Spread footings and soil interaction

[StrEng007]

When designing isolated spread footings along a building line, when do you need to determine if the length of your footing requires consideration of the soil interaction?

For typical spread footings, the ACI design approach is to consider the bearing capacity of the soil and the upward force that induces tension and shear at the footing's critical sections (and of course FOS for bearing, OT, and all that good stuff). However, couldn't it be said that if a spread footing is long enough that soil interaction (soil spring constant) becomes an issue?

I have a building I've designed for several spread footings equally spaced along a single gridline. These footings support gravity and lateral overturn, where the lateral is all pushing the same direction. The design calls for footings to be long enough (parallel to gridline) to where they encroach on each other. At that point, I've now switched my analysis over to looking at a beam on elastic foundation. Interestingly, you get a similar volume of concrete either way.

 

[structSU10]

I would say it is when the footing is getting more flexible with respect to the soil support. At a certain point your footing is not thick enough to avoid it truly acting as a rigid body. It very much depends on the relative stiffness of soil vs footing.

 

[human909]

The quickest check would be a span on depth ratio. For an isolated rectangular footing a maximum width to depth ratio of 5 is a reasonable rule of thumb. So a 300mm deep footing can be up to 1500mm wide until you have to start looking at things a little more closely.

Of course like structSU10 says it certainly depends on the stiffness of the subgrade.

 

[milkshakelake]

If a spread footing is designed properly for one-way and two-way shear, it will generally be stiff enough to be considered rigid. When it starts becoming really long, the thickness starts increasing due to increased shear demand (and maybe flexure to some extent). I generally don't have thin, long footings because they will fail in shear.

The cutoff for me is when footings start taking up more than 50% of the building footprint, which is when it becomes a mat foundation and the soil interaction needs to be considered.