Wooden Skyscrapers – To be or not to be?
When I think of the word skyscraper, I think of steel and concrete structures soaring to the heavens. I would think that is what most people think of. And why not, that is what these structures require to achieve the design concept that architects and designers envision. But does that need to be the case? Can skyscrapers be built with wood?
There have been some interesting ideas that have popped up recently about building tall wood structures. Let’s take a look at what these concepts are, the questions that come up when talking about them and what does the future hold for these types of structures.
Architect C F Moller has recently proposed a 30-story wood skyscraper in Stockholm, Sweden.
Michael Green, an architect in the Vancouver area, has a concept for a 30-story wood building as well.
There are numerous other examples of large wood structures and concepts out there, but these are a couple of recent ideas to get the conversation started. I watched a YouTube video of a presentation by Michael Green and Andrew Waugh on this subject. Much of the info in this post is based on their presentation. You can see the video at http://www.youtube.com/watch?v=O4XLRLY29iw.
It seems that there is a push towards looking at wood as an alternative to concrete and steel in the design of larger structures. Wood is a more sustainable product than concrete and steel and has less of an environmental impact. As an example, if you are concerned with CO2 emissions, the production and use of concrete is responsible for approx. 5% of CO2 emissions worldwide. Wood on the other hand has the ability to store (sequester) CO2. A cubic meter of wood can sequester 1 ton of CO2. Creating buildings with smaller footprints is driving this interest in tall wood structures. Additionally, a big selling point with these buildings is the speed with which they can be erected as opposed to concrete and steel.
How do you build a tall wood structure?
Building these tall structures takes some new ideas and techniques. You can’t build your 2x walls, put a floor box on top and keep on going.
Mass Timber is the answer.
This refers materials such as LVL, LSL, Glu-lam, and CLT (Cross laminated timber).
These are the building blocks for these large wood structures. I’ve heard of the first three materials above. The CLT was something I had not heard much about.
CLT’s are manufactured panels made up of layers of lumber that are glued together – each layer set 90 degrees from the previous. It’s basically plywood on steroids.
Initial development of CLT started in Switzerland in the early 90’s. By the early 2000’s, CLT use began to increase substantially in Europe and is beginning to be used in North America. Panels in North America are available up to 19.5” thick, 18 ft. wide and 98 ft. long.
By using these building materials, it is possible to build tall wood structures.
There at number of concerns that are brought up when talking about these structures.
Codes – Current building codes do not allow for a wood structure of the heights that are being proposed. There will need to be some code revisions required to make these structures a reality.
Seismic – Can these buildings resist earthquake loads? It has been shown that these buildings do perform well under seismic loads. Depending on the height of the building, the design may incorporate some steel to help with these loads. Because the wood structure is considerably lighter than a concrete building, there is less weight to be resisted from seismic forces.
Shrinkage – Wood does shrink and that could be an issue when dealing with tall wood structures. In Mr. Green’s presentation, he talks about their solution to this by building the floors within the exterior structure as opposed to a platform type building.
Moisture – Many people think that the buildings will be sitting out exposed to the weather for long periods and soaking up the rain and wind. Like other wood structures there will be a building envelope on the structure to protect from the elements.
Acoustics – Concrete is very effective in dealing with sound transmission. This is a challenge with a wood building and will require some attention to detailing how this can best be handled
Fire- Based on the building codes, buildings need to be able resist the two-hour fire. They need to be able to stand for two hours to allow for people to get out and firefighters to get in a put out the fire. There are a couple of different ways that Mr. Greene proposed to deal with this issue. First is to put 2 layers of fire rated drywall on the whole structure to get the two-hour rating. Second is a method called “charring.” They can look at the rate at which certain wood burns and calculate how much of a structural member would burn over a two-hour period. If enough of the member is left to still be able to support the structure, then the two-hour criteria has been met.
Past and Present
Tall wood structures have been around for a long time. A good example is the Japanese pagoda. This one stands 122 ft. tall.
These structures have stood for hundreds, even thousands of years all the while enduring earthquakes, typhoons and whatever Mother Nature could throw at it.
I see no reason why we can’t build a wooden skyscraper with the advances in engineered wood products and the new and innovative building practices being explored today.
Stay tuned and let’s see what buildings may start reaching towards the sky.
Bill Hoover – Design Professional
Gould Design, Inc.