Cross-Laminated Timber

https://avc.com/2019/10/cross-laminated-timber/

by abhi3 • 7 years ago

112 77 7 years ago

VectorLock• 7 years ago

The first thing that jumps to mind is wondering how they compare fire-safety wise vs. "traditional" concrete and steel framed buildings?

narag• 7 years ago

Not bad if we believe this video:

https://www.youtube.com/watch?v=YuAya0hRjwU

gnopgnip• 7 years ago

Steel performs more poorly than you would expect in a fire

bob1029• 7 years ago

I actually wonder if CLT/wood would perform better than steel during an all-out fire in terms of the structural integrity, at least for an initial period of time.

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...

josephpmay4• 7 years ago

It burns slowly enough that it doesn't conflagrate like unprotected hardwood or plywood might, so there's much more time to evacuate. If the fire isn't put out quick enough, however, the building may lose its structural integrity

willyt• 7 years ago

Different failure modes.

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.

Tcepsa• 7 years ago

Today I learned why the bottoms of the staircases in the building where I work look like they've been sprayed with porridge. Thank you!!! (I had assumed it was insulation of some kind but could not figure out why they'd put it there)

chipsa2• 7 years ago

Have you ever tried to set fire to a single log?

lquist• 7 years ago

Anybody know how these stand up to earthquakes?

josephpmay4• 7 years ago

The answer is basically that they don't know at this point. (Reference: I did extensive research on CLT as part of a project in LA a few years ago and talked to CLT experts from the Wood Products Council)

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

Merrill• 7 years ago

Is the main difference between CLT and plywood that CLT is a crossed grain stack of sawn lumber instead of peeled veneers?

beat• 7 years ago

As I understand it, the modern trend for buildings that are concrete on the first floor and wood frame for 2-5 floors above that started with earthquake-resistant designs. So they can probably play on that model.

josephpmay4• 7 years ago

That's incorrect. The structural properties are completely different.

mixedbit• 7 years ago

Movies demonstrating how CLT building standup to earthquakes and fire: https://alexschreyer.net/engineering/much-abuse-can-cross-la...

tantalor• 7 years ago

kitcar• 7 years ago

This comment on the article is more interesting than the article itself IMHO http://disq.us/p/24slwwt

floatingatoll3• 7 years ago

Unshortened: https://avc.com/2019/10/cross-laminated-timber/#comment-4643...

mooreds• 7 years ago

JLM and his comments are a treasure. Check out his blog at https://themusingsofthebigredcar.com/

peterwwillis2• 7 years ago

That commenter is doing a lot of gymnastics to avoid talking about reinforced concrete's hidden failures. It's difficult to determine reinforced concrete failures before it gets bad, and repairing it is very expensive. Concrete can get really complex depending on how you make and apply it, and what it's exposed to.

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.

aitchnyu• 7 years ago

Now I want to know how it will recover after fires. I read it will burn/collapse well after occupants have escaped. But will a building with some burnt CLT members be a total loss?

notJim• 7 years ago

Really? It seems like a lot of words to say "unfamiliar material bad." Surely the people advocating CLT have thought of some of these obvious downsides, and have ideas for how they're addressing them. If those are inadequate, then talk about that. Instead we get "In my professional judgment it is driven by a certain "gee whiz, isn't this cool" environmental state of mind." Excuse me if I want facts and not someone's un-sourced, un-backed-up "professional judgment".

hermitdev• 7 years ago

"unfamiliar material bad."

I think its more "unfamiliar material" has different properties than we're used to, and there will need to be some adjustment.

rkachowski• 7 years ago

The comment repeatedly states that CLT is neither new nor unfamiliar, one main issue is that CLT requires many joints with individual properties (many points of failure with manufacturing errors specific to the people that worked there) whilst concrete behaves / accepts load as a singular structure.

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.

rmason• 7 years ago

JLM's comments some days are better than Fred's. The guy has been a high ranking army officer, built a lot of the newer skyscrapers in Austin and now is retired from CEO of a Fortune 500 company.

knodi123• 7 years ago

I can't believe nothing in this article, nor in the articles linked from this article, actually says what CLT is.

https://en.wikipedia.org/wiki/Cross-laminated_timber

SamBam2• 7 years ago

My understanding is that it's identical to plywood, just with thicker layers.

olivierlacan• 7 years ago

No, it's much closer to Glue Laminated Timber whereas plywood tends to be made of wide sheets of thin timber of varying quality (depending on the quality of the wood and number of sheets glued together). CLT and GLT/glulam aren't made in sheets like plywood, they're usually constructed for a specific purpose (beams, etc.)

- https://vicash.com.au/cross-laminated-timber-vs-glue-laminat...

coryrc3• 7 years ago

Yes. Tricky thing is plywood uses veneers (very thin pieces) because wood expands at about a 10x different rate in width versus length, so cross-laminated joints can be subject to an incredible amount of stress. The thin veneers cannot develop enough stress to break the glue joint.

aidenn0• 7 years ago

I can see CLT replacing steel, and possibly CMUs, but concrete in general?

- It can't be poured into arbitrary shapes.

- Much lower acoustic isolation

SamBam2• 7 years ago

I would expect that the vast majority of concrete used in construction isn't poured into "arbitrary shapes," but flat walls. Even if only 50% of concrete is pored as flat walls, that would still be a huge market for this.

bch• 7 years ago

Re: acoustic isolation - having recently dealt w acoustics/concrete (laying a floor in a concrete apartment), I found out there’s not necessarily “concrete in general” so acoustic (and other) properties can vary from concrete to concrete.

floatingatoll3• 7 years ago

99pi episode "Built on Sand" talks about mass timber aka CLT: https://99percentinvisible.org/episode/built-on-sand/

danans2• 7 years ago

My house has some LSL (Laminated Strand Lumber) beams which, like CLT, are engineered, but are built up from wood strands of and a lot of glue instead of from boards like CLT. As a result, they can be made from the remainder products of milling lumber.

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...

hammock• 7 years ago

Where does that glue come from and how good is it for humans/the environment?

jpollock• 7 years ago

a more important question is what happens when it's exposed to water, either through flooding or a leaky building.

cardiffspaceman• 7 years ago

I found this FAQ [1]. According to my reading of it, VOCs and formaldehyde are not a problem.

[1] https://research.cnr.ncsu.edu/blogs/clt-panels/home/

danans2• 7 years ago

This is the primary adhesive used:

https://en.m.wikipedia.org/wiki/Phenol_formaldehyde_resin

newnewpdro• 7 years ago

Since this is basically plywood++, does it have similar failure modes? Like what happens when there's a leaky tub frequently wetting a structural piece of CLT?

edaemon2• 7 years ago

I don't know about all of CLT's failure modes, but Oregon State University is building a structure (for their College of Forestry) out of CLT and had some delamination issues: https://www.oregonlive.com/news/erry-2018/08/4965f0c7df3495/...

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.

buss2• 7 years ago

Here in San Francisco, CLT won't be legal for buildings over six floors until about 2023 because of dumb policies. California uses the International Building Code as its base building code and then modifies it based on particular laws we have here. This modification & ratification process takes about two years. Then San Francisco does the same thing, adding another two to three years (because surely _our_ engineers are better than a consortium of the world's best structural engineers...)

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.

watersb• 7 years ago

I am guessing some of this delay springs from the Loma Prieta Earthquake (1989).

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.

Gibbon1• 7 years ago

Yeah every time there is an earthquake they learn how new modern materials and construction techniques fail and perform worse than the previous ones.

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.

rayiner• 7 years ago

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.

Permits take longer than the building?!

chipsa2• 7 years ago

This is one of the reasons why housing prices in the Bay Area are always increasing. They literally can't build enough, and so what they do build is going to be expensive, because that's what's profitable.

buss2• 7 years ago

Pretty much every time

nwallin• 7 years ago

I subscribe to HN for the insights into technical subjects I might not otherwise have exposure to. My primary takeaway has been "this is why we can't have nice things", whether it's safe air travel, affordable housing, or affordable healthcare.

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.

d-sc• 7 years ago

The process you describes is kinda a safety net. Buildings aren’t like software where you can continuously design and push updates. You get one shot and mistakes are very expensive.

If you want to change the environment right now, build buildings that let people live closer to work and commute less.

abakker• 7 years ago

I mean, someone signed off on the footings of the millennium tower, too. SF is not taking a cautious, informed approach, they’re just burying their heads in the sand.

TylerE• 7 years ago

Especially it seems SF is an area where it would seem wise to be a bit extra cautious...since you're both seismically active and in a marine environment.

maelito2• 7 years ago

I'm looking for a model of the carbon emissions of a concrete house vs a mostly timber house.

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.

cmutel2• 7 years ago

We looked into this with a completely open toolchain [0]. You could email either of the two first authors to get the code (not sure why it wasn't in the paper supporting information).

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

[1] https://brightwaylca.org/

[2] http://www.openlca.org/

[3] https://github.com/JGCRI/gcam-core

[4] https://github.com/iiasa/message_ix

[5] https://bonsai.uno/

maelito2• 7 years ago

Thanks for these great resources.

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.

sjg007• 7 years ago

I thought there was concrete that could absorb co2... but maybe it’s not enough to be carbon neutral.

cmutel2• 7 years ago

The process of making clinker uses heat to separate CaCO3 into CaO and CO2 [0]. This release of CO2 is normally about half the CO2 footprint of making concrete (the rest is mostly combustion of coal to generate heat).

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).

[0] https://en.wikipedia.org/wiki/Calcium_carbonate

kimburgess• 7 years ago

Well this is timely. In case anyone else is from Brisbane, Australia - we're home to the world's largest CLT building: 25 King St.

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....

Cross-Laminated Timber

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