A bridge made of engineered wood; specifically, cross-laminated timber! Even Madison’s hadn’t expected such anon-traditional application of forest products as a building material.
Researchers at South Dakota State University’s J Lohr Structures Lab have tested the strength of a glulam bridge as part of a push to give counties and towns more options when designing new bridges or replacing old ones.
BRIDGES MADE FROM GLULAM
Glulam is making its way from buildings to bridges, becoming a cost-effective, viable and sustainable alternative to steel and concrete, said the Fifth Estate Australia October 17. Australian Bridges on low-traffic county and township roads must be strong enough to withstand great weight – enough to support the typical 15 fully loaded trucks per day that cross these roads, said leading investigator Mostafa Tazarv of South Dakota University.
To prove the glulam bridge’s reliability and strength, the researchers applied a 32-kip – or about a 14,500 kilogram-force – load to the middle of the tester bridge, approximately equal to one lane of traffic, at a rate of one load a second.
Glulam construction costs can also be anywhere from 25 to 50 per cent less than conventional bridges, Tazarv said. As well, they can be installed in a day without specialized equipment or trained personnel.
A sustainable alternative, glulam has low levels of formaldehyde and can be reused to make longer pieces of straight or curved timber. The annual glulam consumption in Australia is approximately 30,000 cubic metres, which is only 0.6 per cent of Australia’s total timber consumption, according to the Glued Laminated Timber Association of Australia.
The town of Neckartenzlingen near Nürtingen in southeastern Germany has needed a new bridge for a few years, now the town is celebrating the opening of an innovative timber structure, said BridgeWeb November 7.
Since the Neckar River curves at the bridge location, it seemed appropriate to choose an S-shaped plan geometry. As far as bridge type was concerned, two options were compared; a two-tower suspension bridge and an alternative with a continuous support beam mounted underneath with a ‘block laminated’ timber beams. This consists of glulam beams which are then glued together to form a block, hence the glueing process is in two directions.
The idea was to emulate the form of timber cantilever bridges, whereby supporting elements are stacked one on top of another on the support point. Towards the end of each cantilever arm, the number of supporting elements is reduced, thus resulting in an aesthetic narrowing towards the centre.
With a total length of 96.3m, it became obvious that dividing the bridge into three spans was a practical system, so that it crosses the Neckar River on a 44.5m-long free span in the middle of the structure, with two 25.9m-long spans, one on each side. For feasibility reasons, it was decided from the outset to provide a two-part cross-section, with a gap in the centre providing space for cable conduits.
The basis for the construction is defined as a simple, continuous three-span Gerber beam, the section height of which varies according to the bending moments. The variation in cross-section, which makes the bridge so visually distinctive, is due both to the statics and to efforts to optimise the production process of the wooden structure. For construction, glulam beams with a decreasing cross-section were simply stacked on top of one another and glued.
The choice of timber for the structure was a deliberate one, with integration into the unspoilt surroundings being one of the most important criteria. Also, the challenging geometry resulting from the two curved bridge elements particularly lent itself to the material. Economical construction can easily be achieved using compact glue-laminated timber construction.
Proposal for Tallest Wood Building in London: Made of Wood
The conceptual 80-storey building called Oakwood Timber Tower “is the largest use of cross-laminated timber (CLT) in the U.K. and in Europe,” Kevin Flanagan said at the recent Green Building Festival in Toronto, ON, according to Daily Commercial News October 17.
Flanagan is a partner at London’s PLP Architecture which is behind the tower that would have as many as 1,000 residential units in about one million square feet in the heart of London. PLP has partnered with engineering firm Smith and Wallwork and Cambridge University’s Centre for Natural Material Innovation on the project.
The tower is engineered in a series of quadrants or bundled columns, much like Chicago’s 1,450-foot-tall Willis Tower, formerly the Sears Tower, Flanagan said.
Flanagan’s architecture firm is no stranger to innovation. It designed The Edge, an office building in Amsterdam that was awarded the Building Research Establishment Environmental Assessment Method’s (BREEAM) greenest office building in the world last year. A world standard, BREEAM rates and certifies the sustainability of buildings.
TALL WOOD BUILDING IN THE US
For years, glue-laminated beams have been allowed in US construction, but now the International Code Council’s ad-hoc committee on tall wood buildings, which has been meeting since 2016, is working to change the International Building Code to allow mass timber products to be used to build taller structures. In May and June, the committee tested five fire scenarios on a two-story, CLT-constructed building and were successful, said the National Real Estate Investor, November 1.
There’s the 18-storey T3 project in Minneapolis, MN, made from pine felled by mountain pine beetles.
The Framework project in Portland, OR, which received approvals over the summer and is slated to begin construction next year, will stand at 12 stories, or 148 feet, tall—63 feet above what’s currently allowed by building code and a record for the United States. It will be constructed out of a mass timber product called cross-laminated timber (CLT), which only a small number of manufacturers in the US can supply. There is also an effort to incorporate mass timber products into new code regulations to build taller structures.
In another part of Oregon, Swinerton Builders is overseeing the construction of the largest-known building that uses cross-laminated timber, according to ENR Northwest. The project, which will house the corporate offices of First Tech
Federal Credit Union Oregon, will be 156,000 sq. ft. in size and five stories high.
Elsewhere, Lendlease, a multinational property developer, owner and builder with many CLT projects in Australia, opened the first CLT hotel in the US, the Candlewood Suites on Redstone Arsenal in Alabama last year. The project saw a 20 per cent faster completion time and was constructed with 30 to 35 per cent less work-force, said Ben Symons, general manager of communities and infrastructure at Lendlease to the Investor.
The four-story hotel, Lendlease’s first US project built of CLT, was a partnership with the US Department of Defense as part of the Privatization of Army Lodging Program. For Lendlease, using CLT for this project was cost-competitive to light gauge steel frame systems, which had been used in other Lendlease hotels, the company said.
ADVANTAGES OF BUILDING WITH ENGINEERED WOOD
In the UK, the Committee on Tall Wood Buildings earlier this year conducted a test at the Fire Research Laboratory in Virginia, a state-of-the-art facility that basically lets scientists set things on fire and then watch what happens, according to BBC October 31. When they set alight two one-bedroom apartments made of engineered wood, the fire raged until it had burnt through the furnishings, then extinguished itself. The contents were turned to ash, but the structure itself charred and remained intact.
Other than preventing the spread of a fire, one of the most important factors is what happens to CLT when it’s heated. In this respect, the material wins hands down over steel and concrete, which tend to melt and weaken.
Then there’s the issue of weight. For a long time Murray Grove, a nine-storey housing block in Hackney, UK, was the highest in the world.
“If we’d made it from concrete it would have taken 900 HGVs [heavy goods vehicles] rumbling through London to deliver all the material,” said Anthony Thistleton, a founding director of Waugh Thistleton Architects who designed the building, to BBC.
In the end it took only 100 heavy trucks.
As well, they’re quicker to build, since even steel skyscrapers have concrete floors which can take weeks to dry. That’s several weeks per floor. On the other hand wood panels can be sliced to exact dimensions in the factory and then slotted into place within a matter of hours.