Using acetylated wood for structural applications

November 23, 2016

All photos courtesy Accoya by Accsys Technologies

By Steve Morrison
Most architects and engineers have historically defaulted to steel and concrete when designing tall buildings, bridges, and other towering structures. Over the past few years, however, commercial designers have begun to reconsider the suitability of wood as a structural building material.

Compared to concrete and steel, wood is less expensive, generally easier to work with, and more sustainable. It is made from carbon dioxide (CO2) captured from the atmosphere by trees and stored in wood, where the carbon will remain locked for the entire lifespan of the material.

As urban centres become denser—and as demand for residential and commercial skyscrapers surges—use of structural wood, particularly in tall buildings, could make a marked difference in the air quality of those cities and in the health of the environment in general.

One of the primary limitations on using wood for structural applications is its vulnerability to fungal attack. Preservatives resolve some of this concern, but raise another: the lingering effects of toxins (such as those released by pressure-treated wood) on indoor air quality (IAQ). The few species of wood that naturally resist fungal attack tend to be in limited supply, fairly short-lived, and difficult to glue. This is one of the reasons architects have shied away from specifying wood for structural use on the exteriors of edifices located in damp climates.

Since the introduction of acetylated wood to the market nearly a decade ago, however, using wood for tall structures has become an increasingly attractive alternative to using concrete and steel. In 2008 and 2010, acetylated wood replaced traditional construction materials on two structurally demanding, heavy-traffic bridges in the Netherlands. Since then, structural beams made from acetylated wood have also replaced concrete foundations in a house in Scotland. The modified wood also stood in for conventional timber for the columns of a veranda at a sustainable, five-storey care complex in Amsterdam, the posts of a prominent, 16-m (52.5-ft) architectural sculpture at the entrance of Dublin City University (DCU), and the structural components of several other architectural structures around the world.

Acetylation’s advantages
A process developed more than 80 years ago, acetylation modifies the wood at a molecular level to alter its reaction with water. This permanently alters wood’s propensity to absorb and release water—the quality that makes it swell and shrink. Taking the place of those water-absorbing free hydroxyls are stable acetyl groups that do not bond with water. Wood naturally contains acetyl molecules; the nontoxic acetylation process simply increases their levels so the wood no longer responds as usual to moisture. This reduces its tendency to shrink and swell by more than 70 per cent.

Acetyl groups are naturally present in all wood species. Currently, acetylation converts sustainability grown softwoods and non-durable hardwood. This means the manufacturing process adds nothing into the wood that does not already naturally occur within in. The end product, acetylated wood, does not add toxins to the environment.

The process also renders the wood inedible to insects and parasites like fungi and far more resistant to rot and decay than unmodified wood. The result is a durable, dimensionally stable product with the natural beauty of wood.

Since this modification permeates the wood, the full cross-section of members up to 75 mm (3 in.) thick remains moisture- and fungus-resistant, even when the core is exposed during cutting and machining—unlike treated conventional wood.

The dimensional stability provided by acetylated wood’s resistance to shrinking and swelling is one of its primary advantages. Changes in the moisture content of the surrounding environment can cause large structural members made from unmodified wood to split—especially at the point where they are joined by steel connection plates. Acetylated wood is less likely to do so. This modified wood also does not suffer from moisture-induced pits and fissures, which can trap water and increase the moisture content of the wood, creating an appealing breeding ground for fungi.

In one 16-year-long test, large stakes—some made of acetylated wood and others of unmodified wood—were planted along the edge of a Holland canal. Over the course of those years, the acetylated wood did not decay, but the untreated wood was destroyed. Other tests in the United States, Australia, and Japan proved the modified wood was up to 20 times more resistant to termite damage. These qualities render acetylated wood virtually free of the need for preventative maintenance, unless a coating is applied for esthetic reasons. In that case, the finish may need periodic reapplication.

The Dutch bridges in the city of Sneek are the first to incorporate laminated softwood elements without relying on toxic preservatives or a protective roof. By curving the two trusses toward each other, it was possible to provide clearance for two lanes of traffic while limiting the overall height of the bridge. The trusses were created by curving thin singly curved beams about their weak axis and then gluing them with a water-resistant timber adhesive. A steel rod concealed within the bottom chord, with large end thrust blocks, is used to make the critical connection between the rafters and tie of the trusses.

Structural potential and examples of successful exterior usage
The dimensional stability of acetylated wood makes it a candidate to be used outdoors on fully exposed bridge structures and ground beams, even in moist climates and near water. Those are applications where conventional timber—including durable tropical hardwoods, which fall short because of their limited supply and glue-averse surfaces—has traditionally been passed over in favour of moisture-resistant concrete and steel.

Still, most acetylated wood is specified for non-structural applications, like decking, cladding, windows, external joinery, and even outdoor furniture. However, some architects are choosing acetylated wood when the project calls for structural wood on an exterior.

Building bridges
The first use of structurally sound acetylated wood on a large-scale, high-moisture project involved the building of two bridges in the northern Dutch city of Sneek, in 2008 and 2010.

These open-truss traffic bridges were crafted from 1200 m³ (42,3380 cf) of Forest Stewardship Council (FSC)-certified, acetylated radiata pine from New Zealand. In part, the wood was chosen for the 16-m (52.5-ft)-tall bridges because of its Class 1 durability, as every bridge built in the Netherlands must have an 80-year lifespan. The architects there favoured a natural construction material over steel, which has an average lifespan of just 55 years. There is no defined science on how long acetylated wood will last, but third-party testing institutes show acetylated wood window frames can have an expected service life of between 77 and 90 years, with the range representing exposure conditions from severe to mild.

Acetylated wood is more expensive than most timber or modified wood, but is less costly than tropical hardwoods such as ipe and mahogany. This is due in part to its durability and long lifespan. The cost to add coatings is lower, and coatings last longer when applied, compared to most other wood.

These architectural light poles are made of tapered, round, solid acetylated wood, and feature glulam construction and a rounded extruded aluminum pole base. These poles are located outside the Hillstone Restaurant in Biltmore, Ariz., and were installed in December 2010. More than six years later, they still look as new as they did when they were first installed.

Specifying conventional lumber on an exposed and unprotected open roof was out of the question. The complex geometric pattern of the bridges’ beams also meant their assembly required gluing. These factors allowed acetylated wood to trump even the naturally durable, fungus-resistant tropical hardwoods, which are difficult to glue and have a lifespan of just 45 years. The material could also support road traffic weighing up to 65 metric tonnes.

Modern meets historic
At a residential care complex in Amsterdam, structural acetylated wood was chosen for the columns on a large veranda—but not only because of its strength.

The FagelCats Apartments compound consists of 22 sustainable apartments in a pair of five-storey buildings, along with four large community homes, near the historic canal ring in the centre of Amsterdam. Surrounding the complex are a number of aging 19th-century homes, whose architectural character was to be mirrored by the façades of the new buildings. To accomplish this, a variety of materials were required, including brick, freestone, tin,
and wood.

A communal courtyard between the pair of buildings features glazed corridors, a garden pavilion, and a veranda with wood columns. Designers selected acetylated wood for those columns because of its structural and sustainable qualities—the manufacturer guaranteed it against rot and decay for 50 years in exterior use—and its appearance. Using modified wood for the columns, the decking of the veranda, and the window frames along the corridors softened the look of the façades in the courtyard.

Acetylated wood was used for the columns of this veranda at FagelCats Apartments in Amsterdam.

High beams
At the entrance of Dublin City University, 54 timber posts—some as tall as 16 m (52.5 ft)—sprout skyward from the brick pavement. This imposing sculpture is the backdrop to a large platform holding the letters, ‘DCU.’ It has created a visual identity for the 20th-century institution, which is home to the Helix—Ireland’s largest
arts centre.

Made from glulam acetylated wood, the sculpture was chosen to adorn the entryway after London-based ZAP Architecture won an international design contest hosted by the Royal Institute of the Architects of Ireland (RIAI). ZAP’s creative director, Pol Gallagher, has called the monument “a bold forest of vertical timber elements, broken up by rich green spaces.”

Moisture-tolerant acetylated wood was selected for the project because the sculpture is uncovered and exposed to Dublin’s abundant rainfall—and because the material is FSC- and Cradle to Cradle Gold-certified.

The sustainable structure is steel-free, as the glulam posts are clamped into the ground.

“This wouldn’t have been possible with any other type of timber, as there would have been far too many concerns about the durability of such an essential structural component,” noted Dominik Niewerth, a project engineer at Schaffitzel + Mieback, the German timber engineering firm that consulted on the project. In addition to the
50-year guarantee previously mentioned for above-ground applications, the project’s modified wood manufacturer guaranteed the material below-ground for 25 years.

Built in 2014, the landmark structural project was the first of its kind in Ireland. Gallagher said the use of a “naturally grown and sustainably harvested, high-performance modified wood” was a must for the sculpture.

Set on a woodland site in the Scottish Highlands, the two-bedroom Dunsmore House was designed to use building materials that were energy-efficient and locally sourced. To that end, its architects specified acetylated timber groundings rather than a traditional concrete foundation. Made from Portland cement, concrete is high in embodied energy and carbon.

MAKAR, an architect-led ecological design and building company located in Inverness, rested the acetylated wood beams on concrete piers, so that the platform floats above ground. This allows for drainage in Scotland’s oceanic climate. Modified wood was chosen for the laminated beams, as its dimensional stability and durability is equal to the challenges of a moisture-heavy environment.

MAKAR architects chose acetylated wood after consulting with Edinburgh Napier University’s Centre for Offsite Construction + Innovative Structures. Although the beams are sheltered under the house, risks of fungal attack and dimensional instability on the damp building site were still worth considering, as they had strong potential to weaken the building envelope. Architects’ evaluation led them to acetylated wood as a solution, given its resistance to water and fungi. This wood also met architects’ need for an easily glued, sustainably sourced product free of toxic preservatives.

For the rest of the home, the designers chose local Scots pine, Scottish larch, spruce, and Douglas fir. Sheep’s wool insulation, south-facing windows, solar water heating, and a wood-burning stove ensure the home’s energy efficiency. To minimize construction waste, MAKAR built many of the home’s components in an indoor workshop and erected them onsite within three days.

Historically, steel has been the best material option for external columns supporting a timbe-framed canopy. Timber columns were often awkward visually, as they required either a generous overhang to keep the base of the column dry or careful detailing to ensure water could not become trapped between the column and the steel base. However, with acetylated wood, the columns can now be timber to match the frame.

Increasing potential
Acetylated wood was the material of choice for the cladding, decking, and structural beams of a sustainable combined house and boathouse in the waterside village of Horning, England. The project showcased the extreme durability and structural properties of modified wood in a challenging marine environment. The curved, glulam screen designed for the project is an excellent example of the versatility of modified wood.

In Istanbul, Turkey, 356 pieces of acetylated wood were curved and invisibly bolted together to create an iconic egg-shaped sculpture that sits on water and doubles as the entrance to the Marmara Forum. Modified wood was selected for the colourful, lighted sculpture—called OVO—because it “ticked all the boxes,” the sculptors said in a statement. “We needed a simple, natural, and durable material for our creation, and it suited our design well.”

On this side of the Atlantic, acetylated wood was also used for lighting poles around a new-concept restaurant in Biltmore, Ariz. Featuring movable glass walls, the Hillstone Restaurant offers indoor and outdoor seating and views of the southwestern landscape. Designers selected a variety of natural, sustainable materials including stone, copper, and wood to tie the outdoor architecture to the indoor look. For the lighting poles, they specified acetylated wood because of its durability, longevity, and elegant appearance.

The growing potential for acetylated wood as a sustainable, structurally sound building material could become clearer as timber becomes a more widely accepted alternative to steel and concrete for tall buildings. As the search for ways to build greener, more energy-efficient buildings continues, architects and builders are pushing the traditional boundaries of building with wood.

An 18-storey student dormitory slated to open next year on the campus of the University of British Columbia (UBC) is the world’s tallest timber tower, according to the institution. This will top the 14-storey Treet in Central Bergen, Norway, and the 33-m (109-ft)-high Cube in London, whose developers call it the tallest cross-laminated structure in Europe. Last year, Canadian architect Michael Green also unveiled a futuristic proposal to build a 35-storey wood skyscraper in Paris. The exterior of the building has been completed, but the interior is still under construction, set to be completed in May 2017. Acetylated wood is available throughout Canada.

The development of high-tech processes for creating ultra-strong, dimensionally stable timber is, in part, driving the trend toward the use of wood in big buildings. Architects, builders, and building owners are likewise sharpening their focus on the environment—and therefore on sustainable materials like acetylated wood for major building projects.

Steve Morrison is head of marketing for Accsys Technologies in North America, manufacturer of Accoya wood. An industry veteran, he has a healthy background in wood products at a variety of large manufacturers throughout North America for Universal Forest Products, International Paper, and Masonite. Morrison can be reached at[2].

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