Timber Buildings

[Mike G]

And that's the point, Matt. It's the stuff in our buildings which burn. The buildings themselves typically aren't the hazard.

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Just to show how much of our fear of fire is misplaced: I once saw a demonstration involving a bale of straw and a lighter. A member of the audience was called up and asked to set a straw bale on fire. He couldn't do it, and used up a lighter-ful of fluid trying. It just would not burn. It was dry, and there was no trick involved.

 

[Mike Jordan]

It was my intention to refer to a couple of obvious errors that had cost lives rather than compare them with mass timber construction. You are quite right in asserting that timber has advantages over steel, particularly when it comes to early loss of strength in steel if not properly protected and expansion causing collapse.
My concern is really about the lack of long term knowledge of buildings of this kind, the char rate of the timber is well known, and as you say has advantages.
In the 60s I worked on a twelve storey block of flats built using what was known as the Larson Neilson system.
As I’m sure you know the internal concrete wall slabs and floors are made on site and lifted into place which the external panels being factory made and delivered when needed, the whole structure being being locked together with poured concrete at the joints.
I thought it was a great system and it was very popular throughout the country, but after a few years a partial collapse of a similar block at Ronan Point in London put a question mark over the method.
A minor gas explosion displaced a few wall panels and progressive collapse caused floor slabs to collapse.
The answer was to remove all gas supplies from the buildings and prohibit portable gas appliances.
The block I worked on is still in use after55 years and must prove the point.

 

[Windows]

The steel & wood thing is interesting and surprising. A quick look suggests that maybe temps between 600F and 700F might be interesting, since I see descriptions of wood combusting at 700F, but charring and taking hours to burn at lower temps whereas steel is described as “starting to lose its strength” at 600F. Is that the gist, that steel loses its strength at temperatures far lower than its melting point of ~2500F and even at temps lower than wood fails catastrophically or is there something else going on?

 

[AJB Temple]

Used to have my office on the 24th floor of a building overlooking the Thames at London Bride. Glazed corner office. In storms over London at that height you could feel the building move. The views were spectacular but I never really liked being high up. When there was a fire escape practice we used the stairs (there were three separate fire escape stair sets in stair wells). It took many minutes to evacuate as traffic jams would form on the stairs. Not ideal. And I've never liked tall buildings for that reason. Bit scary.

When we lived in Capel, a farm neighbour had someone try to burn down an old oak barn (arson / kids probably). Circa 1700. It wasn't used much and just had some hay in it. All the softwood cladding burnt off, and the hay burnt out. A few roof timbers collapsed (it was clay peg tiled) but the fire burnt itself out and the frame was intact. Some bits were charred but to everyone's surprise the oak frame was pretty much OK.

 

[Dr.Al]

The steel & wood thing is interesting and surprising. A quick look suggests that maybe temps between 600F and 700F might be interesting, since I see descriptions of wood combusting at 700F, but charring and taking hours to burn at lower temps whereas steel is described as “starting to lose its strength” at 600F. Is that the gist, that steel loses its strength at temperatures far lower than its melting point of ~2500F and even at temps lower than wood fails catastrophically or is there something else going on?

I had assumed it was to do with steel being stiff and hence not being able to move as much as it needs to as temperatures change to the extremes produced by fires. Steel typically has a coefficient of thermal expansion of about 10 μm/m/°C (depending on grade of steel), so at the typical temperature of a wood fire (about 600°C), it'll have expanded by nearly 6 mm per metre of length. On the scale of steel buildings, that could add up to quite a large length change and if there isn't some way for the movement to be dealt with, you'll get buckling with potentially disastrous consequences. I'm sure the designs include some sort way to deal with movement of the steel, but possibly not enough to cope with 600°C or more (depending on what's in the building and burning).

Note that I know nothing about buildings of any sort, the above is just conjecture.

 

[Mike Jordan]

Matt - In simple terms the time you mentioned probably relates to deciding how many people are allowed to occupy a premises used for public entertainment purposes. Depending on the type of construction the maximum time to clear the building varies up to a maximum of 3 minutes.
Its necessary to calculate how many units of exit width are available, the distance to be travelled, and the type building construction. Together with any stairs, steps involved. As with many seemingly simple jobs it’s actually quite involved and takes in things like adequate lighting, handrails, type of occupancy etc.
In the case of infant schools for example the provision of a staircase handrail at a child friendly height will probably double the speed of evacuation .
It’s easy to see that there is a huge difference between a grand theatre and a wooden scout hut.

 

[Mike Jordan]

On the subject of flammable furnishings , the use of urethane foam was a serious problem until the success of a long campaign which resulted in regulations prohibiting its use and requiring combustion modified foams be used in all furniture from 1988.
It’s worth noting that abut 1000 people died each year in fires at that time. Todays figure is under 300 per year

 

[Mike G]

......Is that the gist, that steel loses its strength at temperatures far lower than its melting point of ~2500F and even at temps lower than wood fails catastrophically.....

Yep, that's it. Steel starts sagging long before wood starts failing. I can't find it at the moment, but I took a photo in an oak framed cottage which had burnt down, in which a steel beam had been inserted. The beam was maybe 4.5 to 5m metres long. The oak posts holding it were fine, and still in place, so the ends of the steel were still up in the air where they should be. The centre of the beam was resting on the floor. It looked like it was made of wet spaghetti. Of course, the collapsing beam has holding up a 1st floor wall which was holding up the roof, so the roof collapsed when the beam sagged. All the oak frame principle timbers were still standing, and in fact, almost all of them were scraped down and retained in the re-built building.

 

[Mike G]

.....a partial collapse of a similar block at Ronan Point in London put a question mark over the method....

It ended it's use in that form. Progressive collapse wasn't on the radar until that happened. I'm sure structural engineers would now be given a lecture on Ronan Point in their first year of uni. Progressive collapse is why the World Trade Centre is no longer standing after a major fire in the middle of it.