April 25, 2024

strap-footing-design-example

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.

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Strap Footing Design Example

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. Both columns transfer out-of-plane moments to the footings. Design the strap footing to support the applied loads satisfactorily per the ACI 318-19.

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 in-plane shears and moments, preventing the tilting of the exterior footing.

Strap Footing Example Input

Go to the Geometry tab and 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.

Use the Materials tab to enter the material properties, such as the concrete and steel strength for the different elements. In this case f'c = 4 ksi and fy = 60 ksi. The soils report has provided the allowable gross bearing pressure as 3.6 ksf.

Go to the Loads tab and enter the loads given in the statement of the problem. Select the load cases option, since the dead, live, and wind loads are known. Finally, go to the Reinforcement tab and enter the rebars information for the footings and the strap beam.

strap-footing-example

Strap Footing Example Output

The At-a-Glance tab shows a summary of the results for a quick overview of your work as you go. Make sure that all checks pass. The Condensed tab shows a more complete set of results grouped by topic, great for a more detailed overview of your design. The Detailed tab shows a complete set of step-by-step calculations with exposed formulas and with references to the design code.

In this example, out-of-plane wind moments are being applied, which will affect the bearing pressures under the footings. The maximum pressure occurs under the interior footing and it is 3.4 ksf, which is smaller than the allowable pressure of 3.6 ksf. Note that the bearing condition is checked for the service load combinations.

The footing thickness will be controlled by shear. In this case 18" thick footings provide a shear ratio of 0.71, so it could be reduced further if desired. Rebars #5 or #6 are commonly used in footings, and the spacing can be selected to optimize the design.

ASDIP FOUNDATION generates the shear and moment diagrams of the strap beam, as shown below. With this information we select the top and bottom rebars, as well as the stirrups. Finally, the software generates construction sketches of the strap footings and beam, showing the rebars for the final design.

strap-footing-example

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 strap footing design example shows that the process can be completed and optimized in 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. For our collection of blog posts about foundation design please visit Structural Footing Design.

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

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