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Sustainable hardwood CLT structure unveiled

The hardwood Smile structure has been unveiled as part of the London Design Festival. It is aiming to highlight the use of hardwood as a sustainable building material.

The engineering team behind the structure is also responsible for the Endless Stair which was built three years ago. That structure was the team’s first venture into using tulip hardwood as a building material, but as Arup associate director in advance technology and research Andrew Laurence explained, the Smile project has pushed the boundaries of the material even further.

“We used it for the endless stair but at that stage we were making tiny panels, but this time we made 4.5m x 14m panels, the largest hardwood cross laminated timber [CLT] panels ever made,” he said.

The 34m long sculpture is in the form of a “smile” with two cantilevers raised 3m up on either side, giving the appearance that it could rock from side to side on its central axis when loaded at either end.

“We didn’t want it to rock, it might be quite fun if you’re inside, but not if you’re standing underneath,” said Laurence. “So underneath is a box containing about 20t of steel weights which counter balances the structure against the weight of the 60 people we’ve designed it to take.” 

Part of the reason for having such a “large” foundation, explained Laurence, was due to the poor ground conditions under the square in the grounds of the Chelsea College of Art and Design in London. 

“The old Milbank prison is directly underneath,” he said. “When we were excavating we exposed some of the remains of it which created the hard spots and soft spots. So we needed a big foundation to spread the loads out.”

Twelve of the large panels varying from 100mm to 220mm in thickness were used in the construction of the rectangular curved box. Cross bracing to stop it racking over has been eliminated, replaced by portal action being formed by tying the walls and the roof into two glulam beams which run along the length of the structure.

The size of the structure was dictated by the size of the panels which were fabricated in Germany by CLT specalists Züblin-Timber, brought to the UK by lorry and assembled on site.

A total of 6,000 screws from 160mm to 500mm in length hold the structure together.

“The point is that you can install these with a battery operated screwdriver so it’s quick and quiet,” said Laurence. “This is a joinery wood so it was previously used for furniture, but what we discovered was that it’s twice as strong but the same density.

“This means that you don’t have to pre-drill a hole. The economics of it is that the cost of building in timber is in the connections, so being still able to use the same self-tapping screws is really important.”

The bending strength of the tulipwood is around 40N/mm2 which translated to the CLT panels. It is this strength which has allowed the structure to cantilever out and create an opening at its centre where the stresses in the panels are the highest.

“The tension over the door and compression underneath is significant, so we needed to make it [the panels] thicker to accommodate the screws in two rows,” said Arup engineer advance technology and research Ishan Abeysekera.

“Basically we started off with a GSA model, but because it’s fibrous it’s difficult to model the panels, so we were more confident to do it by hand.” 

The myth of unsustainable hardwood 

The tulipwood, showcased by the Smile structure, is a fast growing hardwood that was sourced from North America.

“This wood grows twice as fast as spruce and can therefore be harvested after 40 to 50 years,” said Arup associate director in advance technology and research Andrew Laurence. “This is a fast growing hardwood.”

He said that there was a misconception in the industry that hardwood was an unsustainable building material and that there was a need to highlight the difference between tropical hardwood and wood grown in the temperate forests in North America.

“Let’s not confuse the tropical with the temperate forests. There are problems with the tropical rainforests, but this is from North America where it is managed,” he explained.

Laurence added that a third of the forests in North America and in Europe were hardwood trees which were not being used and therefore it was a large underused resource.

“The thing about the hardwood is that they’re not only the most beautiful woods, but they’re also the strongest. A CLT [cross laminated timber] wall panel can be structure and architecture at the same time. So there’s a real logic to using the hardwood.”

Sourcing the wood is a managed process which is very sustainable, according to American Hardwood Export Council executive director Mike Snow. He explained that in the majority of the land from which the timber was sourced is privately owned, with the average plot less than 100ha in size.

“There are around 9M hardwood land holdings in the US. They own it for camping or wildlife or hunting and usually they’ll go in and thin the woods maybe once a year. The timber manufacturers will then buy this thinned timber from around 500-700 of these landowners but then next year they’ll buy it from a completely different set of 500 owners,” he said.

“It’s that relationship between the timber industry and the small land owners which will help to maintain this.”

Thinning the forest, added Snow, was necessary because dead wood on the forest floor not only released carbon back into the atmosphere, but it acted as perfect kindling for forest fires. He also pointed out that thinning by opening out the forest allowed light to penetrate through the canopy and create a more diverse range of species needed for different ecosystems to thrive. This process has allowed the industry to create a growth to removal ratio for hardwood trees of 2.4:1, including natural mortality and clearing for settlements.

“This structure used around 330m3 to 340m3 of American tulipwood and that includes the waste that didn’t make it into the structure,” said Snow. “People often ask how many trees does that equate to, but I think it’s better to say how long it takes to regenerate that.

“In the US it’s five minutes. We have a fantastic growth to removal ratio.”

To manage the amount of trees which were available to the industry, he said that logging companies used satellite data from forest inventory analysis surveys carried out every four years. More recently, this has been supplemented by the US government making data from its military satellites available.

“The data is so accurate that not only can we count individual trees, but individual branches and we’ve actually found that we’ve underestimated the amount by 14%,” said Snow.

Overall, he said that the way the industry was managed stacked up to making this one of the most sustainable and economic building materials available.

“The Smile is made from the lowest grade of tulipwood,” said Snow. “One of the things from the environmental perspective is that the high grade sells very well, but every log produces a high and a low grade so you need to create a market for both. This provides a good use for that.”

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