What is Footing? (In Australian Terms)
A footing, also known as building footings, is a vital component of any building, acting as an intermediary between loads of the house and the ground on which it stands. Without a well-designed and sturdy footing, walls can easily become misaligned or floors uneven due to soil movement, water damage, or even earthquakes.
The Importance of Footings for Building Stability and Load Distribution
Footings play an important role in distributing loads evenly across the ground, anchoring buildings against high winds, and protecting them from expansive soils and soil moisture. In some cases, footings may also house mechanical systems. By ensuring that footings are adequately designed and constructed, we can ensure that our buildings are safe and stable.
What is Foundation? (In Australian Terms)
Foundation is the soil upon which the entire structure rests and must be able to support the weight of the building without failing. Therefore, when constructing any building, it is vital first to assess the type of ground or soil which will act as the foundation. Footings must then be designed carefully, taking into account the specific site conditions and type of foundation.
How Different Foundation Soils Impact Stability and Drainage?
Different types of foundation soil offer different levels of stability and drainage. Generally, footings founded on bedrock will settle the least, while footings founded on sandy soil tend to settle quickly and evenly. Sandy soil also drains well since sand does not absorb water.
Clay soil is notoriously volatile, absorbing or losing water dramatically in response to changes in the weather. In wet seasons, clay soil expands by up to 50%, while during dry months, it shrinks by the same amount. This constant movement can cause problems for buildings on clay soil, as the footing moves unevenly, and cracks appear on the finished surfaces over time.
The Role of Geotechnical Investigation in Reliable Footing Design
A thorough geotechnical investigation will provide all the necessary information about the soil type, behaviour, and limitations. This allows the Structural Engineer to design an appropriate footing for any project. If the footing is designed and constructed properly, problems should not arise; however, if it is not constructed properly, it will never cease to be a problem.
What are Different Types of Footings Commonly Used in Residential Houses in Australia?
There are a variety of footings designed and constructed for residential houses in Australia. Some of the most common ones include:
Strip Footings and Stump Footing System
One of Australia’s most commonly used types of footing is the strip footings and stump footing system. Concrete stumps are often used in this system to comply with construction regulations and design requirements. This system consists of two main parts: a strip of linear rectangular concrete or bluestone, and isolated posts known as stumps supported on pad footings. The strip footing supports external walls, while the stumps support the internal floors.
This system is adaptable and can be built on flat and sloping sites. In addition, it creates a crawl space that can be used for storage. However, one downside of this system is that it is susceptible to differential footing movements, especially when built on a reactive foundation.
Reinforced Concrete Raft Slabs
A reinforced concrete raft slab is a strong and durable footing for any building. Raft slabs are a type of shallow footing commonly used in Australia. The integral concrete beams provide added strength and support and can be spaced and positioned according to the project’s needs. This type of footing is especially well-suited for construction on difficult or unstable ground. Differential movements of a raft slab can also be minimised with proper design and construction. Two different types of raft slabs widely used in Australia are:
Waffle Raft
Waffle slabs have become a popular choice for builders in recent years due to their faster, easier, and cheaper build process. Waffle slabs are typically 85 mm thick and supported by edge beams around the perimeter and internal beams (known as ribs) that run in both directions, creating a waffle-like shape when viewed from below. Waffle slabs sit directly on the ground, either on compacted fill or natural soil. Due to their shallow-founded beams and relatively low stiffness, they have become problematic on highly reactive soils.
Stiffened Raft
Stiffened Raft typically consists of a 100mm thick steel-reinforced slab with stiffened internal and external beams embedded into the Natural foundation soil for extra strength and support. The edges of the stiffened raft slabs range from 300mm to 1100mm thick, depending on the type of construction they support (e.g., clad frame, brick veneer, full masonry, etc.).
Suspended Slabs on Piers
Some sites can be very problematic due to the significant reactive nature of the foundation soil or the presence of deep layers of unstable Fill material that are unsuitable for use as a foundation. The choice of this footing system is heavily influenced by the soil conditions present at the site. In these cases, deep concrete piles are installed to support the concrete raft slabs and take the load down to the Natural soil or bedrock, beyond the depth of the problematic section of the soil. The piles are rarely structurally connected to the concrete slab above, and as such, this footing system is very susceptible to soil heave.
Slab on Ground
Slab on Ground is usually made from reinforced concrete and is approximately 80mm thick. The edge of the slab is usually slightly thicker, which helps to increase the stability of the slab. The slab is typically built on a layer of crushed gravel and a sheet of waterproofing plastic to prevent moisture from seeping up from the ground. This type of footing is generally suitable for non-habitable areas, such as garages and driveways, and is installed on flat sites.
What are the Materials Used in the Construction of Footings?
There are many different materials that can be used to construct footings, but concrete is by far the most popular choice today. Concrete is a very strong and durable material made from a mixture of cement, sand, aggregate, and water. Reinforced with steel bars, it becomes even more robust and can last for many years. Before concrete became the standard material for footings, other materials such as bluestone, bricks, and wood were used.
Most historic buildings from the 1800s in Australia are supported by bluestone footings. This was a popular choice back then because of bluestone’s versatility and aesthetic appeal. However, these footings are pretty shallow and can be difficult to maintain.
In the early to mid-1900s, many houses in Australia were built on timber stumps. This was a popular method at the time due to the abundance of timber and its easy workability. However, over time it has become apparent that timber stumps are not as durable as other materials and are susceptible to rot and borer damage. As a result, they are no longer considered a suitable option for use in the construction of footings.
Footing Depth Requirements in Australia: Soil, Load, and Site Considerations
The depth of a footing is dictated by the type of soil beneath it and the amount of bearing capacity required. The heavier the building, such as apartments, the deeper the footings should be. Also, soils that are more reactive necessitate a deeper footing in order to minimise differential movements. Additionally, design engineers must account for any adverse factors present on site, such as trees, which could potentially have a negative impact on the footing’s performance. The Australian Standard AS2870 Residential slabs and footings provides minimum requirements for footing design; however, this standard should be treated as a general guideline only. Every site must be thoroughly tested and analysed by both a Geotechnical Engineer and a Structural Engineer to ensure an appropriate footing type and depth is proposed.
How Do You Choose the Right Footing Type for Your Project?
There are many factors to consider when choosing a footing type for your project. A Forensic Structural Engineer can advise you on the advantages and disadvantages of different footing types and help you design a system that is stiff enough to work well with the foundation soil on your site without experiencing too much movements. Building deep footings on reactive foundations can be costly, but it can save property owners from a lot of problems down the road.
Important Aspects of Footing Design in Australia
- Footings distribute building loads to the ground, ensuring structural integrity.
- Australian residential construction uses various footing types, each suited to different conditions.
- Different footing systems, including stumps, strip footings, and pad footings, are used based on specific requirements.
- Strip footings are essential for supporting load bearing walls in construction.
- There are various types of footings, such as individual footings, combined footings, strip footings, and raft or mat foundations.
- Soil type and site characteristics play crucial roles in footing selection and design.
- Reinforced concrete has become the primary footing material, replacing older options like bluestone and timber.
- Footing depth varies based on factors such as soil reactivity and building weight.
- Proper footing design requires expert assessment from both geotechnical and structural engineers.
