By: Javier Encinas, PE | June 27, 2017

ASDIP FOUNDATION includes the design of concrete combined footings. This structural engineering software is based on the latest ACI 318 provisions. This post explains the calculation of the soil bearing pressures in a typical combined footing.

A combined footing usually supports two columns. The use of a combined footing may be justifiable under conditions where the distance between columns is short, so a large excavation is not required. There are two common scenarios in practice:

  • A property line exists at or near the edge of an exterior column, so a normal isolated footing would tend to tilt. This problem may be prevented by supporting this column together with an interior column on a common footing.
  • Two columns are so close to each other that two spread footings would overlap. In this case it’s easier to design a common footing for both columns.
combined-footing-geometry

How do you calculate the soil bearing pressures?

When the loads of the two columns are about of the same order of magnitude, generally the combined footing is designed as rectangular, so that the bearing pressures are about uniform. However there are cases where one column load is much heavier than the other, so the footing should be designed as trapezoidal in order to keep the pressures uniform. Another example of a trapezoidal footing is when the available space is limited due to the presence of other existing footings in the area.

combined-footing-types

The calculation of the bearing pressures is relatively simple for rectangular footings in full bearing, but it becomes increasingly complex for trapezoidal footings. Furthermore, the calculation is very cumbersome when a trapezoidal footing is in partial bearing, since the location of the zero pressure line is unknown.

The maximum bearing stress depends on the shape of the bearing area, which in turn depends on the location of the centroid of the bearing volume, which in turn depends on the maximum bearing stress. So, how do you solve this puzzle? One option would be multiple trial and error cycles, until finally the applied load equals the bearing volume, and the location of both the load and the volume centroid coincide. This is time-consuming.

The closed solution lies in the integral calculus field. ASDIP FOUNDATION uses an algorithm consisting of double integrals of the type P = ∫∫z dy dx where z is the bearing stress. The centroid of the bearing volume is therefore located at X = ∫∫x z dy dx / P.  The resulting solution is applicable to any footing geometry, either in full or partial bearing, as shown in the picture below.

combined-footing-bearing-pressures

Takeaway

The calculation of the bearing pressures for a combined footing may become complex and time-consuming. ASDIP FOUNDATION accurately calculates the bearing pressures for any geometry and loads. This is useful when you have trapezoidal combined footings, either in partial or full bearing.

For a more detailed discussion of the design of combined footings, please read my blog post Combined Footing: An Important Structural Design Element.

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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