Cross-Laminated Timber
avc.com
avc.com
I agree with the author that water is now a bigger deal than fire for wood construction, and we need way more testing for taller construction, perhaps even designs tailored to the material.
That's a non-negligible practical problem and a perfectly valid point.
And it's difficult to take the residential approach of "just over-spec" when you're building mid-rise+.
https://www.theb1m.com/video/top-5-the-world-s-tallest-timbe...
- completed in 2015
- completed in 2017
- completed in 2015
- completed in 2017
Mmhmm.
Care to elaborate? Concrete has already been extensively studied and has enjoyed widespread use, including under seismic loads. I don't understand how in this day and age where rc structures have been used for over a century there is any mention of "hidden failures".
For reinforced concrete failures, see here: https://en.wikipedia.org/wiki/Reinforced_concrete#Common_fai... https://www.usbr.gov/ssle/damsafety/risk/BestPractices/Chapt... By the time there's spalling the reinforcement is already rusted and breaking up the concrete. Of course you can say "Oh but we know all that now, so just design it right and you're good!" Sure. Unless you make a mistake, in which case you won't know it until the structure is crumbling. And remember, "it's a monolith", so good luck with those repairs.
I'm not saying CLT is better - but let's not kid ourselves, reinforced concrete isn't perfect. It's just cheap, strong, and easy to install.
I think its more "unfamiliar material" has different properties than we're used to, and there will need to be some adjustment.
Admittedly, this isn't an argument against CLT but instead against anything that doesn't behave as a single structure under load - but it does point out that CLT isn't a suitable replacement for all building materials everywhere. High rise buildings need the safety of reenforced concrete, with it's load handling and simplified calculations (due to lack of joints), and until something can perform in a similar manner it's not likely to be replaced.
The 2018 International Building Code provides codes for tall CLT structures, which won't get adopted in the CA code until late 2020, which won't get adopted in the SF code until late 2022 or 2023. At that point, developers can start applying for permits for CLT buildings, which takes about four to five years. Then add two years of construction.
So SF won't see CLT structures over six floors until about 2030. Makes you realize how unseriously we're taking climate change when the only carbon-negative structural building material in existence won't exist here for another decade.
Lots of structural retrofit there.
Yet I also guess that lessons from big quakes have been a part of the IBC review process. For decades.
??
Kinda of like how joints in steel frame buildings held together with stupid janky rivets would shift and creek during earthquakes while strong modern welded ones wouldn't . Progress! No wait that last bit is bad. The steel at the welded joint buckles unless carefully designed. Which they weren't for the first 30-40 years.
Permits take longer than the building?!
Demolition on the Waldorf-Astoria began on October 1st 1929. Unforeseen financial difficulties slowed progress. Construction began January 22nd 1930. The steel structure of the Empire State Building was completed on September 19th 1930. Construction was completed April 11th 1931.
These days it's more difficult- impossible- to get permitting for low rise medium density housing buildings in a city with significantly more financial resources and significantly more demand for floorspace.
If you want to change the environment right now, build buildings that let people live closer to work and commute less.
When unexpected things happen it doesn't always mean anything was done wrong, or even that anything should be done differently in the future.
They can even be made from rapidly harvestable plans like bamboo:
https://www.sciencedirect.com/science/article/pii/S095006181...
This company appears to be making them:
https://www.moso.eu/en/products/bamboo-beam-panel-veneer/bam...
Suffice to say they're 'quite a bit' heavier than wood.
That weight is the adhesive and/or compression. So we're very much not talking about 'tree wood' at this point.
- https://vicash.com.au/cross-laminated-timber-vs-glue-laminat...
I use this stuff when I design and build LEED-Gold certified hydroponic food production buildings. CLT is made out to panels and then cut to customer specifications. It has the same construction (including 90-degree off-orientation of each layer) and pretty much the exact same manufacturing process.
Glue Laminated Timber has the same orientation all the way through. It is meant specifically for one-direction loading (primarily structural support beam but can be done for joisting as well) whereas plywood and MDF and CLT will handle multi-directional loads (and is why it gets used for the outside skin of buildings framed with wood.)
Which makes me wonder how CLT is supposed to behave with wood movement when atmospheric moisture changes. Is it made from a species of softwood with very little movement? Or is the adhesive supposed to be strong enough to withstand the stress under movement? Or has the wood been treated with some industrial process to reduce the movement? All of the above?
Cross laminated timber goes against everything I know about wood movement as a woodworker. But the only exposure I've had to this stuff was beating 6 inch thick (3 layers) CLT pieces with an axe for firewood. I'm really curious how this stuff can work.
Ditto CLT. My class was making this almost 25 years ago back in high school wood shop, we just called it "Thick ply" and we primarily used it back then for constructing ultra-solid subwoofer boxes for vehicles. The lamellae were cut to 1/4" thick sheets on a lathe (they do make saw-based lathes, we had two in wood shop, one industrial one, one made out of a table saw with jigs for making dowels and such) planed, cut, oriented/glued/layered, veneer layer applied, sanded, then a final heat treating process, and done.
OK, plywood overcomes the stress by having very thin laminates. So how does CLT overcome the stress due to different width vs length expansion rates? Shouldn't the problem be much worse with thick laminates?
It's open to the public this weekend as part of BNE Open House: https://brisbaneopenhouse.com.au/building/25-king-street/.
There's also a talk this evening: https://www.eventbrite.com.au/e/brisbane-open-house-tall-tim....
That is, a program writen in code or text with parameters such as : square meters, number of floors, wood species, distance travelled.
It looks like open source carbon models are almost unheard of yet.
Building open carbon models isn't difficult, it's the input data that normally require licenses. I have built an open source life cycle assessment (LCA) software which has some traction [1], and there are alternatives for LCA [2] and integrated assessment models [3, 4]. However, data availability, especially on the level of completeness and detail you need to answer a specific question like carbon performance of a structure over a given period of time is a challenge. We are working on building a large open database to answer these kinds of questions[5], and Hacker News readers are welcome. Happy to chat via email if you want more info!
[0] https://pubs.acs.org/doi/abs/10.1021/acs.est.5b01735
[3] https://github.com/JGCRI/gcam-core
What I'm trying to do is, I believe, complementary to these LCA tools. I'm building a website https://futur.eco that bridges carbon models with our every day life as citizens.
Sorry, it's in french for now, but I have some hopes that you read french :-)
Behind the website is a database of open source and extremely simple LCA models, expressed in a new redable programming language. All of it resides in this single file : https://github.com/laem/futureco-data/blob/master/co2.yaml.
CO2 can penetrate into the concrete and bond with CaO, over time reabsorbing all the CO2 released during the calcination process. In practice, CO2 doesn't penetrate deeply into concrete, so depending on the concrete type and environment something like 25% of the potential absorption of concrete is realized (this is based on a conversation I had with a colleague at work, the number is not exact).
That's just like plywood. I have to imagine other similar problems affect CLT. Your example of moisture damage essentially has to affect wood, though there are ways to engineer the CLT that will mitigate moisture damage.
The steel structure would more likely survive in-tact, but the wood structure could guarantee a certain minimum bound in terms of evacuation time available to occupants (which might be superior to steel).
From my very novice perspective, I feel like wood does not attenuate in structural integrity as its temperature rises (at least not in the same way as steel), and it seems it would also absorb and conduct heat much more slowly than steel.
On the other hand, I feel that once a wood-based structure gets hit with a big enough fire, the entire building is guaranteed to be lost due to how hard it could be to fully extinguish the fire (e.g. if the steel in a skyscraper could burn openly, what would that be like?).
I did find this related paper which may be of interest to HN: https://www.fpl.fs.fed.us/documnts/pdf2012/fpl_2012_kukay001...
Mild steel becomes ductile at relatively low temperatures, way before it actually melts, hence why you sometimes see steel beams deform and drape over timber beams in a fire. The ductile transformation happens relatively quickly once you get to the critical temperature on the iron carbon phase diagram for typical construction grade steels. Hence why Steel is usually protected with fire rated drywall like boards, intumescent paint or weird spray on insulation that looks like porridge.
Whereas with timber the rate at which it will burn is reliably predicted, so exposed timber structures are designed to be oversized so that the reduced beam sizes will still be effective. This is done using a charring calculation to give the required amount of structural integrity duration which varies depending on how tall the building is and what it is used for.
All types of structure will fail eventually in a fire, the assumption is that the fire brigade will tackle it before it goes too far.
In the UK, the main fire related problem with CLT is that the exposed surface doesn't meet the requirements for the speed of surface spread of flames so it has to be treated with a fire retardant or you need sprinklers if you want to expose the material. Both of which are expensive.
- It can't be poured into arbitrary shapes.
- Much lower acoustic isolation
CLT is stiffer than plywood, and the construction techniques are closer to steel/concrete, so it'll likely need reinforcement or dampering. Traditional wood buildings these days are designed to flex in earthquakes.
But it hasn't been extensively tested for seismic properties yet
Plywood is much weaker essentially because trees evolved to withstand wind and the veneers are cut perpendicular to wind loads.