By: Javier Encinas, PE | October 1, 2019

ASDIP FOUNDATION is a structural engineering software for the design of concrete footings. It includes the design of combined footings, based on the ACI 318 provisions. This document is a step-by-step design example of an eccentric combined footing using ASDIP FOUNDATION software.

## Combined footing example

As an example, consider the combined footing shown below, which is part of the foundation of an industrial building. The left column is very close to the property line and it has a relatively light load compared to the adjacent interior column. Design the size, thickness and reinforcement of a shared combined footing for both columns.

## Design Steps

1.- Enter the geometric dimensions of the footing. In this case, there two possible approaches. One is considering a rectangular footing, and the other one is a trapezoidal footing. For illustration purposes let's try a trapezoidal footing 4'-0" wide at the exterior column and 9'-0" wide at the interior column. Enter an initial footing thickness of 18". The At-a-Glance tab shows a summary of the design for a quick overview of your work as you go. The green check marks indicate that all checks passed.

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2.- Enter the pedestal dimensions and height. In this case the pedestals are 18" x 18" x 2 ft high. Note that the loads are applied at the top of pedestal, which is important if some lateral loads are being considered. In this example we are considering only the gravity loads. The Condensed tab shows a more complete set of results grouped by topic, great for a more detailed overview of your design.

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3.- Enter the loads on the combined footing. In this example the analysis of the structure provided the reactions per load case, as shown in the statement above. The base plates bear on top of the pedestals, and the columns have been modeled as pinned, so no moments in the reactions. The Detailed tab shows a complete set of calculations with exposed formulas and with references to the ACI code. An excellent tool for granular checking.

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4.- Enter the material properties, such as the concrete and reinforcing steel strengths, the allowable soil bearing pressure, and the densities of concrete and soil cover. Verify the bearing results graphically. In this case the maximum bearing is below the allowable value. Note that the service loads have been used in the bearing analysis, and the program shows the controlling service load combination.

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5.- Enter the reinforcement information for the footing. Specify the concrete cover and the bar number and size. In this example we will use #6 longitudinal bars top and bottom. This will provide the bending capacity of the combined footing acting as a beam. In the transverse direction let's specify #6 bars for the footing acting as a cantilever. The Diagrams tab shows the shear and moment diagrams of the combined footing for the controlling factored load combinations.

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6.- Enter the reinforcement information for the pedestals. ASDIP FOUNDATION will generate the interaction diagrams of the pedestals, and will compare this capacity versus the applied loads. The Construction tab shows the design sketches with the footing dimensions and reinforcement, similar to the construction drawings.

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## Takeaway

ASDIP FOUNDATION includes the design of combined footings, with multiple options to optimize the design easily. The design of combined footings may be cumbersome and error-prone. This eccentric combined footing example shows that the design can be completed and optimized within minutes.

For engineering background, please read the blog post Combined Footing: An Important Structural Design Element.