AHEC Europe

Timber Wave

Species: American red oak
Architect: Amanda Levete Architects (AL_A)
Photographer: Dennis Gilbert

London Design Festival is a perfect opportunity for designers to explore materials in new ways. For the 2011 edition of this annual event, the American Hardwood Export Council (AHEC) commissioned Amanda Levete Architects (AL_A) to design a temporary installation called Timber Wave at the entrance to the Victoria and Albert Museum in London. American red oak was used to create an impressive structure investigating the potential of designing in wood, at the same time being a showcase for a timber still unknown in Europe.  Timber Wave is also a demonstration of collaboration on many levels – not only between architects and engineers, but also with the museum and contractors who made and assembled the majestic structure. The result was ground-breaking and overcame everyone’s expectations.

This virtuoso piece of design was the third and the most ambitious collaboration between the American Hardwood Export Council and the London Design Festival. Victoria Broackers, who is head of the Festival at the Victoria and Albert Museum, was delighted at the result: “We are absolutely thrilled to have had a major installation on our doorstep”.
The initial project brief provided by London Design Festival and AHEC, challenged Amanda Levete Architects (AL_A) and engineering company Arup to design a structure which would manifest the strength and beauty of American red oak. “Normally I feel uncomfortable if a design doesn’t have a function at its core, but the purpose here is very clear: to create something that is self-supporting, that is made out of timber with minimal steel connectors, that goes 12,5 m high and doesn’t need to be tied back to something. We wanted to bring the museum onto the street, to make a reference to the form of the arch – from a certain angle the Timber Wave appears to mirror that arch. We also wanted to demonstrate the technical and conceptual properties of the material” – said Amanda Levete, the founder of AL_A.  “It is the most structurally complex project that we have ever undertaken, which was deeply stressful, but ultimately successful”– she added.

Ed Clark, Arup director commented on the project: “Participating in these experimental installations at the London Design Festival is hugely valuable to us. They provide a great chance to evolve new modeling, analysis, design and construction techniques. Design is central to the work that we do as engineers and this installation provides an excellent opportunity to demonstrate the interplay of artistic creativity and technical expertise that is inherent in all design”.

The timber used for the body of the Timber Wave was American red oak, which was donated by eight AHEC members. This species grows abundantly in the eastern part of the United States, making up about 30% of all the hardwood trees. American red oak combines high strength with good workability, which makes it ideal for machining process. In 1998 AHEC and the Building Research Establishment (BRE) tested the structural potential of four species – Tulipwood, American ash and American red oak and American white oak. Of all the species tested, it was actually American red oak that came out the strongest, despite being about 20% less dense than the white oak. This research was published under Eurocode 5, and was invaluable at the design stage.
The installation at the V&A needed to be 12m high to match the scale of museum entrance but AL_A was keen to use members which were no bigger than furniture legs. Arup suggested a simple trussed arch as an efficient structural form, combined with the strength of the red oak to keep the structure as lightweight as possible. However, it was the architect’s inspired idea to wrap the arch into a large circular wave, itself composed of smaller wave shaped components that was to be the defining feature of the project.

The key components of the Timber Wave are chords and braces, presented on figures 1 and 2. However, in order to create the impression of delicate structure, the architect decided to curve all the elements and place the largest at the bottom of the piece, with the lighter at the top.  Curving the elements created significant engineering challenges. In a truss all the members are either in compression or tension; curve the members and those in tension will try and straighten while those in compression will try and fold up. The more curved they are, the larger they will need to be to stop them straightening or folding. By using a piece of parametric design software called Grasshopper, architect and engineer were able to rapidly explore the effect of varying both member size and degree of waviness on the overall appearance and behaviour of the structure. In the end it was decided to keep a constant 150mm ‘waviness’ and to vary the size of the members (from 60 x 60 to 140 x 80) to respond to the level of applied force. This means that visitors could instantly see which members were working harder adding to the dynamism of the piece; the elements towards ground level are obviously working hardest, particularly the outer chords which attract the largest forces as the prevailing wind tries to blow the structure against the museum.

The curved nature of all the members also enabled the design team to explore different ways of forming the wood into curved shapes. It was originally intended to steam bend the members, but at 8% moisture content, the kiln dried lumber was too dry to steam bend; steam bending could also not have achieved the necessary tolerance. Instead, the chords were conventionally laminated in a press but to achieve the 1m radius of curvature, the thickness of the laminates had to be reduced to about 6mm, compared with the 30-40mm typically used for glulam arches. The advantage of laminating is of course that the strong fibres of the wood follow the line of the member.

Each chord was prefabricated in seven lengths.  Steel rods bonded into the end of each piece ( in order to be joined together on site, with a slight twist to accommodate the geometry as they spiral outwards from the museum. Discreet rods were also bonded into the sides of the most highly stressed elements to prevent the laminates pulling apart under load.

Working in wood made it possible to estimate the size of raw material by examining the finished members. The entire work was fabricated from sawn kiln dried boards about 200mm wide and 25m thick. After planing, the boards measured about 20 mm and thus the braces were 60 mm thick, being cut from three 20 mm boards laminated together. Slicing the boards into three, yields the 6mm laminates used for the chords and it was this in turn which determined the maximum curvature to which the chords could be bent (typically wood can be bent to about 200 times its thickness before it snaps). Finally, sawing the 200 mm faces in two more or less determines the maximum 80 mm width of the braces.

Connecting the pieces
Getting the connections right was a crucial step to ensure that the very complex joints work. At a typical joint, there are two chord sections joining end to end, with four curved braces. If these are outer chords, they also have two ties, joining them to their neighbours. The engineers from Arup designed all the connections in stainless steel so as to avoid staining the oak with mild steel due to reactions with tannins in the oak. The team wanted the connections to be as discreet as possible, as the intended visual effect was that of the structure made entirely from timber.

The braces were ‘flitched’ with stainless steel plates at their ends, which means that thin plates of stainless steel were inserted into the components, and fixed to them, so that they could take up some of the stress as well as forming a method of connection. Bolts are fitted through ‘lugs’ at the end of them. The chords have steel rods inserted into them that are connected with a fixing called a clevis. The design of the structure had to take into account not only its weight and the vast loads it imposes in tension and compression, but also external forces such as wind.

When indicative testing was carried out at the works, the timber elements were found to perform better than expected – a tribute both to the quality of material that had been supplied and to the care and skill that went into making the elements.

Challenging installation
The final stage of the project was its installation on site. Cowley Timberwork preassembled six large pieces in the factory and delivered them to site. Each large piece was connected to its neighbor with extreme accuracy. Then Timber Wave was slowly released from the scaffold, allowing it to become self-supporting before the scaffolding and hoarding were removed, lights were fixed and the majestic structure was in place. Timber Wave remained on display from 19 September to 14 October 2011.

The finished structure demonstrated the unique potential of timber to form complex geometries as well as the structural potential of American red oak. Only by utilising the tremendous strength of the red oak was it possible to achieve such a lightweight structure. David Venables, AHEC European director and the driving force behind the commissioning of the Timber Wave, believes that the perception of American red oak is about to change:  “We hope that this very high profile project will be a turning point in changing perceptions and demonstrating what a good looking, high performing hardwood it really is. We believe the relatively low consumption in Europe is down to a lack of familiarity and awareness of its potential. Red oak’s colour is distinctively different in appearance from the white oaks, being a richer colour with red tones which we believe is potentially its greatest asset”.

The architect Amanda Levete was impressed with the unique qualities of American red oak:  “We have never worked with wood at this scale before or used it as a structural material. I love very human properties of wood, the unpredictability of the grain and the pattern that wood has. There is the beauty of working with a natural material – the perfection lies somewhere deeper, not in the evenness. AHEC finds it interesting that American red oak can be given a low impact oil treatment, which makes it more durable for exterior use”.

DESIGN TEAM
Architect: Amanda Levete Architects (AL_A)
Structural Engineer and material specialist: Arup
Contractor: Cowley Timberwork
Lighting: SEAM Design