By: Javier Encinas, PE | October 15, 2018

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

As an example, consider the spread footing shown below, which is part of a building expansion. The new footing is adjacent to an existing footing, and the dimensions are also affected by the proximity of the property limit. As a result, the column will be eccentric in both directions. Design the footing size, thickness and reinforcement.

Design Steps

1.- Enter the geometric dimensions of the footings. In this case, the column is 1'-6" from the border in X, and 2'-0" in Z, therefore the footing should be narrower in X in order to minimize the eccentricity.. Let's try a 5'-0" x 6'-0" footing in X and Z respectively, so that the eccentricity in both directions is 12". The At-a-Glance tab shows a summary of the design for a quick overview of your work as you go.

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2.- Enter the effective column dimensions and eccentricities. The steel column is W12 and the base plate is 20"x 20", therefore the effective column size is 16" x 16" for punching shear calculations.. Since the trial footing dimensions are known from the previous step, we can calculate the eccentricities as 12" in both directions. 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 footing. In this example the analysis of the structure provided the reactions per load case, as shown in the statement above. The base plate bears directly on the footing, and the column has 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.

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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 a small portion of the footing is not in contact with the soil. However, since the overturning is not a problem, this is acceptable.

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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 bars at bottom only. Some engineers might prefer to specify top and bottom rebars to avoid shrinkage cracks at the top on thick footings. Note that the X-bars need to be hooked in order to comply with the development length requirements.

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Takeaway

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

For engineering background, please read my blog post "Spread Footings Under Biaxial Bending: A Complex Design Subject".