By: Javier Encinas, PE | June 11, 2019

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

As an example, consider the strap footing shown below, which is part of a steel retail building, with columns spaced in a 20 ft x 20 ft grid. One exterior column is located very close to the property limit. As a result, the column will be eccentric on the footing. Design the strap footing to support the applied loads satisfactorily.

strap-footing-design-example

The exterior eccentric column would cause an isolated footing to tilt. This can be prevented with the use of a combined footing with the interior column. However since the column spacing is 20 ft, the cost of the large excavation would be excessive. The solution in this case is a strap footing, which consists of two spread footings tied together by a strap beam. In such a system, the loads are supported by the footings and the bearing capacity of the beam is neglected. The beam produces a rigid system and resists the resulting shears and moments, preventing the tilting of the exterior footing.

Design Steps

1.- Enter the geometric dimensions of the footings. In this case, the exterior footing should be wide and narrow in order to minimize the eccentricity. Let's try a 4'-6" x 7'-6" eccentric footing, making sure it doesn't pass the property limit. The interior footing can be concentric. Assign a preliminary size to the strap beam. The At-a-Glance tab shows a summary of the design for a quick overview of your work as you go. Note that all checks pass.

strap-footing-design-example

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2.- Enter the loads given in the statement of the problem. Select the load cases option, since the dead and live loads are known. The Condensed tab shows a more complete set of results grouped by topic, great for a more detailed overview of your design.

strap-footing-design-example

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3.- Enter the material properties, such as the concrete and steel strength for the different elements. The soils report has provided the allowable bearing pressure as 3.5 ksf. The Detailed tab shows a complete set of calculations with exposed formulas and with references to the ACI code.

strap-footing-design-example

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4.- Enter the reinforcement information for the footings. Specify the concrete cover and the bar number and size. Verify the bearing results graphically. In this case the maximum bearing is below the allowable value. Note that the bearing pressures are uniform, since the reactions are applied at the center of the footings.

strap-footing-design-example

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5.-  Enter the rebars of the strap beam, as well as the stirrups. ASDIP FOUNDATION generates the shear and moment diagrams of the beam, as well as a construction sketch of the strap footing.

strap-footing-design-example

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Takeaway

ASDIP FOUNDATION includes the design of strap footings, with multiple options to optimize the design easily. The calculation of the moments and shears in the strap beam 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 the blog posts "Strap Footings: A Very Useful Structural System", and "Strap Footings: How to Calculate the Bearing Pressures".

Detailed information is available about this structural engineering software by visiting ASDIP FOUNDATION. You are invited to download the Free 15-day Software Trial, or go ahead and Place your Order.

Best regards,

Javier Encinas, PE

ASDIP Structural Software