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Comment by ynniv | Hacker News Reader

"Using wood means greater fire safety"

Edit: Outside of how ridiculous this statement is, how does one bring a charred wooden high-rise back up to code?!

There was an article some time back about fire ladders and how wooden ones will still be structurally sound after getting quite burnt, while metal ones can lose their integrity well below temperatures where the metal would actually melt.

I think it was in relation to the San Francisco fire department.

The SF fire department uses wood ladders because they are non conductive, vs aluminum. They operate in an area with a lot of aerial AC power lines.

And worse, 600 VDC trolley lines, which are closer to the ground than uninsulated distribution feeders. Unlike power lines, trolley wire covers a sizable fraction of street area.[1] Trying to raise even a 20 foot metal ladder is dangerous.

[1] https://www.munidiaries.com/wp-content/uploads/2015/11/wires...

Vancouver has a lot of electric trolley buses, makes me wonder what the fire department has standardized on...

This one?

https://gizmodo.com/inside-san-francisos-fire-department-whe...

yup. that's the one

The I-beams used in modern highrise construction are a world different from lightweight aluminum extrusions used in ladders.

Aluminum is one of the butter metals.

I beams are quite safe if the fire retardant remains on (a major cause of the WTC collapse was the fire retardant getting blown off by the impact).

Trusses on the other hand are quite dangerous in a fire.

https://community.fireengineering.com/forum/topic/show?comme...

A truss structure is a design not a material though, are you saying all truss designs are dangerous in a fire regardless of material?

An I beam is also a design not a material, if you want to be pedantic. You can buy aluminum ones.

That's fair, but I know of no aluminum I-beams used in modern highrise construction which is the context of my sentence.

Truss roofs are made from steel of various shapes, even I-beams, and wood. I'm only asking for clarification if all truss designs regardless of material are problematic in a fire.

That was compared to aluminum(!!!!) ladders. Aluminum loses strength far more rapidly when heated compared to steel.

If you're the fire department, and you're moving them around regularly (as opposed to a fixed installation), the weight advantage of aluminum over steel is probably compelling. Wooden ladders are heavier than aluminum, which is part of why you no longer see them for sale at most stores, but they're still lighter than steel.

SFFD uses wooden ladders due to overhead power lines, predominantly.

For the same reason you won't be starting your bonfire with the big logs. It has a huge thermal mass and it takes a lot of external energy to make the fire self-sufficient.

It's also easier to repair, as you can just replace the damaged wood. With concrete that's not so easy, and in some cases it can be hard to even gauge how damaged concrete is from fire damage.

According to this study, it does not compare too bad: https://www.firechief.com/2017/03/14/are-wood-frame-high-ris...

Thanks, this is a good summary of something I wasn't paying attention to. It contains an interesting point:

The standard of workmanship (particularly in the installation of plasterboard or sheetrock) is a critical component to the necessary fire resistance

steel gets floppy before it gets melty. Wood stays rigid as it burns until it snaps.

Many factors when thinking about which is better.

Build the structure with steel pipe. Fill it with water. The temperature can not exceed the boiling point of water until all of the water has been boiled away.

Steam expands, so you're going to have some nasty pipe explosions in there. Also you'd need a way to stop the water corroding it from the inside.

No, the steam comes out. The water won't boil if it is contained under pressure, and it must boil in order to keep the temperature down.

Dirty water and a bad choice of steel will lead to corrosion, so don't use those. Purify the water and/or use a different type of steel.

Went to a talk about this (among other things). Short answer: They size the structural members making an allowance on depth from the surface for char (zero strength) and a further depth of affected material assumed to be weakened. The unaffected wood inside is what supports the structure.

For a dramatic demonstration of how shallow a fire actually penetrates wood over a short period of time and how well it insulates, see here:

https://www.youtube.com/watch?v=YU48ypPTH1E

(Promotional, but informative nonetheless)

Yea, I went looking for the same thing. I find it misleading to claim that steel melts in high rise fires and collapses (only three times in history) but wood chars and becomes fire resistant (at same temperature to melt steel??).

I would never choose to live or work in a multi-Tennant hi-rise due to fire risk.

You don’t have to get anywhere near melting temp to weaken the beam strength of steel.

From the internet: Average building fires reach temperatures of 700 to 1000°C. Steel weakens dramatically as its temperature climbs above 230°C, retaining only 10 percent of its strength at 750°C. Wood generally does not ignite until it reaches 250°C. Once it catches fire, wood develops a protective insulating char layer. After 30 minutes a large wooden beam will have lost around 25 percent of its strength in a 750°C fire and retain structural integrity – a steel beam will have lost 90 percent strength and will have failed.

I'm speculating, but steel takes up less space for the same strength, so you can use that extra space for fireproofing/insulation. How would this affect the calculus?

Absolutely! Its a matter of dimensions, for wood we simply oversize a few centimeters to burn through to make sure it holds long enough to get out of the building. I assume this indeed takes more space than a fire insulated steel beam. The achievement here is the Norsk got over the medieval fear of burning houses and can now use their loved local wood to tackle urban densification challenges!

Wood can be very very low carbon compared to concrete as a building material because the wood itself is a carbon sink.

But what is actually burning in a steel and concrete structure? Drywall and carpet?

Combustible cladding.

https://www.theage.com.au/politics/victoria/smoke-alarm-warn...

https://www.theguardian.com/uk-news/2018/jun/18/fire-safety-...

Some guesses: furniture, bedding, appliances, clothes, papers, trim, flooring, paint, etc.

Usually nothing, because this pretty much doesn't happen. It is nearly correct to say that steel buildings never burn down. The last exception to this rule involved luggage, seats, and lots of jet fuel.

What is burnt may never burn. See Shou Sugi Ban (焼杉板).

Charcoal? Coke?

Production of charcoal and coke both depend on not fully burning the wood or coal, by constraining the oxygen supply, but by heating/baking it to reduce the water content and volatiles. Older processes burn more of the material, in part to help produce the heat, while newer processes try to fuel it mostly of the off-gassing.

But the comment above is referring to a specific treatment of wood by charring the outer layer to make it act as a fire retardant.

My understanding of Shou Sugi Ban is basically to create a charcoal layer on the wood, so its unclear to me how that would prevent it burning, as charcoal is designed to burn. At best I can see it increasing the ignition temperature.

It's not fully charcoal (fully carbonized). Wood is mostly two compounds: cellulose and lignin. They undergo thermal decomposition at different temperatures. By partially burning the exterior, you decompose the cellulose, and when you attempt to set it on fire again, it's harder to light, because you no longer have the cellulose decomposition to provide fuel, but have to get to a higher temperature for the lignin to decompose.

It's charred, but not charcoal. And yes, mostly it just makes the ignition temperature go up. But that's half the battle for fireproofing. You're mostly not trying to stop it from ever burning, but giving people enough time to evacuate, or the firefighters to put it out with water.

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