The shape or footprint of a slab-on-ground foundation has a potentially serious effect on performance. Shapes that have geometric discontinuities, changes in the geometry of the slab, are subject to very high stresses and accompanying high strains. Simple shapes like rectangles are easier to design and more likely to perform better over time.
The Best Practical Shapes
The best shape is a circle or an oval. There are no sharp discontinuities. Rectangles have no real discontinuities. They are easy to model for design purposes. The downside is that rectangles are boring.
Some Examples Of More Complicated Shapes
Let’s look at several more complicated shapes starting with an “L” shape.
Notice the three red lines at the inside corner. These red lines are rebar that is used to control the cracking that is virtually guaranteed to develop over time.
The sketch below is a slab foundation in an “H” shape. Note that there are four inside corners and all have extra reinforcing at each inside corner.
The last example is called a “U” shape. Note that it could be thought of as a half “U”. Once again, notice the red lines indicating rebar at the inside corners.