Footing in Construction: What It Is and Why it's Important
No matter how well-constructed a building is, it doesn't stand a chance without a solid foundation. That's why footings and their different varieties are crucial to the construction process.
Footer construction is a main contributor to effective, long-lasting foundations that all structures depend on. As they are often the very first support constructed on-site, it's important to have a comprehensive understanding of footings.
What is footing in construction?
Footings are structural supports that provide additional stability to a structure's foundation. Found underneath foundational walls or columns, footings are typically made with reinforced concrete and help redistribute a structure's weight from the foundation into the earth and soil below.
Essentially, footings create an additional load-bearing point between the structure and the ground. This is especially helpful when building with shallower foundations. As a result, they help transfer weight into the ground so that the structure doesn't bear as much weight.
Footings vs. Foundations
Despite their similarities, footing and foundations aren't the same. The main difference between the two is how necessary they are to a structure's stability. All major structures are built on top of foundations, but not all of those foundations are supported with footings. However, most building codes mandate footer construction for the majority of structures to ensure foundation stability.
In addition to providing stability, footers prevent differential settlement. Settlement occurs when the soil beneath a structure's foundation shifts, causing the structure to sink, lean, or crack. Water damage, soil disruption, poor weight distribution, and other factors can also cause differential settlements.
Slight shifts are expected in any building, but the significant ones that occur with differential settlement can crack the foundation and lead to building collapse. Footings improve weight distribution by reducing strain on the foundation, which prevents extensive settlement.
Factors to Consider in Footing Construction
The International Building Code (IBC) and the National Building Code of Canada have established guidelines for footer construction. However, structural engineers often offer additional advice to contractors during any type of footer construction. They're especially helpful when designing foundation footers for unique structures and job site environments. In addition to these guidelines, here are a few general factors to consider during construction.
The frost line is the lowest depth at which water in the soil freezes. Footings must be built below the frost line to prevent frost heaving which can cause an upward shift of your structure. This happens because when water freezes into ice, it causes expansion. If ice forms in the ground below your footings, the soil expands, applying upward pressure to your foundation. Once that happens, say goodbye to structural integrity!
Water in the soil located below the frost line won't freeze, so frost heaving won't affect footers built at any depth lower than the boundary. Be sure to check the frost line in your area since it varies in every geographical region. The required distance below the frost line, job site conditions, and the load itself all affect footing depth.
Soil Bearing Capacity
Soil bearing capacity refers to the maximum pressure each class of soil can support, generally measured in lbs per square foot. This specification is the main factor that affects footer construction and required width. In areas where soils have very high bearing capacity, footings aren't always required, as the foundation alone will support the structure.
Silt-like soils tend to have lower bearing capacity, while gravelly soils can withstand more pressure. Soil density tests and penetrometers are a few tools that can assess soil-bearing capacity before construction.
The groundwater table, also known as the water table, is the depth at which underground soil becomes saturated with water. The groundwater table acts as a border between dry and wet earth. Unsaturated soil is found above the water table and saturated soil containing water pockets lies below it.
Water and concrete don't mix well. Concrete footing absorbs excess water from surrounding soil, causing cracks, an upward shift of the structure resting on top of the footing, and reduced load-bearing capacity.
Generally, footing construction should take place above the water table. However, in areas with a high water table, it's difficult to do so. Helical steel piles are corkscrew-shaped footers designed to screw into the ground and work well in areas with high water tables.
Types of Footings
The type of foundation used to support a structure affects what footing you choose. Shallow foundations are built at shallower depths since the soil at that depth can support the structure's load. The following footing types are ones you will find used to support shallow foundations:
Isolated or individual footings have only one individual column or post on top of them. They're used in soils with high bearing capacity and support lighter loads. In this practice, each column has its own isolated footer. They're designed to support structures where columns are set apart from one another, like decks and fences.
There are three types of isolated footings:
- Simple isolated: Rectangular and square-based supports that form an upside-down "T" shape.
- Trapezoidal isolated: Trapezoidal-based supports, which use less concrete.
- Stepped isolated: Stepped-based supports, which are less popular today. Smaller slabs of concrete are stacked on top of larger ones to form a staircase shape.
This is when two or more columns rest on a combined footer. They support a "combined" load, hence the name. Combined footing is used when columns are close together and there isn't enough space to place two individual footers. Typically, this design supports loads in soils with lower bearing capacity since their bases are wider than isolated footers and can transfer more weight into the ground.
There are three types of combined footing:
- Rectangular combined: This design has a rectangular base. They support columns that bear equal weight.
- Trapezoidal combined: This design has a trapezoidal base. They support columns that bear unequal weights. The column bearing more weight rests on the wider portion of the trapezoid's base.
- Strap beam combined: Strap footings have two square bases with one column per base, connected by a concrete strap beam. They're used on property boundaries where the load on one column at the property's edge is much heavier than the inner column's load. Instead of distributing the load's weight into the ground, the strap beam transfers some of the weight from the heavier column to the lighter one.
Strip footings, also called continuous or wall footings, support load-bearing walls. The wall is built on top of the concrete strip, which is generally double the width of the load-bearing wall. They also provide extra support to areas where the load exerts additional pressure.
Raft and Mat Footings
Made from large concrete slabs, raft or mat footings support heavy loads and multiple columns per slab. They work best for loads that exert equal pressure, as the entire slab would transfer this pressure into the ground. This style of foundation is commonly used in soils with lower bearing capacity.
Deep foundations may also require extra support. That's why foundation piles are often used in deep foundations.
Piles are long columns that transfer the structure's load into deeper soil layers or hard earth. They're often made of reinforced concrete, wood, steel, or composite materials. They work best in areas with high water tables and low soil-bearing capacity. Piles can be pre-cast and transported to a job site or cast directly in the ground, a process known as "cast-in-situ" or "cast-in-place."
In addition to being categorized by material and installation, there are two structural classifications for piles:
- End-Bearing Piles: End-Bearing Piles support structures by transferring the load's weight into hard rock or strata beneath the base of the pile, similar to other footers.
- Friction Piles: This is when friction between the soil and the pile transfers the structure's weight into the surrounding soil instead of the soil layer at the pile's base.
Footer Construction in a Nutshell
For seemingly small structural support, footings play a major role in structural integrity. As footings help the foundation transfer weight to the soil that's capable of supporting the load, they are crucial to the longevity of a building.
In the construction process, the type of structure and job site's foundation will have a direct influence on the choice between piles, individual, combined, raft, or strip footing. When designing and building footers, you must consider building codes in your jurisdiction, frost line depth, soil bearing capacity, and the area's water table.
For your next footing construction project, be sure to visit the DOZR Marketplace to search your area for any earthmoving, excavating, or soil compaction equipment you might need!
FAQs about Footings
What's the purpose of footing?
This approach adds stability to a foundation by helping transfer the load's weight into the soil and preventing extensive settlement.
What's the difference between footing and foundations?
Footings are structural supports that are used in addition to a foundation. They help transfer the load's weight from the foundation into the soil. Footers are typically in direct contact with the ground, while the foundation is in contact with the footing.
What are the main types of footings?
The main types include isolated, combined, strip, continuous, and mat footers. Piles are used with deep foundations.
What size do footings need to be?
Building codes provide guidelines on footing sizes, which are generally organized by the structure's height in stories and residential or commercial building classification. Footing depth varies based on the frost line and soil bearing capacity while the width varies based on the structure's weight. The minimum footer thickness is 8 inches.
What equipment do I need to make a footing?
Skid steers, compact track loaders, mini-excavators, excavators, soil compactors, and backhoes are all construction equipment that will help you and your team construct great footings. Rent construction equipment from DOZR today.