May 9, 2024

combined-footing-example

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 real-life combined footing design example using ASDIP FOUNDATION software.

Combined Footing Example

As an example, consider the combined footing shown below, which is part of the foundation of a pipe rack in an industrial plant. A typical bent consists of two steel columns 10'-0" apart that share the same footing. The pipe rack is exposed to dead, live, and biaxial wind loads, whose reactions are shown below. Design the footing size, thickness and reinforcement per the ACI 318-19.

combined-footing-example

Since the column spacing is 10 ft, two spread footings would be very close to each other. An economical solution is using a combined footing to support the loads of both columns on a single shared footing.

Combined Footing Example Input

Go to the Geometry tab and enter the geometric dimensions of the footing. In this case, the distance between columns is 10'-0". Let's try an edge distance of 1'-6", so the total footing length is 13'-0". Since the wind load could be reversed, the footing should be rectangular, not trapezoidal. Let's try a 5'-0" wide footing.

Enter the pedestal dimensions and height. Note that the loads are applied at the top of pedestal, therefore the shear load will produce an overturning moment on the footing. This is the typical condition of footings in industrial plants, where the pedestal protects the steel column from corrosion.

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 3.5 ksf.

Go to the Loads tab and 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. Note that there are horizontal out-of-plane wind reactions. Finally, go to the Reinforcement tab and enter the rebars information for the footing and pedestals.

combined-footing-example-input

Combined Footing Example Output

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. 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 calculations with exposed formulas and with references to the ACI code. An excellent tool for granular checking.

In this example, out-of-plane wind moments are being applied, which will affect the bearing pressures under the footings. The maximum pressure is 3.3 ksf, which is smaller than the allowable pressure of 3.5 ksf. Note that the service loads have been used in the bearing analysis, and the program shows the controlling service load combination.

The footing thickness will be controlled by shear. In this case 18" thick provides a shear ratio of 0.41, 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.

The Diagrams tab shows the shear and moment diagrams of the combined footing for the controlling factored load combinations. Note that the graphical results can be sorted by load combination.

ASDIP FOUNDATION generates 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.

combined-footing-example-output

Takeaway

ASDIP FOUNDATION includes the design of combined footings, with multiple options to optimize the design fast and easily. The design of combined footings may be cumbersome and error-prone. This real-life example shows that the design can be completed and optimized in minutes using the software.

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