Cantilever Footing Design

A cantilever footing is one that usually supports two columns. If a property line exists at or near the edge of an exterior column, an isolated footing would be placed eccentrically under this column and it would tend to tilt.

Overturning of the exterior footing is prevented by connecting it with the adjacent interior footing by a strap beam. Since this beam is subjected to a constant shear and a linearly varying moment, which are the characteristics of a cantilever beam, this system is called strap or cantilever footing.

The use of a strap footing may be justifiable under conditions where the distance between columns is large and a large excavation area must be avoided. It is common practice that the bottom surfaces of the exterior footing, the strap beam, and the interior footing be at the same elevation, but the total thickness of each element may be different, depending on the strength requirements.

The program computes the soil bearing pressures induced by a cantilever footing under the action of vertical loads and bending moments. It designs the reinforcing steel for the interior and exterior footings, and checks the one-way and two-way shear stresses. In addition, the program generates the shear force and bending moment diagrams in order to design the reinforcement for the strap beam.

Input Data

The input data required by the program consists of the interior and exterior footing dimensions, as well as the distances between columns and the property line location, as shown in the figure below.

ASDIP allows designing the foundation for any combination of vertical loads and bending moments. Service loads are used to compute the soil bearing pressures; however, the reinforcement design, as well as the shear check, are performed according to the Ultimate Strength Design Method.

Example

As an example, consider the strap footing shown below, where the property line is at the exterior edge of the exterior column. Equal settlement is assumed for the DL+1/2LL condition at a pressure of 3.3 ksf. Use f'c=3 ksi and fy=40 ksi.

Output

Once the input data is entered, the program computes the soil pressures under each footing and compares them with the allowable value. The shear stresses are checked for the loads and dimensions specified and the reinforcing steel is designed for bending.

Figure at left shows the template of this program with the example data and results.