
Cross-Laminated Timber - abhi3
https://avc.com/2019/10/cross-laminated-timber/
======
kitcar
This comment on the article is more interesting than the article itself IMHO
[http://disq.us/p/24slwwt](http://disq.us/p/24slwwt)

~~~
floatingatoll
Unshortened: [https://avc.com/2019/10/cross-laminated-
timber/#comment-4643...](https://avc.com/2019/10/cross-laminated-
timber/#comment-4643481053)

~~~
peterwwillis
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.

~~~
ethbro
_" The issue with CLT as a building system is exactly that -- its behavior as
a whole when subjected to differing forces as statistically magnified by the
great number of joints. There are a ton of connections in a CLT building.
Thousands of them. One connection failure can create a cascade of failure that
may destroy the building. Each connection has the fingerprints of a single
worker. One bonehead can wreck a connection."_

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+.

~~~
youngtaff
But yet elsewhere in the world they can build multi-storey CLT buildings

[https://www.theb1m.com/video/top-5-the-world-s-tallest-
timbe...](https://www.theb1m.com/video/top-5-the-world-s-tallest-timber-
buildings)

~~~
ethbro
\- completed in 2012

\- completed in 2015

\- completed in 2017

\- completed in 2015

\- completed in 2017

Mmhmm.

------
buss
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.

~~~
rayiner
> 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?!

~~~
nwallin
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.

------
danans
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...](https://www.sciencedirect.com/science/article/pii/S0950061815001117)

This company appears to be making them:

[https://www.moso.eu/en/products/bamboo-beam-panel-
veneer/bam...](https://www.moso.eu/en/products/bamboo-beam-panel-
veneer/bamboo-beams)

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

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

~~~
bdamm
There are many forms of fungi (mold, various rot) that can feed on wet wood.
There are not many forms of fungi that can feed on cured adhesive, which also
functions to prevent the wood that is there from getting wet. Thus, I'd expect
engineered/laminated timber to be better at repelling rot than standard
untreated timber. Done well, it could last longer than steel.

~~~
ethbro
If anyone on HN has never hoisted an LVL beam (or even glulam), I would highly
recommend it.

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.

------
knodi123
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](https://en.wikipedia.org/wiki/Cross-laminated_timber)

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

~~~
coryrc
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.

~~~
mysterypie
> _so cross-laminated joints can be subject to an incredible amount of stress_

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?

------
kimburgess
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/](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...](https://www.eventbrite.com.au/e/brisbane-open-house-
tall-timber-buildings-taking-on-climate-change-tickets-70864384281).

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

------
maelito
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.

~~~
cmutel
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](https://pubs.acs.org/doi/abs/10.1021/acs.est.5b01735)

[1] [https://brightwaylca.org/](https://brightwaylca.org/)

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

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

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

[5] [https://bonsai.uno/](https://bonsai.uno/)

~~~
maelito
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](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](https://github.com/laem/futureco-
data/blob/master/co2.yaml).

------
newnewpdro
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?

~~~
edaemon
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/...](https://www.oregonlive.com/news/erry-2018/08/4965f0c7df3495/problems-
at-peavy-faulty-timbe.html)

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.

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

~~~
gnopgnip
Steel performs more poorly than you would expect in a fire

~~~
bob1029
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...](https://www.fpl.fs.fed.us/documnts/pdf2012/fpl_2012_kukay001.pdf)

------
aidenn0
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

~~~
bch
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.

------
tantalor
See also
[https://en.wikipedia.org/wiki/Engineered_wood](https://en.wikipedia.org/wiki/Engineered_wood)

------
lquist
Anybody know how these stand up to earthquakes?

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

~~~
edaemon
Pretty much, CLT is basically plywood with very thick laminations.

~~~
jcampbell1
True, but worth noting plywood is spiral shaved veneers which are unrolled
glued with the curves counteracting This is quite different from sawn
dimensional lumber. Plywood is entirely cut tangent to the grain / tree rings.

Plywood is much weaker essentially because trees evolved to withstand wind and
the veneers are cut perpendicular to wind loads.

