Properties of Oak?

[Aragorn]

I have a question about the properties of oak.
I remember hearing somewhere that oak is naturally fire-proof (!) and as strong as steel :shock:
That doesn't sound quite right to me, but I can't remember the exact circumstances referred to, so it may be partly true.
Apart from the usual stuff found in e.g. Lincoln's "World Woods in Color" can anyone give me some properties of oak and confirm or correct the above about it's strength etc :?:
I've also heard it is naturally anti-bacterial/anti-fungal... I would love someone that actually knows to help me out here!
Many thanks

 

[dedee]

I do recall reading that after a fire in Hampton Court a few years ago that the original oak beams (500+ years old) although subject to a very fierce fire only had their outer surface charred. The chared surface was then removed without any substantial damage to the integrity of the beams which were, I beleive, left in place.

Andy

 

[mudman]

I remember seeing something similar about the beams in some cathedral roof that had had a serious fire. Apparently the outer charring is like an asbestos coating and prevents the interior from burning.

 

[Neil]

Hi Aragorn,

Re: the antibacterial properties, this is indeed true of several hardwoods. Cliver & Ak from the University of Wisconsin tested various hardwood and plastic chopping boards by adding various bacteria and measuring what happened over time. A summary of their findings is here

Hope this helps,

NeilCFD

 

[Alf]

The charring inhibiting further damage certainly rings a bell here too. I'm not sure it's just a property of oak though, just thick timber beams. One of the drawbacks to modern construction methods using thinner stuff and metal fastenings seems to be that they can be burnt right through before starving the fire of further fuel (which is what the charring effectively does). I think there's some discussion about it in the Encyclopedia of Wood Joints by Wolfgang Graubner, but that's the gist. The chopping board thing is familiar too. I think it was mentioned in GWW some while back, and I've clung to it as a liferaft for wooden boards in this hygenic-mad world! :roll:

Cheers, Alf

 

[Aragorn]

Thanks for your help so far.

I was pretty sure about the antibacterial bit, for example as it applies to chopping boards. Again, I think this is a feature of many (all?) hardwoods, as well as lino used on kitchen/bathroom floors (not vinyl). Probably something to do with them being natural products.

Interesting about the fire-resistant thing. I seem to remember that what I heard was indeed regarding oak beams, so this claim may well be a property of beams in general.

Any thoughts about its strength vs steel??
Thanks again - keep it coming :wink:

 

[Chris Knight]

Aragorn,

Oak is strong but nothing like as strong as steel:-

http://www.efunda.com/materials/common_ ... Mechanical
http://www.woodbin.com/ref/wood/strength_table.htm
http://www.chaos.org.uk/~eddy/project/s ... rials.html

 

[Keith Smith]

Chris, you are right that steel is very much stronger than oak but in a situation where it is exposed in a fire, at a certain temperature the beam will fail catastrophically, whereas a wooden beam will burn in a more controlled rate and still have a good deal of strength even when a good proportion is ash. This is why building regs insist metal beams have fire protection.

Ask a fireman sorry fire officer :wink:

Keith

 

[Chris Knight]

Keith,
Thanks, I did not know that.

 

[Aragorn]

Thanks for the links, Chris. Seems the strength of oak has been greatly exaggerated somewhere along the line: At least in the muddle that I call my brain these days.

Keith - Hmmm, fire regs - what fun... :?

 

[Anonymous]

Yup charing is the woods way of self defense against fire the chared layer act like an insulator to protect itself where as steel will expand and contort and will structurally fail before wood. The wood will eventually fail to prolonged exposure to fire. Although structural steel should have (i think)1 hour fire protection these days via intumescent paint or boxed in with the use of fire retardant/resistent material. fire proof does not really exist a material will resist fire but if it gets hot enough almost all material will fail.

 

[Shadowfax]

Aragorn
Oak, or almost any wood, especially hardwood will char at a certain rate when it is exposed to fire. The greater the cross-sectional area the less likely it is that the timber will fail because after a given amount of charring has taken place the rate at which the timber continues to char will be uniform. This means that the timber effectively protects itself.
I have seen oak beams that have been involved in very serious fire that were about 12 inches across before burning but were still about 10 plus inches across when all the fuss died down a couple of hours or more later.
Steel, on the other hand, if it is unprotected will bend into all sorts of interesting shapes at far lower temperatures.
Give me a good timber beam any day!

SF

Looks like AP got in before me with the same kind of answer. Nice one!

 

[Midnight]

I'll echo the self preserving properties in a fire, adding that you're partially right in thinking that oak is stronger than steel.. the missing part is.... when both are subjected to fire...

A steel beam loses most of its structural integrity at temps approx 650 deg C, plastic deformation occurring shortly thereafter. Oak will happily survive that for all the reasons previously listed.

The anti-bacterial properties are due to the tannin levels in oak (beech too if I remember right). I remember a story in the papers a while ago about the Eurocrats insisting that traditional fish smoking houses be brought into line with current food hygiene standards, abandoning their traditional oak for stainless steel. After extensive research backed by one of the Universities here, they were duely told where to shove their stainless steel, and that the fish curers were quite justified in refusing to lower their standards of hygiene simply to conform to Euro regs.. It seems that when heated, the oak gives up some of the tannins, which act as a natural anti-bacterial agent, both preserving and sterilizing the fish...

 

[tx2man]

Forgive me if i'm wrong, (or not on the same lines ) but isn't oak susceptible to fungal attack when used externally :?

TX

 

[Anonymous]

The anti-bacterial properties are due to the tannin levels in oak (beech too if I remember right). I remember a story in the papers a while ago about the Eurocrats insisting that traditional fish smoking houses be brought into line with current food hygiene standards, abandoning their traditional oak for stainless steel. After extensive research backed by one of the Universities here, they were duely told where to shove their stainless steel, and that the fish curers were quite justified in refusing to lower their standards of hygiene simply to conform to Euro regs.. It seems that when heated, the oak gives up some of the tannins, which act as a natural anti-bacterial agent, both preserving and sterilizing the fish...

I definitely recall a set of laws introduced by food standards agency in this country, forcing all restaurants to ditch their beech chopping boards in favour of polythene ones - the reason given that the polythene boards can be cleaned in dishwashers, and are therefore more hygienic than wood; subsequent research showed that wooden chopping boards are, to a certain extent, 'self cleaning', in as much as chemicals in the wood will kill bacteria on their own; the law forcing the use of polythene chopping boards has subsequently been rescinded.

 

[Chris Knight]

TX,

Oak is one of the very best woods for outside use. It is very resistant to most things and certainly not particularly suceptible to fungus although ineveitably as with any wood outside, it will support populations of all sorts of growing things (like lichens) - which however tend to leave the oak undamaged.