Part 3.4.2 Steel Framing
Acceptable construction manuals
is satisfied for steel framing if it is designed and constructed in accordance with one of the following:
Steel structures: AS 4100.
Cold-formed steel structures: AS/NZS 4600.
Residential and low-rise steel framing: NASH Standard – Residential and Low-Rise Steel Framing, Part 1 or Part 2.
Acceptable construction practice
Compliance with this acceptable construction practice satisfies Performance Requirement for steel framing, provided—
the steel framing is designed and constructed in accordance with–
AS/NZS 4600; or
the frame material has a yield stress of not less than 250 MPa.
The weight of roof or ceiling insulation, particularly if additional ceiling insulation is used for compliance with the energy efficiency provisions, needs to be considered in the selection of plasterboard, plasterboard fixings and building framing.
The steel frame must be protected from corrosion in accordance with the following:
Where the steel frame is within the building envelope, in locations—
more than 300 m from breaking surf; or
not in a heavy industrial area; or
the steel frame must have a minimum coating class in accordance with AS 1397 of Z275 (275 grams of zinc per square metre) or AZ150 (150 grams of aluminium/zinc per square metre) or AM150 (150 grams of aluminium/zinc/magnesium per square metre).
AS 1397 requires the coating mass to be applied to both sides of the framing member. For example, AM150 describes a total aluminium/zinc/magnesium coating mass of 150g/m2 obtained from 75g/m2 applied to each side.
The frame must be permanently electrically earthed on completion of fixing.
The steel frame requirements of this Part should be considered in conjunction with steel frame design and construction advice from the manufacturer.
For the purpose of 184.108.40.206, the building envelope is deemed to be a space in the building where the steel frame does not have direct contact with the external atmosphere, other than for normal ventilation purposes. Examples of such locations are frames which are clad or lined on both sides or frames in masonry veneer construction. Areas not within the building envelope include floor framing members where there is no continuous perimeter subfloor walling or verandah roof framing members with no ceiling lining.
Cut edges on framing components do not constitute a corrosion problem, as the surface area of the metallic coating on either side of the cut edge is far greater than the surface area of the cut edge itself.
Where hole cutting or cutting of members is required, cutting methods that clearly shear or leave clean edges are preferred over those that leave burred edges or swarf.
The adoption of appropriate brick cleaning measures will ensure no damage of any metal or metallic coated components, this would include the shielding of these components during the acid cleaning process. Channels to steel framing should be cleaned of mortar droppings.
Metallic coated steel should not come into contact with green wood containing acidic material or CCA treated timbers unless an impervious non-conductive material is located between the dissimilar elements. The use of kiln or appropriately dried timbers is recommended where contact between the metallic coated steel component and timber is considered.
220.127.116.11 Steel floor framing
The following provisions apply to suspended steel floor framing for single-storey and both floors of two-storey construction:
The two types of suspended floor systems referred to in 18.104.22.168 are—
in-plane systems, such as joist-only systems or systems with integral bearers; and
When used in ground floor construction, all such systems must be installed on stumps, piers or masonry footings complying with Part 3.2. Conventional flooring can be installed on top of the floor frame.
Fibre cement packers or similarly durable and compatible materials may be used when packing is necessary under suspended flooring systems and must be at least the width of the member to ensure adequate bearing capacity.
Table 22.214.171.124 SPANS FOR C-SECTION FLOOR JOISTS
|SPAN||SECTION||Maximum joist spacing (mm)|
|Maximum span (m)|
The size of C-section steel members are identifiable by their description. For example, a C15012 is 150 mm deep and is made from 1.2 mm thick steel.
Table 126.96.36.199 SPANS FOR C-SECTION BEARERS
|Steel Section||SINGLE SPAN||CONTINUOUS SPAN|
|Effective bearer spacing (m)||Effective bearer spacing (m)|
|MAXIMUM SPAN OF BEARER (m)||MAXIMUM SPAN OF BEARER (m)|
Note: For the purpose of this Table:
TYPICAL JOIST OVER BEARER FLOORING SYSTEM
188.8.131.52 * * * * *
This clause has deliberately been left blank.
184.108.40.206 * * * * *
This clause has deliberately been left blank.
220.127.116.11 Installation of services
To maintain the structural integrity of the frame, all ancillary work must be in accordance with the following:
Service penetrations in floor joists must comply with Figure 18.104.22.168.
Plumbing pipe-work in steel framed construction must be run in the following ways:
Pipe-work must be—
run through pre-punched service holes in steel studs; and
extra holes, where necessary, must be located near the centre-line of each stud provided—
the structural integrity of the member is not reduced; and
the hole is not more than 10% larger than the existing holes.
In masonry veneer construction, pipe runs may be located in the cavity and fixed to the studs with full pipe saddles and self drilling screws properly protected against galvanic corrosion in accordance with (v).
In construction where external cladding is attached directly to the steel stud work, piping can be—
installed over the ceiling; or
suspended under the floor; or
installed in accordance with (i).
Plumbing fittings may be attached by—
timber or steel noggings fitted between studs to support tap sets, baths and sinks; and
where a steel nogging is used, the tap set must be isolated to prevent corrosion by a durable non-corrosive material such as timber, cement sheet etc. (see Figure 22.214.171.124).
Copper and brass pipes and fittings must be prevented from coming into contact with the steel frame by one of the following methods:
Where plumbing services pass through service holes, plastic grommets must be snapped into the service hole.
The use of grommets also has the effect of securely fixing the pipe to prevent water hammer.
In other areas where copper pipes may come into contact with metal framing, they must be lagged or isolated with neoprene sheeting or tape.
Electrical cables must be—
run through pre-punched service holes in steel studs (see Figure 126.96.36.199); or
secured to steel framing with—
P clips; or
plastic ratchet straps; or
half saddles fixed with screws or rivets; and
extra holes, where necessary, must comply with (b)(i)(B); and
steel frames must be permanently earthed immediately after the frame is erected; and
backing plates for switches and power points should be fixed at the appropriate positions with suitable fasteners. Where it is impractical to fix directly onto studwork, steel or timber noggings can be fitted between the studs to provide necessary fixing and support.
TYPICAL INSTALLATION AND FIXING OF SERVICES
There are many different types of steel framing systems available. Each of these systems have unique design and installation requirements. Due to this diversity, there are no generic examples of acceptable construction practice for steel wall and roof framing. Accordingly, the design of these systems must be in accordance with the appropriate acceptable construction manual in 188.8.131.52.
Some of the important elements of steel frame design are contained in the following information.
Frames are either in rigid or adjustable form. In the case of rigid frames, minor irregularities in flooring are accommodated by packing.
With adjustable frames, the tensioner assembly on the bracing can usually be adjusted to accommodate these irregularities. After tensioning, bracing straps should be securely fixed to each stud and nogging.
Long runs of external walling may have to be temporarily braced, until the roof members have been fixed. This can be carried out by using lengths of steel, timber or roof battens fixed to the top of the studs and secured to the ground or floor, as temporary props.
Further construction stage bracing may be required to be installed before roof cladding commences. This is required to prevent side sway of the building during construction.
Construction bracing should be provided in the following minimum percentage of required vertical bracing:
40% single-storey slab-on-ground buildings;
40% upper-storey of buildings with suspended floors; and
50% lower-storey of two storey construction.
Trusses and rafters are fixed in accordance with the design details. Generally, the roof members are fixed to the wall structure using conventional building methods.
The fixings may incorporate nails, self-drilling screws, bolts and nuts or shear plate connectors. The fixings should be adequate to ensure that a continuous load path exists from the roof to the foundations for all types of loading including uplift, downward and shear loading.
Temporary roof bracing is generally achieved using one run of roof battens along the full length of the house. It is preferable if the run nearest the roof apex is used for this purpose and fixed as each truss is properly positioned. Next, one run of ceiling battens should be positioned and fitted. This should preferably be the batten run nearest the centre of the building.
Where ceiling battens are not used a bottom chord tie should be installed in accordance with the design details.
Wind bracing should be attached when all trusses have been erected and fixed. Generally all gable roofs and long hipped roofs require bracing in the roof plane. The strap bracing is installed similar to wall bracing and runs from the apex of the roof to the external wall, over the top of at least three trusses or rafters, at approximately 45° to the external walls. The bracing is fixed at the ends, tensioned and fixed to each intermediate truss or rafter.
Connections for steel framing
The following fasteners and connections are acceptable for the assembly and erection of steel framed houses:
Bolts: Bolted connections are used as a means of on-site jointing, particularly where joints are highly loaded and offer a consistent design strength. Bolt design for cold-formed sections is adequately covered in the Australian Standards.
Rivets: Rivetted connections (either pre-drilled or self-piercing) are used for both factory and on-site fabrication and have also been used as elements of proprietary joining systems.
Screw: Self-drilling screws are widely used as a means of connection in almost every aspect of on-site work during the erection of steel framed houses. They are used for connecting wall frame modules, through to attachment of claddings and internal linings.
Adhesives: Adhesives are used in steel framing for attachment of internal linings, including flooring. They are generally used in combination with mechanical fasteners such as self-drilling screws. The screws are primarily used to fasten the linings while the adhesives set, although they continue to act as part of a composite fastening system.
Clinches: Clinching involves the connection of two thicknesses of sheet steel by extruding one sheet into the other using a punch and die, in such a way that the two pieces cannot be subsequently separated. A typical clinched joint used in factory fabrication is usually hydraulically activated whereas clinching systems used on-site are typically pneumatic or electrically driven.
Welds: Welding (typically Mig) has been the most common form of connection during factory assembly for many years. The welded joint strength can vary and the metallic coating is affected in the weld area, the affected area will require post-painting (cold galvanising).
Nails: Hard steel twist nails are used in steel framing for both factory and on-site fabrication. These nails can be used in materials up to 2 mm thick. Nails have also been used for the connection of wall plates to concrete slabs. Where this is done by hand, a timber starter block is normally used. More recently, power actuated nails have been used.
ACCEPTABLE PENETRATIONS TO STEEL FLOOR JOISTS