Timber, steel and concrete are three of the most common structural materials in the world, and each material has pros and cons. We asked industry bodies representing each to argue their case.
“Timber has no disadvantages, only design challenges,” said Andrew Dunn, chief executive of the Timber Development Association. “A forgotten knowledge of fire and durability has limited timber use but that is changing. The key aspect for timber is that its properties are well known and very predictable.”
Structural type, location and intended service life are all factors when determining the appropriate use of timber, but in general terms it is lightweight, easily worked, very adaptable to offsite manufacturing and generally cheaper than other materials.
Its main market is in low-rise residential structures but there is a growing interest in the mid-rise sector for apartments, office buildings and school buildings.
- Timber has higher structural efficiency as carried load per unit weight compared to reinforced concrete and steel structures
- A common stud used in house construction has similar compressive strength to general purpose concrete
- Many timbers are either naturally durable or can be easily treated to make very durable.
Wood, the raw material of structural timber products, is made from energy from the sun and carbon absorbed from the air (from the carbon dioxide in the air). Half the dry mass of timber is carbon absorbed from the air.
- Timber used internally provides a healthier environment for occupants as it helps maintain a better relative humidity
- The vast majority of structural timber is sourced from sustainably managed forests and plantations.
Dunn argues timber offers more architectural design flexibility, which creates more design options without sacrificing structural requirements.
“The natural warmth and beauty of wood is aesthetically pleasing,” he said.
The Metropol Parasol in Seville, Spain, a 5,000 square metre, four-storey plaza cover, is an example of how timber has been used creatively recently.
The Treet or ‘the tree’ shows how far timber has come. A 14-storey (49-metre) apartment building in Bergen designed to comply with the Passivhaus sustainability standard. When completed it will be the tallest modern timber apartment structure in the world.
So what’s next?
“The sky is the limit literally,” said Dunn. “There is a growth of timber building pushing the height limits. Canada has three tall timber buildings in the pipeline, a 13-storey Quebec City project, using cross-laminated timber and glulam, a 12-storey building in Ottawa and a project in Vancouver targeting 18 storeys. Not to be outdone, the highest timber building planned so far is a 23-storey office building in Sweden.”
“Concrete has been used for structures since Ancient Greece and Rome,” said Marianne Fourie of the International Federation for Structural Concrete. “Technological improvements and innovations over the centuries have refined its use. Structural concrete today allows engineers and architects to design and achieve striking edifices that are as robust as they can be aesthetic. Advances have also led to the more widespread use of precast concrete, which offers great benefits of cost and speed of construction.”
When it comes to larger edifices, such as tall buildings and bridges, Fourie argues that nothing comes close to structural concrete for sheer strength and durability.
Part of its attraction, she says, is also its versatility.
“There is no reason to limit design to one material only,” said Fourie. “Assimilating other materials into concrete structure design is simple. However, structural concrete is not only used in conjunction with other materials but also incorporates them, for example, with fibre-reinforced concrete.”
Fourie says although other materials have become ‘trendier’ for their perceived environmental friendliness, structural concrete has made great strides in sustainability. It also has inherent ecological benefits since it is made from the most commonly available minerals (sand and limestone), and has excellent durability, thermal mass and minimal waste.
She cites two projects, which both won an ‘Award for Outstanding Concrete Structures,’ as shining examples. The Centro Ovale concrete shell (Chiasso, Switzerland), illustrates the versatility of the material, while the Bella Sky Hotel (Copenhagen, Denmark) is an example of the innovative use of precast concrete.
“The aesthetic possibilities of structural concrete are limitless,” said Fourie. “One innovation is the use of pigmented admixtures that allow designers to create facades in a great variety of colours. A recent trend is the use of white concrete, which lends a pristine quality to edifices. The reinforcement of concrete also continuously advances, with all matter of material being incorporated to enhance strength and increase the architectural possibilities.”
“Steelwork in major construction is on the rise as builders become more attuned to the advantages of using the material in easing onsite risks, speeding development for earlier returns and environmental benefits,” said Alan Marshall, communications manager at the Australian Steel Institute.
The US-based Council on Tall Buildings and Urban Habitat recently reported the number of composite multi-level projects over 200 metres that typically comprise a steel frame with metal decking, rose 54 per cent worldwide in 2014.
“Australia has experienced a similar resurgence,” said Marshall. “This is particularly the case along the eastern seaboard where, during the same period, about 20 composite multi-level projects are underway, such as at 480 Queen Street, which is the first steel framed building of its size in Brisbane. It utilises a parallel beam flooring system to achieve five-day floor cycle times, along with many other benefits gained from composite design and build programs.”
Marshall argues the case for steel under three key criteria: speed and efficiency; the reduction of on-site risks; and sustainability and waste reduction.
Speed and efficiency
- Computer modelling before fabrication on computer numerically controlled (CNC) equipment means the components are produced right first time and there is minimal rework
- Earlier construction provides faster completion and payback commencement
- Faster construction means the builder’s staff can be released earlier to start the next project
- Steel is a structurally efficient building material and so buildings are lighter and often foundations can be smaller as a result
- Steel is fabricated in controlled conditions driven by 3D modelling and CNC equipment increasing safety and reducing MTIs and LTIs
- A test certificate is available for all steel used and this can be made traceable through to the finished product
- Design for standardised bolted connections and repetitive floor plates can increase speed of construction
Reducing onsite risks
- Steel use reduces the number of workers onsite (approximately 10 to 20 per cent of the labour needed for concrete construction), reducing accident liability for builders
- Preassembled steel packages can be lifted straight from the truck in sequence
- Offsite fabrication relieves congested and hard to access sites
- Reduced noise and dust and construction times ease disruption to current occupants and neighbourhoods
- For domestic housing, steel framing, roofing and cladding offer more termite-free and fire resistant abodes than with other commonly used building materials
Sustainability and waste reduction
- Waste removal is significantly less than for a concrete building
- More than 95 per cent of all structural steel is recovered and reused or recycled
- Steel buildings inherently lend themselves to structural addition and modification easily
- Independently certified steel fabricator members of the ASI’s Environmental Sustainability Charter attract an extra Green Star point for projects
- Savings in freight and materials through not having to use and remove temporary formwork
- The design can be future proofed for longer life such as provision of large beam penetrations to accommodate future services
A compelling cases can be made for any of timber, concrete or steel. So which is your structural material of choice?