Structural Steel in Residential Construction in Australia

What it is

Structural steel in housing is everywhere now. Open plan living rooms run a PFC beam over the kitchen, double garages without a centre post need an RSJ across the door head, alfresco roofs cantilever off SHS posts, and modern coastal homes use steel portal frames where timber wind bracing cannot keep up. Once a build moves beyond AS 1684 timber framed limits, AS 4100 takes over.

AS 4100 is the Australian Standard for steel structures. It sets the design rules for members, connections and stability. AS/NZS 5131 sits beside it as the fabrication and erection standard, calling up four construction categories CC1 to CC4 based on the consequence of failure. Residential work is normally CC1 or CC2. Welding work is governed by AS/NZS 1554, with Part 1 covering general structural welding for Category SP and Category GP welds.

The NCC pathway is the structural performance requirement in Volume Two Part H1, with AS 4100 referenced as the Deemed-to-Satisfy solution for steel members in Class 1 and 10 buildings.

Why fabricator qualifications matter

A residential steel post or beam is only as good as the welder who put it together. Under AS/NZS 1554.1 the fabricator must:

• Hold a Welding Procedure Specification for each weld type used.
• Have welders qualified to AS/NZS 2980 or equivalent for that procedure.
• Operate under a Welding Quality Plan that nominates a welding supervisor to AS 2214.
• Demonstrate visual inspection of every weld and non destructive testing on a defined percentage of butt welds in Category SP work.

A backyard fabricator without a Welding Procedure Specification cannot lawfully produce SP welds for a residential portal frame. The inspector should ask for the WPS, the welder qualification record and the welding supervisor sign off before the steel leaves the shop.

CodeMark and Performance Solutions

CodeMark is the third party product certification scheme administered by the Australian Building Codes Board. A steel product carrying a CodeMark Certificate of Conformity is accepted by every state building surveyor as evidence the product meets the nominated NCC clauses. Engineered steel beams and proprietary connectors used in houses commonly hold CodeMark.

Where a job uses a Performance Solution under NCC, the fire resistance of the steel, the corrosion protection class and the connection detail all need a performance based justification report from the engineer. Without it the surveyor cannot issue an occupation certificate.

Inspection hold points for TradeLens

Residential structural steel has five hold points worth witnessing:

1. Shop drawing review against the structural engineer drawings and against AS 4100 connection design.
2. Pre delivery shop inspection: welds, hole positions, surface protection class against AS/NZS 2312 or hot dip galvanising to AS/NZS 4680.
3. Site setout and base plate level check before erection.
4. Connection inspection after erection: bolt grade and pretension, weld leg length, plumb and line tolerance.
5. Final corrosion protection touch up of cut ends, bolts and weld zones before cladding closes the steel in.

Common defects

The defect pattern is consistent across AU residential steel jobs. Undersized fillet welds where the leg length on a 10 mm plate connection measures 5 mm against a 6 mm specification. Bolts installed snug tight where the engineer called for tension control. Base plate grout omitted leaving the column bearing on packers. Galvanising burned off by site welding with no cold gal touch up applied. PFC beams seated on timber wall plates with no bearing plate causing crushing of the top plate over time. Each one of these is an avoidable failure when the inspection hold points are run properly.