Timber! How to Build a Wooden Skyscraper
Wooden buildings are growing, both in size and popularity.
Architects, engineers and environmentalists around the world are getting increasingly excited about the potential for tall timber structures to transform the skylines of the world. The overwhelming majority of large, modern buildings are built using concrete and steel, which are strong, flexible and stable materials.
Although it poses a significant engineering challenge, building a skyscraper out of wood holds a great deal of potential for our cities.
How Can a Skyscraper Be Built Out of Wood?
Building a skyscraper using wood requires rethinking the material itself. If a standard wooden structure rises too high, the wood at the bottom begins to crush under the load above. Traditional timber may not be up to the tall task of building a skyscraper, but timber can be reconstituted into a material that is.
These materials are referred to as mass timber. The most common of these is cross-laminated timber (CLT), created by placing thin layers of wood at right angles, laminating them with fire-resistant glue and then feeding them into a press.
This process results in massive panels, approximately 10 feet wide by 60 feet long and of varying thicknesses. These panels are reportedly equal in strength to steel and concrete.
Architects, with mass timber now available in their toolboxes, are designing more and more wooden buildings, taller than could have previously been imagined. These structures are taller, in fact, than most current regulations permit.
In the face of these regulations, design contests such as the US Tall Wood Building Prize Competition stipulate that entrants must obtain early permission from respective authorities before beginning construction.
As part of this revolution, architect Michael Green and engineer Eric Karsh have published a document called Tall Wood, in which they make the case for tall wood buildings. The report provides in-depth research and case studies outlining how mass timber offers a safe, economical and environmentally friendly alternative for building tall structures. It is available under a Creative Commons license, which permits the sharing and adapting of the research for non-commercial purposes.
At the heart of their proposal is the FFTT (Finding the Forest Through the Trees) solution, a unique system for simply and cost-effectively constructing tall wooden buildings. This system uses a strong column-weak beam structural approach, using mass timber panels for the floors, walls and core.
The integration of steel beams into this system makes the weak beam solution possible, giving the system the flexibility to reach significant heights while still being predominantly constructed of wood. The animation below illustrates one of the proposed structural configurations utilizing the FFTT system.
The structural configuration of a 30-story FFTT building. (Video courtesy of the author.)
Thinking beyond mass timber, researchers like Dr. Michael Ramage, director of the Center for Natural Material Innovation at Cambridge University, are attempting to redesign wood at a molecular level. Plants derive their structure from the cellulose wall that surrounds their cells and researchers are impregnating these structures with polymers to boost their strength, stiffness and longevity.
Bamboo presents a particular point of interest because it has been shown to have a stiffness similar to timber, but up to five-times greater tensile strength and a five-times faster growth rate. Engineered bamboo can be reconstituted into slabs, I-beams and other shapes for use in construction.
Why Use Wood?
Using wood for construction rather than concrete and steel is much better for environment, thanks in large part to a process known as carbon sequestration. Trees naturally capture carbon dioxide from the atmosphere through their biological processes, so a forest is effectively a large reservoir of carbon. When trees decay or burn in forest fires, this captured carbon is released back into the atmosphere.
If, however, the tree is used for construction, the carbon dioxide remains trapped. One cubic meter of wood stores between 1 to 1.6 tons of carbon dioxide. A typical North American timber-frame home holds about 28 tons of carbon dioxide.
The idea of using wood for structures, however, can seem to be environmentally unfriendly simply because it will increase the demand for timber, which in turn will require cutting down more trees.
Sustainably managing forests through long-term replanting of removed trees and species diversification is critical for making this idea work and it is possible. Canada alone has 397.3 million hectares of forest and other wooded land, less than 1 percent of which is harvested annually, and its rate of deforestation ranged from 0.01 to 0.02 percent between 1990 and 2007.
CLT also makes it possible to use lower-grade lumber that would otherwise be unsuitable for structural applications.
The benefits of using wood are not just environmental—there are significant economic and construction benefits as well. For example, wooden sections can be prefabricated in a factory to practically any shape and dimension. Because CLT is much lighter than its steel and concrete equivalents, these sections can be transported and assembled more easily.
UK architect Andrew Waugh claims that CLT structures can be constructed 50 percent faster than concrete, because they require fewer deliveries to the site and construction workers can put them together with cordless screwdrivers and nail guns rather than grinders and jack hammers, cutting labor costs.
CLT modules are very light-weight and can be made with fully furnished interior surfaces that are fully prepared with fittings, which means construction faces far fewer weather delays and installation takes half the time.
Architect Anssi Lassila, designer of the Finlandia Prize-winning Puukuokka apartment building in Finland, the tallest wooden apartment block in the country, explained that the prefabricated modules of CLT were “plugged in like Lego pieces.”
The Puukuokka timber housing block during construction. (Image courtesy of OOPEAA Office for Peripheral Architecture.)
There are also advantages to living in a building made of wood. Wood helps maintain a good indoor climate and helps regulate the indoor temperature. Wooden structures typically have great acoustics and can be left exposed without the need to be covered in plaster or other expensive materials.
Wooden environments might also have a positive psychological effect on people; in 2009 the Austrian Joanneum Research Institute found that children working in timber classrooms were more relaxed, displaying lower heart rates.
Won’t a Wooden Skyscraper Burn Down?
Fire is the most common knock on wooden structures, the pervading sentiment is that a building made of timber will be extremely susceptible to fire. It’s partially due to this concern that many countries placed height limits on wooden buildings in the first place.
The reality is that wooden structures don’t burn as easily as people think. In a TED talk on the subject, Green likens it to trying to light a log on fire with a match. Giant slabs of timber with fire-resistant lamination don’t burn easily.
Wood is actually more fire resistant than steel and concrete due to the fact that 15 percent of its mass is water, which needs to evaporate before the wood burns. Additionally, heavy timber structures have improved fire resistance because the mass of wood is great enough for a char layer to form, which protects the core structure.
Another advantage of wood is that it burns very predictably, so architects can better design buildings to sustain in the event of a fire. Lassila explained, “It has been estimated that, when realized in accordance with the regulations, a wooden apartment building is 50 times more safe than a similar apartment building made of concrete.” Architects working with mass timber are convinced that the fire hazard of wooden structures is more a problem of a public perception than an engineering one.
The tallest timber building in the world today is an apartment block in Bergen, Norway, standing 14 stories tall. However, its days at the top are likely numbered as the seeds for wooden buildings are being planted all around the world. This includes the 80-storey, 300-meter-high “Oakwood Tower,” which was recently presented to the mayor of London by PLP Architecture and Cambridge University’s Department of Architecture.
The race is on: the present may belong to concrete and steel, but it looks like the future might be made of wood.