ASDIP Foundation – Combined Footings Design

ASDIP Foundation
About ASDIP Foundation:   Overview | Benefits | Features | Documentation
Modules:   Spread Footings Design | Strap Footings Design | Combined Footings Design


A combined 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 supporting the two columns on a common footing.

The use of a combined footing may be justifiable under conditions where the distance between columns is short and the stability of an exterior isolated footing would be compromised. It is common practice to size the combined footing so that the resulting soil bearing pressure is uniform. To accomplish this, the footing shape is sometimes trapezoidal or rectangular, depending on the loads. A combined footing is usually analyzed as a beam in the longitudinal direction and as a footing in the transverse direction.

This module computes the soil bearing pressures induced by a cantilever footing under the action of vertical loads and bending moments, per the latest ACI design criteria. It designs the reinforcing steel, 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 in the longitudinal direction. The concrete design is based on the Ultimate Strength Design Method of the ACI 318. Load combinations per the ASCE 7.

ASDIP Foundation Combined Footings Design Input DataInput Data

The required input data includes the footing and columns dimensions, the materials’ properties, the allowable soil bearing pressure, and the acting service and factored loads. In addition, the program accepts a number of load cases, such as dead, live, snow, wind and seismic, to be combined internally. Alternatively, you can input a set of pre-combined loads.


Consider a combined footing to support two columns 15′ apart. subjected to the following factored loads: Pext=30 k, Pint=200 k. Per the soils report, the gross allowable soil pressure is 3.0 ksf. Find out the footing dimensions and design the reinforcing steel.


ASDIP Foundation Combined Footings Design Example

ASDIP Foundation Combined Footings DiagramsOutput

The software checks the footing stability in overturning, sliding and uplift under the service combined loads, and performs the concrete design, which includes the one-way shear, punching shear and bending under the factored combined loads. In each case the controlling load combination is identified and reported.

For a quick overview of the design results click the “At a Glance” tab. A more detailed step-by-step calculations are available at the “Detailed” tab, which is updated with every new change. In addition, ASDIP Foundation uses a pre-formated colorful text-with-values output for easier identification of the problem areas. Use the Print Preview command to see a preview of the pre-formatted report on-screen, which includes the color graphics generated by the software.

ASDIP Foundation generates a graphical view of the designed footing and the resulting pressures and forces, as shown. The program also generates the moment and the shear diagrams for the controlling combination, as well as a view of the construction section and elevation with the reinforcement information.

ASDIP Foundation Combined Footings Design ReinforcementASDIP Foundation Combined Footings Bearing










For a more detailed information on the design of combined footings please refer to our blog post Combined Footings: An Important Design Element.

Explore ASDIP Foundation

About ASDIP Foundation:   Overview | Benefits | Features | Documentation
Modules:   Spread Footings Design | Strap Footings Design | Combined Footings Design