A concrete footing is a poured foundation that supports the weight of a building or other structure. It transfers the weight from below ground level to solid ground, typically by means of footings. The depth and thickness of these foundations are dependent on geotechnical conditions as well as soil type and bearing capacity. A proper footing provides stability for a building or structure, while also keeping it safe from potential hazards such as earthquakes.
Concrete footings can take many forms depending on what they will be supporting: walls, columns, slabs-on-grade, etc., but in all cases they support the vertical load placed upon them with respect to gravity). Concrete footings are also considered foundation walls because they are load bearing.
As an example, let’s say there is a 4-inch thick layer of sand on top of undisturbed soil. A column is placed on this sand and its weight overcomes the force of gravity acting upon it. The sand below the column begins to compress until it reaches its maximum capacity – at which point all the weight of the column (which happens to be 100 lbs) will be supported by it. The foundation wall system will be made up of the 4-inch sand layer plus another 6 inches of undisturbed soil.
How Do Concrete Footings Work?
Concrete footings work because concrete is very strong in both shear and compression. This means that it can support great loads if layered onto thick enough supports, which are usually solid rock or stone. Thanks to this ability, concrete footings play a role not only in home building, but also many other uses where large vertical loads need to be supported, such as highway overpasses, dams, or even oil rigs.
This strength is also why concrete footings often fail catastrophically- they are unable to flex and absorb energy before suddenly breaking apart under stress. A great example of this is the famous video of the Washington state highway bridge collapse in 2007. Concrete footings simply cannot absorb and dissipate energy; we will always be left with a sudden collapse if the support becomes too weak or breaks.
Concrete Footings Can Fail
A common saying among masons is that “concrete footings fail more often than they succeed.” This is because of the problems inherent with concrete, namely its lack of flexibility and brittleness under stress. This makes it difficult to construct a solid and durable foundation using standard practices alone.
A concrete footing’s ability to hold weight is also dependent on the soil underneath. It can only support as much weight as its foundation below will allow, so it might crack or even sink if the underlying soil isn’t strong enough to withstand the load.
This happens most often when constructing a footing over loose or saturated soil, or in areas where the ground is naturally weak. Coarse soils are also problematic because they have trouble retaining water in their soil grout, which can cause it to shift or wash out when the footing is poured.
The Importance of Formwork
One of the main reasons for this is formwork. As any home builder knows, foundations are often poured in sections, and typically very quickly so as to not allow for much time between batches. This means that the first section of footing may be strong enough to hold weight, but as more is added adjacent sections will become increasingly unstable. The foundation wall may begin to bulge and sag, and the integrity of the footing below will be compromised. If this goes on long enough, even a small amount of pressure could cause it to snap apart- resulting in a sudden collapse.
This method of concrete pours is wasteful and unsafe because formwork must be removed from over poured areas before they dry. It also allows the wet concrete to dry in layers, which creates uneven and unstable supports that never cure together properly.
This process can be mitigated by using a reusable plastic form called a “tilt up,” which is left in place and used like a mold to create many sections of footing at once. The tilt up allows for each section to be poured and cured together, so that they can never sag or move independently from one another.
Do I Have a Concrete Footing in My Home?
If you have a footing in your backyard, there’s a good chance that it rests on top of very hard and solid rock or stone. The kind of soil we generally consider to be “good,” with high clay content and enough gravel to resist compaction, is often too soft for footings because it compresses easily and has low bearing strength. This means it can support weight, but not very much of it, so footings often rest on top of solid rock instead.
Slab on grade foundations are often the same. If your foundation is on sand, it’s more likely that you’re actually standing on top of sand (think about how sand castles work), not dirt or rock. Soil bearing capacity varies significantly, but in general it’s far lower than that of rock or stone.
Finding this out can be important to home buyers because concrete footings are very expensive to repair or replace if they become damaged or unsafe after the sale. For example, according to an article by The New York Times, the average cost to repair a footing in New York City is $8,400. Needless to say, it’s probably not an expense any homeowner wants to face.
Soil bearing capacity
A concrete footing is far better able to handle changes in soil moisture, shear stress and tension than standard footings that rely on the inherent strength of the ground. Cracks are sealed from underneath with a thin layer of gravel or stone, making it much more resistant to erosion from water runoff.
The traditional foundation method involves digging a hole in the ground and filling it with gravel, sand or stone. This is known as “grading,” and it’s often done by layering materials of different sizes to ensure that they compress evenly while still allowing for drainage.
The effect this has on bearing capacity is that it changes how much weight the soil below is able to support. Soil that is loose and easy to compress, like sand or gravel, can hold more weight than soil that is dense, heavy and tightly packed by years of erosion.
Concrete footings are built with a combination of crushed stone, cement, water and steel reinforcing rods known as “rebar.” The rebar helps the footing support more weight by distributing load across the entire footprint.
Are Concrete Footings Safe?
Concrete footings are generally safe if they’re built properly and installed on a stable foundation that can support their weight. However, footings must be properly reinforced and poured with the right mix of concrete and other additives in order to achieve a level of safety. This is because:
- Concrete footings will not flex or bend if there isn’t enough soil beneath them to allow for movement without cracking; they fail just like any structure that can’t absorb and dissipate force.
- Concrete footings can’t support a lot of weight without extra reinforcement, and they need to be properly tied into the supporting beams beneath them in order to stay together if a pressure is applied from a certain direction.
- Above-ground footings are also subject to erosion by wind or water, which can cause damage over time that may go unnoticed until it becomes critical.
What to Look For When Buying a Home
If you’re buying a home with an above-ground footing, there are some questions you can ask the seller to find out what kind of condition it’s in. This allows you to do your due diligence and protect yourself from spending more than necessary on a remodel project, and also find out if the home is on a solid foundation. For example:
- How old is the footing? Footings can last for decades if they’re properly made and maintained, but they may need to be replaced or repaired after twenty years or more.
- Is there evidence of water damage? This can be a sign that the footing is damaged, either due to erosion or improper installation.
- Do you know who poured the footing? The best contractors are properly certified and insured, so ask for documentation that shows that they’re qualified to do this kind of work.
What to take into account when choosing the right footing for your building or structure .
You need to think about these important factors before choosing the perfect footing for your building. Choosing the right footing means you’ll have proper support!
- What are you using it for? If you are using your footing to hold up a wall or structure, then an insitu footing is perfect. If not, look at other types of footings .
- Is the soil suitable? Before choosing the right footing, you need to think about your soil and its bearing capacity. If it is not suitable, you will need a concrete footing mat .
- Do I want an insitu concrete footing? This type of footing is usually used when building a new structure or when replacing old concrete footings .
- Is there enough room for a footing? You need to have a clear idea of the site where you want your foundation before considering this.
- Does it need to be frost-proof? If your structure is going to be exposed to below zero temperatures, then a deep ground socket may be more appropriate.
- How much bearing capacity does my soil have? The bearing capacity of your soil is the maximum load it can safely support. The value depends on many factors like soil density and water content so make sure you check with a structural engineer and a geotechnical engineer before considering footings. For example, soft clay soil may only be able to support half the load of coarser soil so you may need an engineer to help you choose the best footing for your building.
- What is the correct concrete strength? If your concrete is too weak, it won’t hold up under pressure so make sure you check with a structural engineer before choosing .
Now that you know some of the basics of what to look for in a footing, it’s time for you to take action. There are some great resources available online that will help you find the best way forward with your project.
Just as crucial as choosing the correct footing is making sure that it’s installed right. When you have skilled labor working on your home, they should be able to provide all of this information and more so look for a family-owned company that cares about their craft. A good contractor will be able to find out what kind of a structure you have and give you the best solution for it.
Your best bet is to talk to an expert who knows exactly what to look for. Call us today to get a free consultation on your next project. We are available any time to hear your ideas. We also have a number of cost-effective options that can help you get the most out of your budget while still providing the support you need.