Hacker News Reader

Comment by 3pt14159 | Hacker News Reader

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.

View on HN

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.

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.

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.

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).

Reply on news.ycombinator.com