The Ground Up: Understanding Structural Bases
Every successful construction project begins with a solid foundation—literally. Understanding the interaction between soil, footing, and frame is the first step in mastering construction fundamentals.
1. Site Classification & Soil Mechanics
Before a single cubic metre of concrete is poured, a residential site must be classified in accordance with AS 2870. This standard defines how reactive the soil is—that is, how much it will move with moisture changes.
- Class A: Stable, non-reactive sand and rock sites.
- Class S: Slightly reactive clay sites.
- Class M / H / E: Moderately to Extremely reactive clay sites.
- Class P: Problem sites (soft soils, loose fill, landslip, mine subsidence).
Expert Tip: Never assume a site class based on the neighbour's build. Subsurface conditions can change drastically over just a few metres.
2. Foundation Systems: The Basics
The foundation transmits the load of the building to the ground. The choice of system depends heavily on the site class and topography.
Stiffened Raft Slabs
The most common modern method. The entire floor is a single concrete element reinforced with steel mesh and bars.
- Waffle Raft: Uses polystyrene voids to create ribs on top of the ground. Fast, uses less concrete, but critical attention is needed for drainage to prevent heave.
- Conventional Raft: Beams are dug into the ground. Better for sloping sites or highly reactive soil where stiffness is paramount.
Strip & Pad Footings
Common for timber-floored homes or extensions.
- Strip Footings: Continuous concrete strips supporting load-bearing walls (brick or timber).
- Pad Footings: Isolated concrete pads supporting point loads, such as steel posts or brick piers.
3. Advanced & Specialized Systems
When the ground conditions are tricky or the design demands it, standard footings won't cut it.
Piling Systems
Used when surface soils (Class P or E) are too unstable to support a slab directly.
- Bored Piers: Deep holes drilled to reach stable bedrock or stiff clay, filled with reinforced concrete. The structure effectively sits on "stilts" underground.
- Screw Piles: Steel shafts with a helix screw tip. Installed quickly with minimal spoil—perfect for tight access or renovation work.
Cantilever (Strap) Footings
Essential for boundary walls. You cannot pour a footing that encroaches on your neighbour's land. A strap footing uses an internal beam to balance the eccentric load of the boundary wall, preventing it from tipping over.
Basements & Retaining
Going underground introduces a new enemy: hydrostatic pressure.
- Retaining Walls: Whether blockwork or concrete sleeper, they must be engineered to resist the wedge of soil trying to slide down. Drainage behind the wall is non-negotiable.
- Basements: Think of a basement not as a hole, but as a boat in reverse. Waterproofing (tanking) must be perfect, and sometimes redundant (membrane + drainage cell + pump system).
4. Framing Systems
Once out of the ground, the frame provides the skeleton. The two dominant players in Australia are Timber and Light Gauge Steel.
Timber Framing (AS 1684)
The traditional standard. Easy to work with on-site, forgiving of minor errors, and renewable.
- MGP10 / MGP12: Machine Graded Pine. The number refers to the stiffness (Modulus of Elasticity).
- LVL (Laminated Veneer Lumber): Engineered timber for long spans (bearers, lintels).
Steel Framing
Gaining popularity due to timber shortages and termite immunity. It requires high precision—steel doesn't shave or plane like timber. If the slab isn't level, the steel frame will show it instantly.
The Golden Rule of Structure
Load Paths must be continuous. A roof load must transfer to a stud, which transfers to a bottom plate, which transfers to the floor structure, and eventually to the footing. Interrupting this path (e.g., placing a heavy roof strut on a non-load-bearing internal wall) is a recipe for deflection and failure.