Wooden Skyscrapers are, Well,..On Fire

February 17, 2020


“To get attention, you have to build tall,” said Øystein Elgsaas, a partner at the architecture practice behind the record-breaking tower, Voll Arkitekter, in a video call.

“And when you have the world’s tallest building made of timber, everybody says, ‘Wow, what’s going on in Norway?'”

“People are interested, and that is actually the most important part of this building — to showcase that it is possible, and to inspire others to do the same.”

The record-breaking feat was realized thanks to a type of engineered wood called cross-laminated timber, or CLT. Part of a larger group of materials known as mass timber, it is produced by gluing strips of laminated wood together at 90-degree angles to one another, before they’re compressed into huge beams or panels under extreme pressure.

The resulting wooden towers — sometimes dubbed “plyscrapers” — were once the preserve of conceptual designers. But thanks to changes in building regulations and shifting attitudes towards the material, they are quickly becoming a reality.

A slew of new timber high-rises is set to break ground or open in 2020. HoHo Vienna, a mixed-use development just five feet shorter than Mjøstårnet, has just opened for business in Austria. And while Europe has traditionally led the charge, North America is quickly catching up.

In Vancouver — a city already home to a 174-foot-tall wooden student residence — the Pritzker Prize-winning architect Shigeru Ban has designed a “hybrid” condo complex comprising a steel and concrete core with a timber frame that will open this year. Meanwhile in Milwaukee, Wisconsin, work on a 238-foot wooden apartment block, Ascent, is set to begin in June.

Advocates for mass timber claim that, compared to existing alternatives, these towers are quicker to construct, stronger and, perhaps most surprisingly, safer in the event of a fire. It may, however, be their green credentials that explain wood’s rising popularity in recent years.

The construction and operation of buildings accounts for 40% of the world’s energy consumption, and approximately one-third of greenhouse gas emissions. But while concrete emits a huge amount of carbon, trees instead absorb it throughout their lifetime.

If those trees are then turned into mass timber, that carbon is “locked in,” or sequestered, rather than returned to the atmosphere when the tree dies. Studies suggest that 1 cubic meter of wood can store more than a ton of carbon dioxide.

The developers of Milwaukee’s Ascent apartment complex, for instance, claim that its use of timber represents the equivalent of taking 2,100 cars off the road.

“Trees store carbon, so if you harvest them at the right age when they can’t absorb much more or grow much further, then it’s a better solution to use them as a building material,” said Elgsaas, adding that, if buildings are designed with longevity in mind, they could keep the carbon out of the atmosphere for generations. “It prolongs the trees’ lifespans (before they decompose) by maybe 100 or 200 years, if done correctly.”

But while these architects clearly believe in mass timber’s structural potential, there remain very practical barriers to the realization of such projects: building regulations.

The latest update to the International Building Code (IBC), which many countries and US states use as a base model for their own regulations, will allow timber buildings to rise to 18 stories for the first time. The decision is significant given that, before 2018, when Oregon became the first US state to allow 18-story wooden buildings, nowhere in America permitted anything higher than six.

The changes will come into effect in 2021 — though they are only advisory. Some countries, such as Norway, already has looser height restrictions in place, while other countries and US states may opt for tighter building codes than those outlined in the IBC.

What traditional buildings can teach architects about sustainability

And there remains limited data about how large wooden towers will respond, in the long-term, to a variety of risks, from extreme weather to termites and damp.

The most contentious question remains fire risk. The National Association of State Fire Marshals, for instance, opposed the recent update to the International Building Code, citing a lack of requisite fire testing, among other concerns. In a statement, the organization said the changes were the result of “professional judgment” rather than science, adding that allowing larger wooden structures “without proper testing and justification” was “premature and would impact the fire suppression environment significantly.”

The concrete industry has also been a vocal critic. According to Build With Strength, a US coalition formed by the National Ready Mixed Concrete Association, cross-laminated timber is “an unproven material that poses major fire risks, especially in high-rise construction.” In addition to concerns about deforestation, the group says sprinklers are ineffective at preventing blazes from spreading through wood buildings. It also cites research suggesting that exposed CLT panels can lead to the “re-flare and re-growth” of fires.

Supporters of mass timber, however, contend that it’s not only safe — it’s actually preferable, as wood burns in a more predictable way.

Studies have also shown that a seven-inch-thick CLT floor has a fire resistance of two hours, which the US Department of Agriculture’s Forest Department says “will address concerns about the fire performance of wood buildings and help take them to new heights.” Steel, on the other hand, is prone to sudden collapse, said Elgsaas. At certain temperatures it can “lose its load-bearing capacity and turn to spaghetti.”

Green compares mass timber to a big log placed on a campfire — it doesn’t catch light immediately, and it burns slowly once it does.

“In a big catastrophic fire, generally, if you ask firefighters to go into a heavy timber building versus a steel building, they would much rather go into (the former),” he said. “Because although the beams are charred, they can quickly tell how much char, and therefore how much leftover wood, there is.”

Regulations invariably lag behind technology, Elgsaas added, with each completed tower helping to ease concerns around efficacy and safety.

“The more buildings we see that push the limit, the easier it will be to propose new building codes and raise the bar on what’s possible,” he said.

13 Responses to “Wooden Skyscrapers are, Well,..On Fire”

  1. terrydonte Says:

    It will make a nice fire when not if it catches fire in the future. It will burn much better than those British high rise structure which had less fuel in them.

    • Did you actually read the article or are you just a bot?

      Given your Gravatar I suspect the latter

      • redskylite Says:

        This respondent has had more handles than I can remember.

      • terrydonte Says:

        I read the article, 2 hours is not very long if one considered the timber burning is holding up the building which is going to fall down a long time before the two hour burn through test. As to steel if heated it does weaken a whole lot, that is why one sprays a heat resistant coating on the steel, actually it is a fibrous cement mix. My point is that buildings have a problem with fire blocking exits and spreading on the floor it started on and to upper floors of the building. This building would need an even greater fire suppression system than a non combustible steel frame building and there is no indication in the story that was considered in the design except for the nonsense about 2 hour fire resistance.

  2. Interesting item fron February last year. 7:43 addresses the fire safety issue and shows timber performs as well as or better than conventional materials in seismic and blast tests

    • terrydonte Says:

      The problem with fire safety with a combustible building is the same one which killed those people in the British high rise fire. The owners renovated the building by putting a combustible exterior on the building which met all the British codes of the time. This wooden building is inherently combustible so you would need fire suppression, sprinklers in the entire building , in every room and every corridor which would need to be supplied with water from a source to give at least 20 minutes of water per current US codes. That is one huge tank on top of that building if one is simply sprinkling one floor, even larger if more than one floor.

  3. dumboldguy Says:

    Don’t think wooden “skyscrapers” are ever going to be the rage. 10 to 20 story high rises are a different story, and the building in Norway is quite something even if a bit ugly—it’s in a beautiful location.

    We see a lot of 4 and 5 story all wood condo and apartment buildings being built in the DC area, and they regularly have nice blazes that appear to be more frequent and severe than those in similar brick-steel-concrete structures. One under construction here just burned in a huge and spectacular blaze that the TV folks loved.

  4. Bill Ramsay Says:

    There’s one under construction here in Wellington. Featherston Street, by Robert Jones Investments.

  5. Brent Jensen-Schmidt Says:

    Can always build normal size dwellings with wood despite them being boring.
    Storing carbon, long time in wood, beats the hell out of energy intensive mechanical extractive boondoggles.

    • rhymeswithgoalie Says:

      I often wonder how much carbon is sequestered in old neighborhoods with A-frame houses and wooden furniture. Pine tree crops are an old way to pull carbon out of the atmosphere.

      • dumboldguy Says:

        My house is not an A-frame, but it does date from 1972 and is sequestering a lot of carbon in its wood. Some termites found it attractive a few years ago, and chewed some of it up—-I hope there wasn’t much methane and CO2 released in termite farts when that happened.

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