10 Key Elements of a Concrete Foundation

In the previous post (see 7 steps for pouring a Concrete Foundation)I described the basic steps for building, or as they say in the industry, pouring the foundation. In this post I describe a few of the elements important in building a poured concrete foundation.

1) Excavation
The excavation is the hole that in which the foundation will be built and then filled in. It is very important for two reasons: Safety during the building of the foundation and safety of the building. During the building of the foundation the workers will be moving in and about the excavation so appropriate sloping of the excavation walls is needed to ensure the safety of the workers.

The safety of the building, and ultimately the inhabitants is secured by ensuring that the foundation is placed on ground that can support the weight. Most of the time this is accomplished by placing the footings on undisturbed earth.

2) Footings
The footings receive the load of the house through the load-bearing walls and posts. The type and size of footing is based on a number of factors including soil conditions, size of the house and slope of the site. there are three kinds of footings wall, column and stepped. This cottage foundation has all three. It is obvious that wall footings support walls. Column footings support posts which in turn support beams that support floors for instance. On steeply sloped sites, like at this site, steeped footings are used to keep the footing below the frost line.

3) The Frostline
All building codes require that the footing be placed on undisturbed soil that will be always below the frostline. Frost is nasty thing. Water in the soil if it was below the footing and froze it would expand, thus heaving and possibly cracking and otherwise destroying the foundation.

4) The Key
The key is a slot in the top of the footing to ensure a structural join with the concrete wall above it. The footings are poured and cured - usually a 2 day process before the foundation walls are poured.

5) Rebar
Other material are used to strengthen both concrete footings and walls. Long metal rods call reinforcing bars or rebar for short, usually made of cast iron and is given ridges for better frictional adhesion to the concrete, are placed in the concrete as it is poured or just after it is poured to add strength. Concrete is a material that is very strong in compression, but virtually without strength in tension. To compensate for this imbalance in concrete's behavior, rebar is cast into it to carry the tensile loads.

6) Poured Concrete Wall
This is the most visible part of the foundation. It takes the load of the house and provides the walls for most basements. Like the stepped footings, concrete walls often are stepped, roughly following the shape of the site. The missing parts of the walls are later built using lumber.

7) Window insert
How do you put a window in concrete? This is done by inserting the frame of the window with a special supporting structure temporarily replacing the glass part of the window. So the once the wall concrete forms fo the walls are taken off the window is stuck in place.

8) Anchor Bolts
Everywhere the concrete foundation meets a wood structure above it anchor bolts are embedded in the concrete so that the wooden walls can be bolted to the foundation securely.

9) Knee-wall
The part of a foundation wall, really just a thinner part of the footing that is completing buried in the soil, is often referred to a knee-wall. The knee-wall depth is determined by the depth of the frost wall. Commonly in this area knee-walls are 5 feet deep. The part of the foundation wall that seems to missing in the picture actual has a a knee-wall buried, because of the slope of the site. Remember this is the Cottage on the Edge!

10) Floor Ledge
Eventually the basement floor will be poured in concrete. that will happen after the cottage is closed in a bit more to ensure rain doesn't damage the finish of the concrete floor. The concrete will be at least 4 inches thick and will be poured above and on the portion of the footing that sticks out under the wall. But with the knee-wall the footings are 5 feet under the ground -- of a notch is created in the top of the knee-wall to act as a ledge for the concrete floor.

NEXT: finally I get to use my hammer...