Design Options for Mid-Rise Timber Structures 2

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Tuesday, September 27th, 2016
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From a design perspective, one of the most significant areas of opportunity which came out of changes to the National Construction Code in 2016 revolves around the enablement of mid-rise timber buildings under a deemed to satisfy (DTS) solution.

With the new provisions having come into force in May this year, it is timely to look at what they mean and to examine some of considerations and options they raise.

At the core of the provisions is a new clause (C1.13) which has been inserted into Volume 1 of the Code. Essentially, this enables class 2, 3 or 5 buildings of up to 25 metres of effective height to be constructed of ‘fire protected timber’ so long as sprinkler systems are installed throughout the building, insulation is non-combustible and cavity barriers are introduced.

In order to be considered to be ‘fire protected’, timber must meet requirements under a new specification (Specification A1.1). For general timber (traditional wall stud construction), this means it must be protected to achieve the required FRL of the element and either

  • have a covering which resists the incipient spread of fire for at least 45 minute, or
  • contain two millimetres by 13 millimetres of fire grade plasterboard.

For massive timber construction, the timber must be protected to achieve the required FRL of the element and must either be protected to achieve a temperature of less than 300 degrees Celsius for a time specified in a new table or contain fire-grade plasterboard.

Cavity barriers are either solid wood or polythene sleeved mineral wall which are installed in cavities around the building in order to prevent the passage of flame and hot gasses beyond the compartment of origin. According to a new specification (C1.13), these must be in place next to separating walls, at separating floors, around doors and windows and at minimum spacing intervals when mid-rise timber is used.

For timber mid-rise buildings, it also possible under a new specification D2.25 to use timber mid-rise stairs provided these meet timber stair and landing requirements, there is sprinkler protection throughout (including within the stair) and there is fire protection below flights and landings at lower levels.

Of course, it should be recognised that although the DTS provision extends only to eight storeys, timber buildings which exceed this height can still be delivered via a performance solution – as Lend Lease did with its Forte building at Melbourne’s Docklands.

That said, given the greater ease and predictability associated with using the DTS solutions, it is expected that many of the mid-rise timber buildings which are indeed constructed will be built within the DTS provision.

So what are the options and considerations?

According to Nick Hewson, a technical manager at cross laminated timber provider XLam, opportunities provided by the new provision are significant – especially in the growing market of multi-residential dwellings in the inner and middle suburbs of major capital cities.

As well as being lighter, being faster and easier to construct and install and having a lower carbon footprint, benefits include good airtightness and enabling opportunities for use of exposed timber to create a positive aesthetic feel.

In addition, the computer-controlled cutting techniques associated with products such as CLT enable ‘mass customisation’ with regard lengths and shapes and therefore enable significant levels of opportunity for more ‘architecturally interesting’ forms of design.

When it comes to working with timber, Hewson says it one of the most basic decisions revolves around whether to use a traditional stud frame and top plate arrangement or a form of mass timber or engineered timber such as cross laminated timber. Whereas traditional types of construction might work for any building of up to six storeys, Hewson says, CLT, glulam or LVL posts and beams with CLT floors would be a better option for buildings above this.

In terms of approach, Hewson says it is critical to set out the building at an early stage in such a way as to keep your options open as well as to understand the basic quirks of the product in question. Whereas concrete is largely molded into shape, the ‘kit and parts’ nature of timber means that building with wood requires careful considerations about the size of their parts as well as their limitations, he says.

“It’s setting out the building at an early stage so that it could be any materials – not necessarily forcing yourself down one path of a concrete or a timber or a steel building but rather keeping your options open,” Hewson said.

In absence of any kind of concrete reinforcement, timber is best also suited to spans of six metres or less, and is often therefore well suited to projects such as hotels, which involve a considerable number of shorter spans. When setting out the layout of apartments, therefore, it is critical to do so in a way which allows for these shorter spans and the consequences they create.

Internal walls and partitions are more likely to need to be load bearing CLT walls than would be the case under traditional concrete construction where they are more likely to be non-load bearing metal studs and plasterboard. Because of their load bearing nature, the placement of these spaces should be given careful consideration up-front.

Other considerations revolve around acoustics and fire protection. With regard to the latter, as outlined above, mid-rise timber buildings will need to have sprinklers throughout and meet requirements to be considered to be ‘fire protected’. For acoustics, it’s about providing sufficient attenuation between parts – a phenomenon which Hewson says can be addressed via measures such as screeds and denser finishes.

The new NCC provisions have opened up important design opportunities.

Capitalising on these will require careful planning and thought.

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2
  1. Aswan

    Great to see codes adjusted to suit the latest developments in construction technology – massive timber is now a perfectly safe and sustainble solution for mid-rise apartments.

  2. Greg Ellevsen

    Thanks for another great article Andrew. Love your work and learning from you