the footing's center of gravity to align with the resultant load.

How to Calculate Combined Footing Dimensions

To use this calculator, follow these simple steps:

1. Enter Column Loads: Input the axial loads (P1 and P2) for both columns in kiloNewtons (kN).
2. Define Distance: Enter the center-to-center distance between the two columns.
3. Specify Soil Capacity: Provide the Safe Bearing Capacity (SBC) of the soil at the site.
4. Self-Weight: Usually, 10% to 15% of the total load is added to account for the weight of the concrete footing itself.
5. Calculate: The tool will output the required area, the resultant load position, and the recommended length and width (L x B).

Frequently Asked Questions

Q: What is the most common shape for combined footings?
A: Rectangular and Trapezoidal are the most common shapes. Rectangular is used when the loads are similar or space is available, while trapezoidal is used when one column load is significantly higher than the other.

Q: Does this calculator include reinforcement details?
A: No, this tool focuses on the sizing (Area, Length, and Width). Structural design for steel reinforcement requires a more complex analysis of bending moments and shear forces.