Engineering wood for use in multistorey buildings

Wood is a variable and versatile material, but it requires an innovative approach, often lacking in Ireland especially when it comes to construction in engineered wood such as CLT.

Donal Magner

12 July 2017

Engineering wood for use in multistorey buildings

The 53m Brock Commons student residence Vancouver is the tallest cross-laminated timber (CLT) building in the world. Building regulations in Ireland limit heights of timber buildings to 10m. \ KK Law from Timber in Multi-storey Construction, COFORD.

Magnus Modus commissioned by the National Gallery of Ireland was designed and made by Joseph Walsh Studio, Riverstick, Co Cork, using multiple layers of laminated ash wood. \ Courtesy of the National Gallery of Ireland

Wood specifiers including architects, engineers and designers from countries with strong wood cultures understand both the versatility and variability of wood.Variability in wood is challenging for architects and engineers who are used to working with steel and concrete especially for large-scale projects. As a result, timber at best makes its way into light timber construction systems in Ireland but this is not the case in countries with vibrant wood cultures.

Wood specifiers including architects, engineers and designers from countries with strong wood cultures understand both the versatility and variability of wood.

Variability in wood is challenging for architects and engineers who are used to working with steel and concrete especially for large-scale projects. As a result, timber at best makes its way into light timber construction systems in Ireland but this is not the case in countries with vibrant wood cultures.

Solid sawn wood is used in virtually all buildings in Ireland, which has restricted its use to mainly one- and two-storey building systems. Multistorey buildings require engineered wood of which cross-laminated timber (CLT) is the most widely used.

CLT is a wood-based panel consisting of up to seven layers of sawn wood glued at right angles to one another, which provide the desired strength, stability and high load-carrying capacity to support multistorey buildings.

“To date, over 40 buildings between five and 14 stories tall have been completed across the globe,” according to Timber in Multi-storey Construction published last week by the Council for Forest Research and Development (COFORD). The 53m Brock Commons student residence in Vancouver is the tallest CLT building so far. Just 6m short of Liberty Hall, Dublin, the 18-storey building is about to house 404 University of British Columbia (UBC) students.

UBC stresses speed of construction and environmental benefits as deciding factors to build in wood. “The structure was completed less than 70 days after the prefabricated components were first delivered to the site,” a spokesperson for UBC said. “Wood is a sustainable building material that stores, rather than emits, carbon dioxide, leading to a reduction of 2,432 metric tonnes of carbon dioxide compared to other construction materials.”

A small number of architects in Ireland are now beginning to explore CLT but progress is painfully slow as the COFORD publication outlines the main constraints which relate “to the national building regulations concerning fire safety”. The regulations currently have limits on the use of combustible material which uniquely in Ireland limits the maximum height of a building to three storeys.

Countries such as Canada, Norway, Austria, Sweden and Denmark have addressed all the issues regarding fire safety in CLT buildings, but not Ireland. In the UK alone, over 100 educational buildings in CLT were constructed between 2003 and 2011, according to COFORD.

“Many countries are moving towards a performance-based design approach,” the report states. “In the case of fire design, this would mean that minimum national fire performance targets will be specified and the designers will be required to demonstrate that the building meets those targets. In light of the speed of development of new construction products and technologies, this approach would allow the Irish construction sector to take advantage of these developments.”

However, the COFORD report authored by Annette M Harte, Bill Robinson, Andrew Macilwraith and Malcolm Jacob, maintains that the adoption of performance-base design “is not likely in the short-term”. It states that “a proactive approach to disseminating knowledge on fire-safe design of mid- and high-rise CLT buildings should be undertaken”. The COFORD report does not say who should undertake this “proactive approach” or where the responsibility lies in changing attitudes towards building in wood in Ireland.

Surely it rests with the Department of Housing, Planning, Community and Local Government, which could – and should – play a far more active role in promoting timber as a safe, economic and sustainable building material in medium- to high-rise construction.

The COFORD report Timber in Multi-storey Construction identifies the national building regulations on fire safety as the main constraint to the widespread adoption of CLT mass timber construction in Ireland. The UK has a far more enlightened approach in this regard to wood construction.

GreenSpec, supported by the UK government, promotes sustainable building products, materials and construction techniques including CLT. It states: “One of the major advantages of CLT is its inherent fire resistance. CLT can be designed to accommodate substantial fire resistance and unlike steel remains structurally stable when subjected to high temperatures. CLT panels can be produced with fire resistances (FR) of 30, 60 and 90 minutes. The measure of FR is the time elapsed from the start of the fire up until the point where the material fails to function.”

GreenSpec explains how timber reacts to fire: “Typically, resistance is expressed in minutes, eg.FR 30, 45, 60 or 120. CLT’s fire resistance is provided through ‘charring’. As the face of the timber panel is exposed to a fire that ramps up to a temperature in excess of 4000C, the surface of the timber ignites and burns at a steady rate.

As the timber burns, it loses its strength and becomes a black layer of char. The char becomes an insulating layer preventing an excessive rise in temperature within the unburnt core of the panel. It is this unaffected core which continues to function for the period of the fire resistance. To achieve the designed fire resistance period, there must be sufficient virgin solid timber remaining behind the char layer to sustain the loads applied. Therefore, each CLT panel within the building must be designed for the fire resistance period and the specific loadings applied to that panel.”

For further information on CLT and sustainable timber usage, visit the following websites: GreenSpec (www.greenspec.co.uk), the “Talking Timber” section of Wood Marketing Federation (www.wood.ie) and COFORD (www.coford.ie).

The National Gallery of Ireland launched Magnus Modus, a large-scale, 7m tall, free-form sculpture by Irish designer, Joseph Walsh last month. Designed, engineered and assembled in the Joseph Walsh Studio in Riverstick, Co Cork, the work is made of multiple layers of laminated ash wood.

The meticulously crafted curved sculpture is now a permanent feature of the new gallery courtyard in the refurbished National Gallery of Ireland. Resting upon a small Kilkenny limestone base, Magnus Modus almost defies gravity as it first reaches upwards and then outwards. Most of Walsh’s large-scale works have been commissioned by international clients who acknowledge their originality and uniqueness. Magnus Modus is a well-deserved vote of confidence by the National Gallery in Walsh and his creative team as well as an acknowledgement of timber as a major sculptural medium.