Wooden buildings reach new heights

Materials World magazine
,
27 Feb 2019

Timber structures are rising in popularity and proposed building regulation changes will allow for even taller wooden towers in some countries. Ceri Jones reports.

The sky’s the limit for the timber industry, or so it seems, due to widespread interest in wooden high-rise buildings.

Timber First and Wood Encouragement Policies are cropping up world-over, with architects keen on the warm, natural aesthetic. Countries including Canada, the USA, Australia, New Zealand, Sweden, Norway and even Wales have been citing sustainability, recyclability and availability of materials as top factors for embracing the material.

‘The use of timber in construction is not only using a material that has low embodied carbon – i.e. the manufacture of timber has a very low or negative carbon burden – but also, the use of timber in long-term applications means the carbon in the material is locked away for many years keeping it out of the environment, while trees are being planted to replace the felled trees and these are continuing to sequester carbon from the atmosphere,’ Bangor University Head of Materials Dr Graham Ormondroyd told Materials World.

But is timber more carbon efficient than producing steel? ‘I think that the jury is still out on this one,’ he adds, saying that while ‘the timber industry will tell you it is cheaper to build with timber, the steel industry will tell you it is cheaper to build with steel’.

Cost is not the only concern as when considering a wooden tower block, most people’s thoughts are immediately drawn to fire safety.

Coming under fire

Advances in engineered wood are helping frame it as a reliable and safe material for large, complex structures. However, there are still concerns over fire safety, but are they valid?

Late last year, the International Code Council (ICC) in Washington, USA, announced its approval of planned changes to the International Building Code 2021. The amendments would effectively double the permitted height of some wooden buildings in the USA and the countries signed up to the programme, enabling them to go from nine to 18 floors. Years of research and testing for strength and fire safety have resulted in the ICC recommending a new building group called Type IV-A, which would allow for 18 storeys with gypsum wallboard on all mass timber elements.

‘Current best practice for building with timber passes current fire regulations, and there are certainly examples of tall buildings that supersede these regulations. Eurocode 5 takes fire into account when setting out design parameters,’ Ormondroyd says. ‘One important factor to remember is that we know how timber reacts to fire and it is predictable, other materials used in tall buildings, less so.

‘CLT [cross-laminated timber] is in itself inherently fire resistant. As a panelised system with a thick cross-section, it is designed to char slowly and maintain its structural integrity, to add an inert board to the front of this just adds to the safety – it may not be as pleasing but if it increases confidence in mass timber building that is an advantage.’

Tall orders for timber

The fervour to build the world’s tallest timber tower is helping challenge the restrictions around engineered wood. Ormondroyd explains, ‘In terms of tall buildings, it has been shown that timber has a good strength-to-weight and strength-to-stiffness ratio when compared to steel and concrete. This is massively important when building tall buildings and self-weight becomes very significant in the design calculations.’

Evidence of this is clear. Australia already has a handful of timber high-rises, the current tallest being the nine-storey 25 King in Brisbane, built by Lendlease, and Fitzpatrick + Partners’ 16-storey block is underway. These projects are supporting the wider use of timber as well as production. In Australia, there have been large investments in CLT and glue laminated timber production capabilities, including a AUS$20m factory in Queensland that came online late 2018, and a AUS$30m in Victoria due to open in mid-2019.

Sweden also reported record highs in timber production last year, but even this could escalate as national firm Anders Berensson Architects was commissioned to design a wooden skyscraper city along the waterfront at Masthamnen, Stockholm. On behalf of the Sweden Center Party, the project would see 19 low-rise blocks interspersed with 31 narrow CLT skyscrapers of 25-35 floors, with rooftop parks connected via a network of bridges and skywalks.

In the UK

Some local authorities are seeing this trend as a means to an end. In Powys, Wales, the council has committed to a Wood Encouragement Policy to meet the need for social housing newbuilds and job creation. But will we see comparative levels of confidence in timber residences and high-rises in the UK?

‘New, post-Grenfell legislation is now in place. The law applies to all new housing, student accommodation, registered care homes, hospitals and boarding school dormitories over 18m tall and states that only materials with European fire rating of Class A1 or A2 may be used in external walls, which excludes all wood products. This legislation includes the use of CLT – the only viable option in the UK for tall structures,’ Ormondroyd explains. ‘It is not always legislation that prevents the construction of new mass timber buildings, as even Willmott Dixon was forced to decline a 15-storey build as the project could not be insured.

‘Grenfell has left a lot of scars and it will take a long time for the dust to settle and the good legislation to be brought out of the reactionary.’