Multi-Residential Timber Framed Construction – from Forest and Wood Products Research and Developmen

Topics Covered

Background

Structural Performance

Sound Insulation

Fire Separation

Fire-rated Timber Cladding System

Background

This type of construction has been used successfully overseas for a number of decades. Australian research demonstrated that timber based wall and floor/ceiling systems are able to match and in some cases, exceed, concrete and masonry in the same application. MRTFC was introduced to the Australian market in 1994 following an amendment to the Building Code of Australia.

This construction method makes use of a timber frame over the full height of the building. The basic premise behind MRTFC is the utilisation of fire and sound-rated timber framed wall and floor systems to provide for vertical and horizontal separation between dwellings.

The whole system - cladding and framing - must be detailed to provide:

•        Sound insulation from one unit to another

•        Fire protection and compartmentalisation from one unit to another

•        Appropriate structural performance.

Structural Performance

All timber used in MRTFC must have adequate structural performance. Wall framing is designed to have sufficient strength to resist the wind forces (acting in bending), and the weight of the structure and loads above that level (acting in axial compression on the studs). In order to keep the wall cladding flat, there are requirements on the straightness of the wall-framing members.

Floors also have strength requirements. In multi-residential construction, the noise insulation requirements through the floor and ceiling makes deep pile carpet and heavy underlay highly desirable. As well as strength requirements, it is important that floors have adequate stiffness. Flexible floors can be uncomfortable to walk on. Vibration can also be a problem in floors that have not been properly designed and detailed. Vibration problems in residential floors mean that a person sitting in a chair will shake as someone else moves around on the same floor.

MRTFC makes use of structural timber with no particular requirement to have good appearance. Connections are structural elements and will not be made with any special care to visual features. As most members and connections will be hidden below the linings, inspection and maintenance of them will be difficult. Detailing for durability is very important. Currently, most members in a multi-residential building must be designed by a structural engineer.

Starting at ground level and working upwards:

•        Under the slab-on-ground is a protective barrier against termites.

•        On the slab is the bottom plate of the wall frame.

•        Studs are fastened to this bottom plate and are stabilised at mid-height by noggings.

•        The top of the wall frames is a top plate.

•        On top of the lower floor wall frames is the suspended floor system. This consists of flooring over the top of joists.

The joists can either sit on top of bearers, frame into the side of them, or, sit directly on the wall frames underneath. While the floor loads are the same at each level, the vertical loads carried by the walls that support the floors decrease as we go up the building. The studs in the lower floor are larger and stronger than those in the top floor. For lower floors, sufficient strength in the studs can be achieved by using studs made by nailing a number of pieces together making up a single thick, nail-laminated member, or in this case, stronger timber species can be used.

Often wall frames can be successfully prefabricated off-site, delivered and assembled in a day.

Trusses can be used extensively in the roof system. The top chord of roof trusses is known as a rafter. The cladding is supported by battens or purlins that run parallel to the line of the ridge.

Sound Insulation

The BCA requirements for MRTFC include provisions to control the amount of airborne sound (speech, musical instruments, load speakers) and impact sound (footsteps and moving furniture) transmitted between dwellings.

Separating walls must be constructed to reduce sound transmission. This can be achieved by:

•        Increasing the thickness, number or density of linings

•        Using double stud or staggered stud walls

•        Filling the cavity with absorptive insulation material such as cellulose fibre, glass fibre or mineral wool

•        Using non-rigid or resilient connectors

•        Sealing gaps in walls and floors through which electricity or plumbing services pass

•        Designing building layouts with laundries, service shafts, stairs and other noisy areas in buildings situated away from sensitive living or sleeping areas

Fire Separation

In order to achieve a satisfactory performance in fires, all materials (including steel and concrete) require special attention to fire detailing.

Certain buildings and/or elements of buildings are required to have fire resistance levels (FRL), which are given in the Building Code of Australia (BCA) and are a function of the intended use of the building. A structure must remain safe for long enough to evacuate the occupants and to allow the fire brigades to fight the fire in safety. Fire compartments must also contain the fire throughout that time, which will prevent the fire spreading to adjacent properties. Fire resistance, expressed as a time (minutes) that a structural member or part of a building must achieve with respect to:

•        Structural adequacy – maintain stability and adequate load bearing capacity

•        Integrity – resist passage of flames, smoke and hot gases

•        Insulation – maintain a specified temperature on the surface not exposed to fire.

FRL are expressed in the above order (eg. FRL 60/60/60 for separating walls)

The fire ratings can be achieved with timber by using a combination of all or some of the following:

•        Fire grade lining – fire grade plasterboard or a combination of fire grade plasterboard and fibre cement. The timber takes longer to get to ignition temperature, and can remain functional for a longer period while the fire is burning.

•        Fire stop – fire grade material used to close a gap or imperfection of fit that occurs where a service passes through a fire rated element or system to prevent spread of fire.

•        Intumescent sealant – a fire resistant material used in fire grade linings (at joints, penetrations etc) which expands when exposed to fire to fill and/or seal gaps.

•        Fire resistant Mineral wool – compressible, non-combustible, fire resistant material used to fill cavities or restrict the passage of smoke or gas.

•        Over-sizing. – The timber can be oversized so that allowing for loss of material charring throughout the burn period, there will still be enough timber remaining in the cross-section to give it the required strength.

Fire-rated Timber Cladding System

A new fire rated plywood wall system has been developed that uses the weather resistant plywood cladding as an integral part of the wall’s fire resistance. Levels of fire resistance for external walls may vary depending on distance from fire source or whether the wall is load bearing or non-load bearing. The requirements are defined in the BCA.

For use in MRTFC, the new plywood system comprises a number of layers of materials working together to achieve an FLR of 90/90/90 in load-bearing applications and -/90/90 in non-load bearing. Exterior linings comprise a 16mm moisture resistant, fire grade plasterboard fixed to studs. The exterior plywood cladding must be at least 12mm, exterior grade plywood and preservative treated to Hazard level H3.

Internal linings must be two layers of 13mm fire grade plasterboard.

This system offers architects, designers and builders a cladding material to feature on the exterior of MRTFC projects. Structurally, there is a reduction in the load on footings or transfer slabs due to the elimination of masonry cladding. The system speeds up construction by eliminating a wet trade within the construction cycle.

The plywood system also provides the opportunity to combine external cladding materials to create feature panels, incorporate returns and a whole range of architectural and design elements that are timber based.

Source: The Forest and Wood Products Research and Development Corporation

For more information on this source please visit The Forest and Wood Products Research and Development Corporation

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