During the construction process, many steps are taken that are crucial to the structure’s integrity but don’t occupy center stage. If you weren’t involved in the building construction, you may never know they are there.
Footings are a great example of this. Hidden below eye level, these are present in both residential and commercial buildings.
But what role do they play? Why are they so important? What are they made of? Are there different types? How can you ensure that you choose the right footing for your project?
These questions are answered in this in-depth guide to footings.
Let’s explain this in two parts: what it does and what it looks like.
A footing supports the foundation and distributes the load to the soil. In other words, it serves as an attachment from the foundation to the ground.
This weight distribution prevents differential settlement, increasing the structure’s stability.
Footings are one of the main components that extend a structure’s longevity.
Footings are made of concrete and are at the very base of the foundation. They are wider than the foundation wall and help to support it.
12 inches below the frost line, footings are made by pouring concrete with rebar into a dug trench or hole.
Footings can come in many different shapes and forms. Just as there are various types of foundations, there are also multiple types of footings.
The footing design will meet the needs of specific features of each project, including the soil, load, and lay of the land.
When one footing supports one column, this is considered an individual footing, also known as:
Individual footings are used to support a shallow foundation. They should only be paired with light loads on soil with a high bearing capacity.
There are three types of individual footings to choose from:
The most common type of individual footing is called a simple footing. They are either square, rectangular, or circular in shape.
Having been gradually phased out over the years, stepped individual footings are constructed by stacking squares in descending size in a pyramid fashion.
An individual footing with slopes on a 45-degree angle on all sides is known as sloped or trapezoidal footing. Their significant advantage is the use of less concrete than the alternatives.
A footing supporting two or more columns is called a combined footing. Their bases are wider than individual footings, making them suitable for soil with low bearing capacity.
Combined footings are also utilized when columns are close together, not leaving enough room for individual footings. They are also helpful when the foundation construction is close to sewer pipes or fence lines.
Like a sloped individual footing, trapezoidal combined footings support the columns with an angular base.
On the opposite end, rectangular combined footings come into play when both columns support equal loads. These footings are constructed with one rectangular concrete slab as the base.
When there are uneven loads because of the property line, strap beam combined footings are your best bet.
These foundation footings are also known as continuous or wall footings. Strip footings support long masonry walls, such as retaining walls. They’re also used when heavy loads are anticipated on the floor above. Strip footings are typically twice the size of the wall above.
Also known as cantilever or raft footings, mat combined footings are used for heavy loads on poor soil-bearing capacity. A mat footing supports all the columns with one single footing.
When you need extra support for deep foundations, piles are your answer. Piles are long columns that transfer the load into the deeper soil layers, and there are two types to choose from:
Although many people aren’t aware that a footing is separate from the concrete foundation, they are, in fact, two distinct parts of the structural support.
A footing is a foundational component, but not all foundations will have a footing. For example, lighthouses don’t need them unless they’re built on uneven or clay soil.
Slab-on-grade foundations will lay on the ground. For other foundation types that use footings, the bottom of the footings will be partially in the soil. The foundation distributes the load to the footing, which distributes the load to the earth.
Both play a role in supporting the structure and are made from concrete.
The relationship between a foundation and a footing can be likened to a leg and a foot. The leg is the strength of the lower body and bears most of the weight. However, without the foot, it would be very unsteady.
The structural engineer makes the footing calculations based on the following:
One of the biggest factors affecting the footing specifications of a project is the soil conditions.
First, you need to determine the soil type you are building upon. There are many different types of soil with various load-bearing capacities. However, we’ll consider only the four most common versions:
Softer soils like soft clay, soft silty sand, or soft silty or sandy clay will need a different approach, like wide strip footings or raft footings.
The soil’s water absorption rate and drainage properties are more reasons that soil type is so important when designing the proper footing. For footings to do their job, the soil must be compacted underneath them.
The footing depth must be built below the frost line. Otherwise, the water content below the footings can freeze and expand, causing shifting and damage to the foundation.
Since concrete absorbs water, placing the footing above this line is preferable. Water will cause cracks in the concrete or an upward shift in the soil, damaging the foundation.
The weight of the structure placed on the foundation (determined by the size of the building, the materials used, and any unique design) will be a significant factor in footing specifications.
Another consideration, especially in the placement of footings, is the location of load-bearing walls.
Colder climates are prone to frost heave due to the soil’s frost/thaw cycles. Areas prone to earthquakes or high winds will need extra reinforcement, deeper footings, and better spreading of the load.
Before footings can be constructed, you must figure out the best footing parameters, including:
The soil strength and frost line determine footing depth, while the thickness is between 8-12 inches.
After the dimensions have been established, you can begin to build the footings.
First, dig a trench deep and wide enough for the footing dimensions. You’ll also need to prepare the soil.
Next, you will install the formwork for the footings. Sometimes, they’ll need to be constructed from wooden boards. Other times, the soil itself is compact enough to act as a form.
If you’re using precast footings, you can skip this step. Otherwise, proceed with installing reinforcements.
Before the concrete is poured, add rebar according to the engineer’s instructions.
Ensure the footings don’t budge during pouring.
Precast concrete footings should be placed in the dug hole or trench, and the soil must be compacted around them. Wet concrete should be poured into forms to the correct height and then leveled.
After the concrete is poured, it will begin to set or cure.
This curing process takes about a month or more, but you can begin to build on the footings and foundation when the concrete reaches half its overall strength, which will take about two weeks.
Even though footings are meant to reduce issues with the foundation and integrity of the structure, they can cause more harm than good if they’re not correctly constructed.
Below are some common footing problems and how to ensure these are not repeated in your project.
A misplaced footing can pose a severe threat to the stability of the foundation. Luckily, there are means to rectify the situation.
In high-load-bearing soils, excavate under and around the footings and fill these areas with compacted gravel.
In weaker soils, you’ll need to excavate, drill holes into the side of the footing, place epoxy steel dowels into the hole, and then pour more concrete to extend the footing.
If there are soft spots in the soil beneath your footings, you have several options:
Jump footings are the fix to changes in elevation in the soil. Since the footings won’t be continuous, you must reinforce the foundation walls with rebar between jumps.
This only works for a maximum of a four-foot span between footings and a maximum of a five-foot change in elevation.
Dealing with a high water table makes installing footings difficult.
To mitigate this problem, place large rocks into the footing form before pouring the concrete. Use a pump to remove the water before filling the form. You may also need to widen and thicken your footing.
The purpose of a footing in construction is to stabilize, reinforce, and support the foundation. However, for a footing to fulfill its purpose, it must be properly constructed.
This requires accurate calculations and building plans — and a construction team that will follow those plans with exactness.