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Cross-Laminated Timber and the Race for Wooden Skyscrapers (bloomberg.com)
43 points by Osiris30 on July 29, 2016 | hide | past | web | favorite | 22 comments



"...there’s a great deal of evidence to suggest mass timber is actually safer in a fire than steel.

For example, in a fire, a thick plank of wood will char on the outside, sealing the interior and protecting it from damage. Wood burns slowly at approximately 0.02 inches/minute and the char created on the wood surface as it burns helps protect and insulate the unburnt wood below and maintain the structure.

This is because the build-up of carbon on the surface will limit the oxygen supply to the wood below and act as insulator. Therefore, the wood below the charred level will be cool and retain 85 to 90 per cent of its structural integrity.

Metal, on the other hand, begins to melt when it reaches a critical temperature (around 1300 degrees C) so will fail catastrophically. B.J. Yeh, from APA described steel being “like spaghetti” at this stage."

From: http://www.internationaltimber.com/news/timber/how-does-timb...

This is an intriguing argument, but aspects of it bother me. For example, steel does not function well as fuel. Other things must burn in order for it to reach melting temperatures. With wood, even if the inner core remains cool and protected by a layer of char, the fire will be able to spread by using the outer layer of the structural wood as fuel. A building with steel supports may catastrophically fail when its supporting structure melts, but the fire may well expend all available fuel before reaching that point. With a wooden supporting structure, the fire could keep spreading and burning until it is put out. So while CLT may well be superior to Steel in how long it can bear weight in a fire, it may also sustain longer and more intense fires than a building with a steel support structure.

While I'm concerned that massive wooden buildings may be more prone to producing out-of-control fires, this is still a technology well worth pursuing. Wood provides a nearly optimal blend of being low-cost and environmentally friendly.


> Metal, on the other hand, begins to melt when it reaches a critical temperature (around 1300 degrees C) so will fail catastrophically. B.J. Yeh, from APA described steel being “like spaghetti” at this stage."

Heat a piece of steel up to 800 degrees Fahrenheit (427 C) and you can easily bend it by hand. There's no need for it to actually melt before it loses its integrity. That's one of the main properties that makes steel useful to humans.

Edit: Not a 9/11 truther or anything - I just know this from first hand experience with metal working.


>Edit: Not a 9/11 truther or anything - I just know this from first hand experience with metal working

Isn't the knowledge that steel loses structural integrity at a temperature way lower than that required to melt it a sign of not believing in a 9/11 conspiracy (because 'jet fuel can't melt steel beams')? I'm confused.


Good point. What I should have said was, "I noticed some guy a few comments down talking about 9/11, and in no way should you construe this comment as being related to that silly, silly discussion. Please. I don't care if you do or do not believe that the ghost of Nixon is in a CIA satellite reading your lips and writing down everything you say. There's another forum for that somewhere."


From a safety and structural engineering standpoint, I'd assume that the goal of fireproofing isn't to make buildings that can withstand a fire, but to make buildings that can resist a fire for enough time until the building is evacuated and the fire department arrives.

Hence the reason why fireproofing is rated in hours, at least in the US.


That argument is interesting. I follow the logic, but I am not sure it's correct. To put it simply, my experience tells me that wood burns. I have seen houses that have burned, and barns that have burned, and trees that have burned, and they don't look so good. Do they have a demonstration?

Build a small structure out of the stuff, and light it on fire. Build one out of metal, light it (its interior framing? Carpet? Gas leak? Wood floors?) on fire and compare the results.


It sounds like we have the same concern. A free-standing steel frame does not readily burn while a wooden frame will. This is obvious.

In a complex building things may not be so simple. Assuming that everything around the structure is great fuel, it's possible that a wooden frame won't significantly increase the speed at which the fire spreads and may actually remain structurally sound longer than a steel supporting structure would. In this case, you might have more time to put out the fire before the building collapses.

Of course, the obvious question is: why not reduce the amount of combustible building material so that fires don't spread so quickly and there's simply not enough fuel to heat a steel support structure to the point of failure? My intuition says that an ideal building shouldn't readily burn to the point that it's structure fails. If part of it catches fire, the fire should burn itself out before this point is reached. I am not an engineer though, and my intuition may very well be wrong. Obviously, fires do happen in glass and steel buildings.

We can build steel-framed buildings that will readily burn to the ground. Can we build wood-framed buildings that won't? This is a complex question to which I don't know the answer.


I sincerely apologize for linking to a 9/11 denial site, but:

http://www.serendipity.li/wot/other_fires/other_fires.htm

We have plenty of steel skyscrapers that have burned for a LONG time and have not collapsed. Huge fires. Towering infernos. This is a good thing, not only for the residents but also for the neighbors of said structures.

I simply do not believe that you can build a building with wood supports that is going to withstand the conditions in those photos without collapsing. Wood skyscrapers that burn are going to collapse with considerable collateral damage.

This whole debate is an example of not remembering the past and being doomed to repeat it. Corporate greed to save a few bucks during construction will eventually result in preventable tragedies.


Typical mix of everything under the sun... here's a better page on skyscraper fires: http://911encyclopedia.com/wiki/index.php/Building_Fires

I made a 40min documentary on the WTC fires: http://s3.amazonaws.com/nasathermalimages/public/video/Prete...


Some countries used wood for ablation shields for some re-entry vehicles.

http://fas.org/nuke/intro/missile/basics.htm

Google Books https://books.google.co.uk/books?id=2900AQAAMAAJ&pg=SA4-PA4&...


Usually the timber itself is encapsulated with several gypsum layers adding a usual fire and charing resistance between 60 and 150 minutes. So that the rooms are forming cells in which the fire is contained for a definite time - allowing the safe rescue of people.

The jump of fires between floors via the facade is countered by constructional rules e.g. using firewalls (protruding fire diverting metal, wood, concrete bars) or having no burning sequments on the facades.

Also node worthy metal and concrete structures have a very unclear burning behavior and do not allow for a safe estimate of the structural integrity. But wood has constant and know charing rate so that an estimate of the structural integrity is far safer and easier to estimate.


Also, structural steel is sprayed with fiber or cement-based fireproofing.


The article mentions proposed 100 and 40 story tall buildings made of wood. This would be a huge leap since it seems like the current tallest building is the 14 floor Treet in Bergen (and an 18 story building is under construction news.ubc.ca/2015/10/01/new-ubc-student-residence-to-be-among-worlds-tallest-wood-buildings/ )

and I found a paper that says that Cross Laminated Timber isn't part of the main load bearing system in Treet anyway: http://link.springer.com/article/10.1007/s00107-016-1022-5



Since this is HN and we're obligated to talk about the website over the article, can I just say that I literally can't read this article thanks to a 'you're using an ad blocker' message. No I'm on my phone and have no ad blocker and can't view this from my PC since my internet is out. Thanks, Bloomberg, great user experience.


I will follow your rabbit trail. I got the same message here with Chrome on the desktop without any ad blocking extensions installed. However, I was able to close that popup and keep going. Odd


Firefox mobile with ublock origin and ghostery does a good job of ad blocking. No popover here.


Glued laminated timber has been around for decades. It's sometimes used for roof arches.[1] I once saw a Safeway supermarket with a roof built that way. The arch elements were about a foot square in cross section.

[1] https://en.wikipedia.org/wiki/Glued_laminated_timber


> Glued laminated timber has been around for decades.

It says that in the article.


Many garages have such a timber across the top of the door (you can't see it under the siding and paint). Those things are expensive.


How exactly is this considered "greener"?


> Wood sequesters carbon from the environment, while producing concrete emits carbon




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