By: Javier Encinas, PE | April 15, 2020
ASDIP FOUNDATION is a structural engineering software for footing design, such as spread, strap, combined, and wall footings, as well as pile caps. Usually footings support either concrete of steel columns, and sometimes a steel column is on top of a pedestal. But how does this affect the design of the footing? This article discusses the different scenarios of a typical footing subject to a combination of vertical and horizontal loads, and how to model steel columns in ASDIP FOUNDATION.
Where are the column reactions applied?
The first image below shows the case of a spread footing supporting a concrete column, as normally occurs in concrete buildings. In this case the loads are applied at the top of the footing. Dowels should be provided at the interface for load transfer.
The second image below shows the case of a spread footing supporting a steel column, as usually occurs in commercial steel buildings. In this case the loads are applied at the top of the footing, and the base plate and anchor rods transfer the loads directly to the support.
The third image below shows the case of a spread footing supporting a steel column on a concrete pedestal, as commonly occurs in industrial structures. In this case the loads are applied at the top of the pedestal, which in turn transfers the load to the footing. Note that any horizontal reaction in the column will produce an additional overturning moment that must be considered in the footing design.
Footing and concrete column
Footing and steel column
Footing and pedestal
What are the effective column dimensions?
The effective column dimension will determine the location of the critical section to calculate the shear and bending in the footing. This is particularly important for the punching shear calculations.
For a concrete column or a concrete pedestal, the effective column dimensions match exactly the actual column dimensions. For round columns or pedestals, the effective equivalent square column should have the same section area.
For steel columns supported directly on the footing, the effective column dimensions are defined as the average dimensions of the steel column and the base plate. For example, if the steel column is 6"x 6" and the base plate is 12"x 12", then the effective column is 9" x 9". The images below show graphically the effective column concept for concrete and steel columns.
How do you model steel columns in ASDIP FOUNDATION?
In previous releases the user had to specify the effective column dimensions in the software. This created some confusion and comments from the plan checkers, particularly because the program showed an equivalent concrete column, even if it was actually a steel column resting directly on the footing.
This feature is now out of the box, as it was implemented recently in ASDIP FOUNDATION. Now it's possible to model either a concrete column or a steel column, and the steel column may be supported on a pedestal or directly on the footing. The software does the required adjustments in the calculations as explained above, including an updated set of images in the reports showing the steel column and base plate if necessary.
The screen shot below shows the graphical user interface corresponding to this new feature in the Spread Footing module. Similar graphical controls have been added to the other modules as well.
ASDIP FOUNDATION now includes a feature to model either concrete or steel columns supported on footings. Steel columns may be either on top of pedestals or directly on the footings. The point of application of the column reactions, as well as the effective column dimensions are considered in the software accordingly.
For software usage, please read the blog post How to Design Spread Footings Using ASDIP FOUNDATION. For a footing design example, please see the blog post Spread Footing Real-Life Example Using ASDIP FOUNDATION.
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.
Javier Encinas, PE
ASDIP Structural Software