Designing a Combined Footing-Worked Example

Spread foundations supporting two or more columns are classified as combined footings. Sometimes, it becomes necessary to combine the footing for two or more columns for two reasons:

• If columns are so close to each other such that constructing single footings will result in an overlap.
• When columns are very close to a property line such that a single footing cannot be constructed without projecting beyond the line.

In designing combined footings, it is always best to proportion the footing such that the resultant of the applied loads pass through the centroid of the footing area. This allows a uniform bearing pressure under the footing and helps to prevent differential settlement. However, sometimes, the footings are constraint from projecting in a certain direction due to the property line or site restrictions. Thus, it is not possible to proportion the footing for uniform pressure distribution, the eccentricities must, therefore, be calculated and the footing designed for varying earth pressure.

Alternatively, a strap foundation may be used where a property line or site constraint exists. However, a strap foundation might not be economical for closely spaced columns. The design of strap foundation has been the subject of a previous post, see: Designing a strap foundation to Eurocode

Design Procedures

The design procedure of a combined footing is almost the same with that of pad foundation. The only concern is proportioning the footing for uniform pressure distribution. Generally, the steps are as follows:

• Determine the required size of the footing using the soil bearing resistance and the loads at serviceability limit state.
• Proportion the footing for uniform pressure distribution by finding the location of the centroid and adjusting the footing appropriately.
• Determine the bearing pressures at the ultimate limit state.
• Assume a suitable thickness for the footing, calculate the effective depth and verify punching shear at the column face.
• Analyze the footing in the longitudinal and transverse direction.
• Design the bending reinforcement in the longitudinal and transverse direction.
• Verify shear at the critical sections including punching shear at the basic control perimeter 2.0d from the column face.
• Verify detailing requirements.

The analysis of the combined is idealized as an inverted overhanging beam subjected to the earth pressure at the ultimate limit state.

Worked Example

A combined footing is required in a proposed office building for four heavily loaded closely spaced columns. Design the footing completely assuming a presumed bearing resistance of 150kN/m2 using C30/37 concrete with 460mpa bars. The design data for the columns are presented in Table 1 and Figure 1 shows a part of the column layout.

Serviceability limit state.

Let’s start by determining the actions on the footings.