Spread Footing Design

The foundation is that part of the structure, which is usually placed below the ground surface and transmits the load to the underlying soil. If satisfactory soil directly underlies the structure, it is merely necessary to spread the load, by footings or other means, in order to reduce the bearing pressure and therefore prevent excessive settlement or rotation.

The SPREAD program computes the soil bearing pressures induced by a square or rectangular spread concrete footing subject to vertical load and biaxial moment. In addition, the steel reinforcement is designed and the concrete shear capacity is checked.

Footing Types

There are four possible types of spread footings according to the soil stress distribution, as shown in the figure to the right and described below:

Type I - Footing with all its area in compression.
Type II - Footing with one corner unstressed.
Type III - Footing with two corners unstressed.
Type IV - Footing with three corners unstressed.

Type I footings are the most commonly found in practice. However, under large eccentricities it is sometimes necessary to design the other types of footings.

Input Data

The required input data includes the footing and pier or column dimensions, the materials' properties, the allowable soil bearing pressure, and the acting service and factored loads, as shown in the figure to the left.

Example

As an example, consider the spread footing shown to the right with a soil cover of 2 ft. subjected to the following loads. The allowable soil pressure is 3.0 ksf. Find out the adequacy of the footing dimensions and design the steel reinforcement.

Output

Once the required data is entered, the program automatically computes the soil pressures indicating the footing type and the percentage of the total area in compression, as well as the pressures at corners. In this manner, the designer may vary the dimensions to obtain an adequate design.

Figure to the left shows the template related to this program with the data of this example. The footing is Type II with 84% of its area under compression. The maximum pressure is 92% of the allowable stress. The program also computes the bar spacing required for the reinforcing bars selected.

Procedure

For the reinforcement design, the program computes the required spacing for the specified bar sizes at top and/or bottom.

The program uses the service loads to compute the soil bearing pressures. However, the reinforcement design, as well as the shear check, are performed in accordance to the Ultimate Strength Design Method.