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Endless Stair, Tate Modern: Step in the right direction

A drive by the American Hardwood Council to boost its sales of timber has led Arup to take on and deliver the challenging task of making architect’s dRMM’s Endless Stair design a reality. Max Thompson finds out how the engineering team did.

Under the looming brick edifice of Tate Modern on London’s South Bank visitors will find another, albeit temporary, modern marvel; architect dRMM’s Endless Stair.

Made of cross laminated tulipwood, the higgledy piggledy Escher-like structure is formed of 15 interlocking flights of stairs that seem to effortlessly and almost impossibly hold one another up.

Commissioned by the American Hardwood Export Council (AHEC) as part of this year’s London Design Festival, the 187-tread, 7.7m-tall installation is the brainchild of dRMM co-founder Alex de Rijke.

Described by de Rijke as “a three-dimensional exercise in composition, structure and scale” it fell to consultant Arup to engineer the fully demountable sculpture. But as senior engineer Helen Groat explains, working out the stresses and strains of such an ­irregular formation proved almost too much for their in-house modelling software.

“A non-linear analysis depends on enormous matrices and when you factorise and multiply them there are any number of ways of doing it,” says Groat.

“The model kept breaking so we had to fiddle with the parameters all the time. If it wasn’t for the Arup programmers downstairs I don’t know what I what I would have done,” she adds.

While cross-laminated timber (CLT) has been around for a decade or so, it has so far only been available in softwood. cross-laminated tulipwood is a new innovation. “When I arrived on the project the wood was ‘on the water’,” says Groat, referring to the fact that the one-inch thick planks of tulipwood were in the hull of a ship en route from North America to Italian manufacturer Imola Lengo.

Those inch-thick planks can come in a variety of widths and lengths from 95mm to 350mm wide and 1.8m to 5.5m long. This is because pretty much all of a fast-growing tulipwood tree can be used for CLT. This includes the uppermost sections that in many timbers are usually regarded as cast-offs because they are too thin and weak.

As well as being low on wastage, tulipwood is fast growing and abundant, making it incredibly sustainable, according to AHEC .

The model kept breaking so we had to fiddle with the parameters all the time

Helen Groat, Arup

In Imola Lengo’s workshop the lengths were planed down to a 20mm thickness and butted up to one another with the grain running in alternate directions. They were then edge-glued along a wavy dog-tooth joint to make large sheets.

“It’s like enormous plywood,” says Groat. “Each sheet is glued at 90˚ to each other and in our stair we used three sheets; it’s usually an odd number.”

The finished CLT sheets are 60mm thick, a size that Groat says on first glance, seems “dangerously thin”. Extensive testing at the University of Trento, in Italy, proved otherwise.

Arup project director Adrian Campbell says a large chunk of that time at Trento was spent trying to understand how tulipwood CLT performed in “rolling shear”.

“When stressed in one direction, which is perpendicular to the fibres, they have a tendency to deform and roll over each other,” explains Campbell.

“We found tulipwood CLT’s rolling shear properties were substantially stiffer; it rolls much less; and it has about three times the stiffness and strength of [spruce] softwood CLT which allows us to keep the elements more slender.”

The cross-laminated panels were then taken to Switzerland where they were made into the individual flights of stairs by Swiss firm Nussli. From there they were trucked to London and the whole installation was completed in under a week.

When NCE visited the stairs, Arup’s engineering theory was being thoroughly tested by a 20-strong group of teenage tourists who had decided to mass on one of the flights looking out over the Thames.

“We knew this would happen… so we took pattern loadings of much higher intensity in certain places,” says Campbell.

“If you took just standard loading from the code you get something like 4KN/m2 for a big crowd. But we thought this is a sculpture… so there was a lot of debate about how many you could get on the sculpture.

“We watched people walking round Tube stations and up and down escalators to see how close they get to one another and we could see most people don’t want to get closer than one tread. So we have a full loading case of one person every other tread but we also looked at pattern loading like this a lot,” says Campbell, pointing at the tourists. “And doubled it to one person at every tread.”

The load from all those teenaged tourists - Arup reckons up to 93 people could be on it at any one time - and from the structure itself, is transferred to the ground via the arching flights. Compressions are carried through the solid balustrades on one side of each flight and also via the treads. The balustrades and treads are screwed and glued along a dog-tooth mitre joint into an L-shape, repeated throughout the structure.

There are no stringers between the treads, just blocks of laminated tulipwood. Groat says the strength of those blocks, which Campbell describes as “large Battenburg cakes”, was crucial to the stairs’ structural integrity.

“The direction of the grain was really important in determining the stiffness,” says Groat. “That was the thing I could play most with. It was quite fun. ”

Being on the highest point of the structure is also “quite fun”; a stairway to the sky that ends with a transparent Perspex barrier that allows punters to ponder the not insignificant drop.

Less fun is Campbell’s demonstration of the “entirely predicted” degree of lateral movement, which he demonstrates by shaking the structure from side to side.

Alex de Rijke

During their education, engineers need to demand more knowledge about timber construction processes

Alex de Rijke, dRMM

Back on the somewhat stiffer level below, Campbell executes a drop-heel test which results in obvious vibrations. “The frequency of the structure is about 3.3Hz,” he says. “So you would have to move at three steps a
second to match it which is higher than we thought it would be; so we’re getting more stiffness than we expected.”

At the base of the last tread on each flight is a narrow concrete plinth, which as well as footing the structure also keeps the feet dry. Originally the foundations were going to be tied together with ratcheted crane strops, but two weeks before the build that option was ditched as it was deemed too disruptive to the landscaping.

Instead, Campbell says made ground conceals a pad foundation designed to take the vertical loads and lateral thrust created by the arching flights.

Judging by the reaction of the people clambering up the structure, and the attention it has received in the press, the Endless Stair has been a success for London Design Festival. But those involved in the construction have even greater aspirations.

dRMM helped pioneer the use of softwood CLT back in 2006 with its Naked House scheme in Oslo. It has also used CLT nearer to home, more recently at Clapham Manor School in London.

But for hardwood CLT it is still early days. For instance, the size of the panels used in the Endless Stair were limited by the size of Imola Lengo’s press to just under 1m in height.

Nonetheless, de Rijke is convinced that the successful realisation of the Endless Stair heralds a “new chapter for hardwood CLT”, and he reveals that he is drawing up plans for a couple of buildings that he hopes will use huge 5m by 45m wall panels.

“The message to get across is that during their education, engineers need to demand more knowledge about timber construction processes,” says de Rijke.

For de Rijke , who believes tulipwood CLT could in many cases be a substitute for concrete, the opportunities offered by the material are, well, endless.

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