Wooden high-rises change city skylines as builders ditch concrete | Hacker News Reader

Wooden high-rises change city skylines as builders ditch concrete

theguardian.com

48 pointskawera59 comments

Here's a video about a 14-story building like this in Norway. The video is from last year, but it was completed just this month.

https://www.youtube.com/watch?v=e5XsqauBCX4

For those of you who understand Norwegian or are willing to use Google Translate, there's an article about the building here[1], which also mentions some of the same projects as the Guardian article.

[1]: http://www.bt.no/nyheter/lokalt/Canada-sikler-pa-bergensk-ve...

I'm bit worried with this getting too popular, because of fire safety. Fire safety of a material is about two things: Fire load(kJ) and fire-resistance(hours). Wood increases fire load, but it has surprisingly good fire-resistance. Better than steel. The elevated fire load is usually compensated by sprinklers, and so far wooden skyscrapers have been safer than concrete ones because of this. Currently fire-resistance is lot more important, because that pretty much dictates loss of life in regular fire.

But imagine city where you don't have just few wooden skyscrapers, but like 90% of the buildings are wood. Then earthquake hits. Now the sprinklers don't work anymore, and the fire can spread from building to building. Whole cities burning was relatively common before 20th century, partially because they we're mostly built from wood.

>"So wood structures make sense in seismic-heavy regions, especially in the Pacific Northwest, where a 9.0 magnitude earthquake is expected to hit in the next 50 years."

>"Ideally there comes a day when people can choose between mass timber, steel and concrete. We don’t see mass timber replacing those, it just becomes another option when certain parameters and goals apply to a project."

I hope these cancel each others out. And there will be regular buildings in between to act as firewall of sort.

Interesting but considering how cities exist today, specially large cities that are basically concrete/steel jungles, I guess it would take decades for wooden buildings to reach, say, a 20-25% share.

An earthquake that shuts off water to a whole city seems just as liable to render uninhabitable all those concrete buildings (assuming they don't outright collapse). Wooden structures seem more likely than concrete to survive the initial quake, even if they're less likely to survive the resultant fires, so it may be a wash in the end.

Historically the fires have been often even more hazardous than the quakes.

https://en.wikipedia.org/wiki/1906_San_Francisco_earthquake#...

Something like 30% wood might be the best. But it creates interesting problem, who gets to live in a wooden building? Who gets to build one?

Many of those fires were set intentionally for insurance claims. Many other fires were caused by dynamite, gun powder, or artillery used to create fire breaks.

I trust that the vast majority of the destruction in the aftermath of the earthquake was caused by fire, but what I'm curious about is the death toll of the fires themselves, which I don't see in the article. The reason that it matters is because an earthquake can easily cause structural damage to a building that leaves it standing but renders it uninhabitable, forcing you to demolish it anyway. Even structures that are specifically built to withstand earthquakes aren't necessarily safe to occupy after the quake. From this (simplistic) perspective, wood is no worse than concrete because either way you'll be forced to rebuild the vast majority of the city after the big one hits.

You have good points. My only counter point is that rescuing people from ruble is easier if there are not much fires around.

The cynic in me suggests if cities still burned down every few decades, we'd have a chance to fix entrenched mistakes in the planning/zoning/etc.

I'm a former structural engineer. I can explain any questions that anyone has. But in short, wood is one of my favourite building materials. It's repairable, light, stronger during a fire than steel (although is burns), and it literally grows in trees!

While it's strength is lower than steel, for buildings less than 10 storeys tall it's a very good choice, especially if you can do the lower floors and basements with steel and concrete.

I've got a tough one for you. Some guys from material science lab claimed that with massive wooden structures the strength doesn't scale when size goes up. But they were unable to explain why. They just claimed that welded steel is somehow lot better for very large stuff. Have you heard about such? And why does it happen?

I'd imagine it would go other way around, as knots are typically the weak spots. And with bigger structure knots would be averaged out.

One possible problem is connections. You can make very large wood beams and columns, but then it becomes difficult to transfer the loads from one member to the other, because even a pretty big bolt will act on a fairly small area of wood.

It's would be very good guess, but that problem occurs with small scale wood too.

Only a guess: The strength between the grains of the steel has less variation than the strength between the grains of the wood. So as you increase the cross sectional area the likelihood of splitting goes up.

Yeah, that is only easy reason I can imagine. You can cross the wood grain and turn it into "veneer". That would quickly average out natural wood grain out of it. But you can't cross the glue planes.

Now if you scale that up, the likelihood of having faulty glue plane could go up. It still doesn't sound like static stress problem. More like fatigue causing delamination.

I'm still bit suspicious because this would be too easy to explain.

although is burns

The article also notes that this kind of wood is "fire resistant." It also says that, "But the road to approve CLT as a sound building material in the US has been arduous, with engineers and architects working to prove that it complies with codes and is resistant to fire and seismic events." Despite that, though, it still seems like fire is a threat. Is there something I'm missing? Are the other materials in the buildings sufficiently fire-retardant to make the issue a small or unimportant one?

When you set a large beam of wood on fire, what typically happens is the outside chars but the fire never reaches the core, which stays intact. This won't happen so much with typical dimensional lumber like a 2x4, but when you look at timber framing using 5x5 or 5x10 beams as a stock element, the effect can be quite real.

Fire is always a threat. The question is how fast the burning spreads and the structural capacity during a burn. Fire retardants which are pressured into the wood, begin to react when the temperature approaches normal wood burning points (around 275 C) the chemicals convert the wood tars to a carbon char which provides insulation from direct heat while also allowing the water to evaporate out of the wood as it more slowly heats. This gives you a much longer horizon to get people out and firetrucks in. You would need a sustained burn for a couple hours, at which point an exposed steel building would have fallen anyway.

A typical high rise building will experience some small fires during its lifetime that aren't structurally threatening. Is the wood building going to require more repairs or inspections in these cases?

There would have to be extended contact (more than just a small kitchen fire). Usually the beam will be repaired or replaced anyway because of aesthetic reasons, and much of the wood structure will be behind fireproofing where it's 100% safe for at least a couple hours.

Could you explain how wood is stronger than steel in a fire? I would have thought steel's high melting point make it impervious to fires, whereas wood, as you say, burns...

At high temperatures, steel loses strength long before melting. By 600 C (~1100 F), steel has lost half of its strength.

But isn't wood turned to ash at 1100F?

Not if it's being kept relatively cool by a layer of carbonized wood (but you need a pretty thick piece of wood for that to happen; naked 2x4s burn like kindling).

Can you comment of the lifetime (length of service, distinct periods) of a significant wood building are, and how it compares to others, like concrete or brick ? What special treatment or considerations it requires ?

We don't really build load bearing structures with brick anymore, it's too weak. Any brick you see is mostly done for moisture control or aesthetics (although in Canada you can reduce the wind load by 10% to your actual structure if you have brick as the outer wall).

Wood buildings are significantly cheaper to maintain. Significantly cheaper to build (at least in Canada).

The down side of wood is that it doesn't handle wetness well and it gets damaged more easily. For larger projects concrete starts getting financially competitive also partially due to the fact that it's more friendly to just leave exposed while you're building. You can't really do that with wood. Even the OP's original building was being built in sections.

Also, wood isn't as strong as steel + concrete. So if property prices are really high concrete and steel are going to win just so you can build higher and lose less space to structural members.

Thanks for the thoughtful response.

> The down side of wood is that it doesn't handle wetness well

So, vapour barriers are a Very Big Deal then, right ? Or do we have better solutions to the wood/water problem ?

I was going to ask "will this bear out over 50, 100(+) years", but I realized I'm putting my desire of how long a building should last... so, question: How long should a (say, 4-10 story apt) last ? Can we get 100 years out of modern one that are being put up ? What do people in the industry punt-around as a figure for the life of a building ?

I have a great love of wood -- it seems to work in compression, and in tension, has interesting failure modes, seems easy to work with and is versatile... but I do like the perceived permanence of concrete. Makes for interesting judgment of buildings...

The question isn't whether something will last for 100 or 1000 years. The question is "is it cheaper / safer to get something to last for 1000 years with wood or concrete?" And for most buildings wood is far, far easier to maintain.

And yeah, moisture is a really big deal. Most of building science is around moisture and heat. Humans and building materials don't like getting too wet.

> for most buildings wood is far, far easier to maintain.

Interesting. I always thought of it as (relatively) delicate.

Shame they didn't mention the 18 storey wooden residence they're building at my alma mater, UBC. I wish I knew more about civil engineering and how to visualize structural loads.

Link: http://news.ubc.ca/2015/10/01/new-ubc-student-residence-to-b...

The article doesn't really get into the obvious question of fire risks. This has coverage on that issue: http://thoughts.arup.com/post/details/302/would-you-live-in-...

Curious what species of timber they use to make this. There's a lot of structural and and resiliency differences between white pine, Doug fir, hemlock, and the various hardwoods.

I'm trying to figure out where this product fits into the construction, as well. I'm reasonably familiar with the general principles of stick-built and post & beam wooden construction, structural steel construction, traditional brick and masonry, and poured concrete. Do you just put these panels up as full walls? Does it still need an internal structure, or do the panels have sufficient rigidity? What do you do for insulation?

It depends on the design, but you can build something using the panels (+ gypsum board) as walls and floors, with almost no other structural members, at least for residential.

For insulation, the standard approach seems to be using rigid panels on the outside, either rock wool or foam. This stuff is a good bit more expensive than stick built construction, so I figure they count the expense of outside-only insulation (vs. batts or cellulose between the studs) is just a cost that goes with it. If you're building high-end, you may as well insulate high-end.

How well does this material resist water damage? Recently on British TV there have been loads of programmes about people building cheap houses (eg [1]). They tend to be prefabricated off-site from softwood and quickly assembled on site. I wonder how long such buildings will last if the roof ever leaks.

[1] http://www.bbc.co.uk/programmes/b03bjqk4

All roofs and buildings leak. There are probably more wooden timber roofs built during the 13-15th centuries (and earlier) that are still around than you might expect.

Wooden houses and roofs have been built for long time. I'd be more worried about leaking plumbing.

If you bring piece of wood indoors and let it dry, it's water content will drop to about ~17%. If you take it outside and hang it somewhere so that it catches rain and then dries, it's moisture content will average about ~20% in most places on earth. Wood has evolved to do capillary action. This means it's very good at drying, but it's also very good at staying wet if there is moisture around.

https://en.wikipedia.org/wiki/Capillary_action

Most wood eating fungi need moisture content of at least 30%. And this has to be long term, wet week doesn't do anything. But there is fungus called dry rot, that can cope with 20%. This leaves you practically no buffer, as the indoors natural is 17%. The catch here is that dry rot needs certain minerals to ever start growing. Cement and mortar have these minerals. If you build effective barrier between cement and wood structures and keep the moisture below 30%, you don't have to worry about rot ever.

Insects are another problem. But it's rare that you get insects if you didn't get rot first. In those rare occasions, there has been some visible defect in the surface of the wood. Or alternatively some very infested thing nearby. Regular paint goes long way.

UK has traditionally had lots of problems in build quality. Mostly because they didn't take into account dry rot.

Source: Deterioration and preservation of timber in buildings. G. A. Scott 1968. And no, this info has not dated since 60's.

Historically, though, wood structures have been built (intentionally or not) such that the wood is dried by the heat escaping from within. Modern wood structures are paradoxically designed for energy efficiency, which is in conflict with the above. You can't have an efficient building where the interior heat escapes constantly. Since we're still in the early decades of this paradox, modern wood buildings tend to be designed to be water-tight, and when that inevitably fails the structure is destroyed.

Old adage of construction, if you think "but how could this ever get wet?" you're fucked. Everything will get wet at some point, so everything has to be self drying.

The plastic film stuff has been tested pretty well for current structures. But as the insulation thickness increases there might not be big enough pressure gradient across the structure.

Concrete does not get a pass though. Here in Finland we have had bit thicker insulation for a while. Even if you don't get wood rotting, you can get mold. That ruins indoor air quality.

Hopefully the people building that stuff will have learned from Vancouver's leaky condo crisis: https://en.wikipedia.org/wiki/Leaky_condo_crisis

If your roof leaks badly enough, even steel is unsafe: http://www.macleans.ca/news/canada/elliot-lake-how-could-so-...

Ideally you want a really good roofing system, tip-top windows installed with the best practices, good walls with vented cavities behind the siding and a good roof overhang to protect the walls and windows against the rain.

There's also a big mass-timber office building that just started construction here in Minneapolis called T3:

http://www.t3northloop.com/

Given the amount I keep hearing about the concept I'm guessing it'll be a lot more common in just a couple of years.

I have been a fierce antagonist of modern architecture for many years.

Then I saw some modern architects using natural materials (wood and stone) and realized that much of my antagnoism was due to the synthetic materials they were using, and less the style itself.

There is something deeply humanizing about wood and stone.

Wood framed structures are horrible for dampening the noises from people walking (or stomping) around. I will never live in a wood structure with people living above me, its horrible. This is a non issue with steel and concrete structures.

Reinforced concrete is not that good either. The best floors are made so that you have separate floor and ceiling and some foam in between. It's called "floating floor".

https://en.wikipedia.org/wiki/Floating_floor

It can be done with wood too. But I don't know how effective that is.

We're not talking about an early-20th-century timber frame. This is a different material entirely.

A two-second Google search puts actual numbers on this: https://fpinnovations.ca/media/factsheets/Documents/cross-la... (page 8). Seems a standard suspended-ceiling type arrangement provides damping as good as a well-built concrete structure (by comparison to http://www.aia.org/practicing/awards/AIAB025071).

HN comments seem to have a bad tendency to dismiss interesting and potentially cool ideas out of hand (especially about anything that isn't software) by citing the first difficulty that comes to mind, without doing even a little bit of research. As though the many highly qualified engineers putting this stuff together wouldn't have thought of something as basic as sound insulation.

> HN comments seem to have a bad tendency to dismiss interesting and potentially cool ideas out of hand (especially about anything that isn't software) by citing the first difficulty that comes to mind, without doing even a little bit of research.

I agree and was just pointing out something similar in another HN discussion. It's detrimental to do the easy thing and shoot down innovations rather than the creative work of imagining the possibilities. Most of the tech people are using to post these comments, and probably most of their employers and YC members, wouldn't exist if they listened to the naysayers.

It's also a waste of my time to read the noise of yet another naysayer. I thought that behavior was explicitly discouraged at HN. Sigh.

The problem isn't really sounds, its the vibration and shock waves which travel thru the structure. I could never hear my neighbors, but I could hear and feel every heal strike. Loose metal attached to the structure would vibrate (air vents, kitchen items). Wasn't trying to tear down this technology or anything, I guess I just had a really bad experience living in a wood constructed apartment (constructed in 2013, 5 floors) and I pretty much vowed to never live in a multi-family wood constructed structure again.

Edit: page 9 in the parent link addresses this. Extremely poor vibration dampening.

Naked CLT sucks at impact damping, but so does naked concrete. You really need to think things through (methods of attachment for vents and pipes, flanking problems, etc), and to add some stuff to dampen the impact (on that same page they suggest adding a suspended ceiling with resilient mounts, giving an IIC of 62, which is pretty good).

I wouldn't feel comfortable in one of these buildings. I designed and built a timber frame home, but I look at this "panelized wood" and see a glued product produced by corporations which are more interested in profit. There has been a string of failures in similar products as corporations extract their profit and don't really care how their wood performs twenty-five years down the road. Glued wood is tricky by nature, and both complex safety and health implications are introduced by its use.

> produced by corporations which are more interested in profit

how is this different than any other material? or anything manufactured, for that matter?

fwiw, glu-lam wood has been used in huge structures for decades, and it works fine.

What about termites?

Termites need somewhere to live. They won't come along to eat a wooden house in Manhattan, unless there's some habitat for them nearby.

Sabotage, buddy. Think about it.

You simply spray for termites at the base. I believe most houses in tropical climates in the US get sprayed yearly as part of a basic termite insurance policy. I believe I was paying $110 every two years for my house in Florida.

Is thermal expansion of the wood not an issue?

Yes. Movement created by volume changes due to changes in moisture contentcan tend to dwarf it however. In climates with highly variable humidity the technical challenge can be substantial.

This is pretty interesting. What if our housing wasn't built to last indefinitely, but had a life cycle of maybe 50-100 years? As cities change, their housing stock could more naturally change with them.

That said, I'm not sure if this style is appropriate for Manhattan today. We don't need more ten-story buildings in Chelsea for multi-millionaires, we need more 50-story buildings (still for millionaires, but hey).

The majority of housing in the US isn't even built to last 100 years (shingle roofs need replacing after ~25 years and a typical wood building will quickly deteriorate once the roof fails).

Exactly! That's why I suggested wood buildings might be interesting. Lots of the apartment buildings in Manhattan were solidly built in the late 1800s or early 1900s and are not going anywhere.

Unfortunately the residents of Chelsea seem adamantly opposed to taller buildings, in the few areas where they're not already prohibited by zoning or landmarking. West of Tenth seems to be the only area where taller (10+ storeys) seem to be accepted, even as they box in the High Line.

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