The Endless Stair

Pioneering the use of hardwood CLT, The Endless Stair is a bold installation in American tulipwood born from a series of collaborations between the London Design Festival and AHEC. Taking inspiration from the drawings of Escher, dRMM and Arup created this extraordinary towering structure.

Vision

The latest in a series of collaborations between the London Design Festival and AHEC, the ambition for this project was to pioneer the use of hardwood for a CLT installation at the London Design Festival 2013. 

CLT is typically made from softwood, yet this project aimed to demonstrate the real potential for using tulipwood, an abundant, relatively inexpensive and structurally impressive American hardwood.

Design Process

The Endless Stair is an installation that seemed to challenge the rules of perspective, took its inspiration from the drawings of Escher. But unlike the works of the Dutch graphic artist, whose designs were famously mathematically impossible, the Endless Stair was not only realisable but actually achieved. 

It consisted of a series of ‘handed’ timber stairs, some veering to the right and others to the left, offering a number of routes to a top flight that culminated in a viewing platform.

Alex de Rijke of de Rijke Marsh Morgan is committed to designing structures that do not waste materials. So part of the thinking behind the project was a desire to make the elements as environmentally friendly as possible, with each flight of stairs built up from standard elements, as little waste as possible in construction and the ability to re-use and relocate the design either in part or as a whole. 

‘We have tried to dimension the panels according to the size that can be laminated,’ he said. ‘They can be simply cut with no waste.’ Rijke says. 

The challenge of using timber in a new way seemed therefore made for him, particularly as he had written that ‘Swiss, Austrian and German development of laminated mass-timber construction techniques (with increasingly fine consequences) are now challenging the preconception that timber is modern architecture’s poor relation.’

Making

The Endless Stair was much more than just a piece of art: it pioneered the use of hardwood in CLT. A fast-establishing technology, CLT is usually made from softwood, but AHEC believes that there is real potential for using tulipwood, a sustainably abundant, relatively inexpensive and structurally impressive American hardwood.

To test this idea AHEC worked on the Endless Stair with architect dRMM and engineer Arup, to create a design that was not only visually exciting but that also tested the potential of this new form of timber. Manufacturers in Italy and Switzerland made the elements, which were assembled quickly and efficiently on site. 

CLT is an engineered timber product that is used increasingly to create the walls and floors of buildings. It is of a ‘sandwich’ construction, normally with an odd number of layers in the sandwich. On each successive layer the fibres of the timbers run in opposing perpendicular directions, so that if you could look through the CLT from above you would see a kind of grid of fibres. It is orthotropic – that is, it has different properties in three directions. This is important because timber is strong along the directions of the fibres, and less so in the cross direction.  Modern offsite manufacturing methods mean that CLT panels can be made in a factory and then delivered to site for assembly in a fast, rapid and accurate manner, cutting down on the time needed for construction and the risk involved.

By assembling the material in layers the structural impact of individual pieces is averaged out across the panel, ensuring better control of material properties and also allowing lower quality material to be used that would otherwise be unsuitable for this kind of structural joinery. The use of No 2 Common grade (NHLA visual grading standard) tulipwood resulted in a 40% saving in cost compared to the FAS grade normally used for joinery applications.

The manufacturers of CLT panels, most of which are based in northern Europe, use softwood – most commonly spruce. The question that AHEC posed was, would it be feasible to use a stronger hardwood to make the panels?

Solid CLT panels have inherent fire resistance and therefore can be left exposed in finished structures without applied fire protection. Hardwood CLT therefore offers the benefits of a fire resistant material combined with the visual appearance and inherent strength of the American tulipwood. 

Outcome

A towering Escher-like structure made from American tulipwood CLT installed outside Tate Modern for the London Design Festival 2013. Endless Stair was both a sculpture and research project, advancing the knowledge of timber technology and sustainability. 

Equivalent to two-storeys in height, the network of stairs allowed visitors to enjoy the capital from different viewpoints, looking at the art gallery, the Thames River, the Millennium Bridge that crosses it and, on the other side, St Paul’s Cathedral. It also encouraged social interaction.

Source: American Hardwood Export Council