Deep Dive: Structure of a Deck - Foundation
The Foundation of a structure is the part of it which supports the rest of the structure and anchors it to the earth. With decks, we usually do this through concrete piers dug into and poured in the earth.
Any time a structure is placed on the earth, it imposes a downward force due to gravity, compressing the soil. Most of us think of the ground as pretty solid, but with large compressive forces like a deck or house, long time spans (years and decades), and with weathering and changes in moisture content, freezing, etc, soil can become a lot less predictable.
Soil type plays a role in how soil expands and contracts with moisture changes, how it reacts to freezing, etc, but all that is a subject for another blog post.
The main 2 factors we’re concerned with when building a deck are:
1) Distributing the weight of the deck and potential loads (people, furniture, snow, etc) over a large enough surface area of soil given the strength, or Bearing Capacity, of the soil.
Bearing Capacity of Soil - the maximum average contact pressure between the foundation and the soil which should not produce shear failure in the soil. In simpler terms: how much weight the soil can hold over a given surface area without shifting significantly.
2) Preventing Frost Heave - Placing foundation supports deep enough that they won’t be affected by freeze/thaw cycles, i.e. below the Frost Line.
Distributing the Weight of the Deck comes down to taking the square footage of the deck, multiplying it by the load requirements (60 lbs/sq ft, or psf, for decks in our area) and then dividing that load into the tributary loads (explained below) carried by each deck footing (usually a cylindrical or rectangular block of concrete embedded deep in the soil; used to anchor a deck to and distribute loads into the earth).
Footing - for decks, these are commonly pier footings (we like to use bell pier footings) usually a cylindrical or rectangular block of concrete embedded deep in the soil used to anchor a deck and spread loads into the earth. In houses, this often takes the form of a strip footing, a continuous rectangular footing that spreads the load of a structure over a large area of soil.
Tributary Load - the area load of the structure that is supported by a particular structural member, such as a beam, post, or footing. Calculating the exact load per footing can get complicated, but here is a photo that shows the general idea.