4 ms·

How is wood more fire resistant than steel? What are the metrics used for this?

jacquesm 9y ago
Because it does not lose its strength as fast in a fire. Wood doesn't flow, it simply burns from the outside in and the inner parts not exposed to the fire are just as strong as they were before the fire. By contrast: steel is an excellent conductor of heat and the interior of a beam is just as hot as the outside of the beam exposed to the fire.
oldandtired 9y ago
See the comment above by phonon.
gmueckl 9y ago
The key is that steel tensile strength goes down quickly as temperature rises. It softens dramatically long before it starts to melt. Wood does much better. My understanding is that wood weakens only by burning off on the surface. The unburnt core essentially seems to retain its mechanical properties for the most part.
cwkoss 9y ago
Jet fuel doesn't melt wood beams.
jaclaz 9y ago
The way a wooden structure behaves is very different from the way a steel (or steel reinforced concrete) one behaves, as the materials itself behave very differently. Let's take as an example a "main" wood beam to support a normal floor (or roof). It has been calculated to be sufficient with a cross section size of (say) 20x20 cm ( Width by Height, in some countries "square" wooden beams are normally used, in some other countries sizes with width half the size of height are commonly used), the dimensional calculation takes into account "exceptional" static loads, like snow (if a roof) or crowd (if a floor), and dynamic conditions (like earthquake) so they are over-dimensioned in normal use. What happens in a fire is the same as if you were reducing the cross section by scraping some materials off the foot amd sides of the beam, let's say removing 5 mm at a time, so , after some time the cross section will be 19x19.5, and then 18x19, 17x18.5, 16x18, 15x17.5, 14x17 etc. What gives a value of the resistance of a wooden (or however homogenous material) beam is its moment of inertia that is expressed in cm^4 and equates to WxH^3/12, so progressively you have: 20x20^3/12=13,333 19x19.5^3/12=11,740 ... If you graph it, you will see a progressive decline of resistance proportional to the reduction of the cross section. Also you have to consider how carbonized wood (char) represents in itself a "defense" of the wood against fire, once the first layer of wood is carbonized by combustion, it becomes harder/slower to burn: http://www.mace.manchester.ac.uk/project/research/structures/strucfire/materialInFire/Timber/default.htm http://www.mace.manchester.ac.uk/project/research/structures... Here a graph of charring rate is given: http://www.mace.manchester.ac.uk/project/research/structures/strucfire/materialInFire/Timber/Charring/standardFires.htm http://www.mace.manchester.ac.uk/project/research/structures... In practice, any wooden structure is fire resistant for 30-60 minutes, and the solution for increasing the fire resistance (if the wood cannot be protected by other means) is simply that of increasing the cross section. "Modern" wood, like composite beams, plywood, etc., may behave differently given the presence of glues/resins, etc. With steel the resistance is given by its tensile strength, that decreases very rapidly from around 300-400 C (easily reachable in a fire and roughly equivalent to the ignition temperature of wood): https://www.steelconstruction.info/Fire_damage_assessment_of_hot_rolled_structural_steelwork https://www.steelconstruction.info/Fire_damage_assessment_of...

Because it does not lose its strength as fast in a fire. Wood doesn't flow, it simply burns from the outside in and the inner parts not exposed to the fire are just as strong as they were before the fire. By contrast: steel is an excellent conductor of heat and the interior of a beam is just as hot as the outside of the beam exposed to the fire.

oldandtired 9y ago

See the comment above by phonon.

gmueckl 9y ago

The key is that steel tensile strength goes down quickly as temperature rises. It softens dramatically long before it starts to melt. Wood does much better. My understanding is that wood weakens only by burning off on the surface. The unburnt core essentially seems to retain its mechanical properties for the most part.

cwkoss 9y ago

Jet fuel doesn't melt wood beams.

jaclaz 9y ago

The way a wooden structure behaves is very different from the way a steel (or steel reinforced concrete) one behaves, as the materials itself behave very differently. Let's take as an example a "main" wood beam to support a normal floor (or roof). It has been calculated to be sufficient with a cross section size of (say) 20x20 cm ( Width by Height, in some countries "square" wooden beams are normally used, in some other countries sizes with width half the size of height are commonly used), the dimensional calculation takes into account "exceptional" static loads, like snow (if a roof) or crowd (if a floor), and dynamic conditions (like earthquake) so they are over-dimensioned in normal use. What happens in a fire is the same as if you were reducing the cross section by scraping some materials off the foot amd sides of the beam, let's say removing 5 mm at a time, so , after some time the cross section will be 19x19.5, and then 18x19, 17x18.5, 16x18, 15x17.5, 14x17 etc. What gives a value of the resistance of a wooden (or however homogenous material) beam is its moment of inertia that is expressed in cm^4 and equates to WxH^3/12, so progressively you have: 20x20^3/12=13,333 19x19.5^3/12=11,740 ... If you graph it, you will see a progressive decline of resistance proportional to the reduction of the cross section. Also you have to consider how carbonized wood (char) represents in itself a "defense" of the wood against fire, once the first layer of wood is carbonized by combustion, it becomes harder/slower to burn: http://www.mace.manchester.ac.uk/project/research/structures/strucfire/materialInFire/Timber/default.htm http://www.mace.manchester.ac.uk/project/research/structures... Here a graph of charring rate is given: http://www.mace.manchester.ac.uk/project/research/structures/strucfire/materialInFire/Timber/Charring/standardFires.htm http://www.mace.manchester.ac.uk/project/research/structures... In practice, any wooden structure is fire resistant for 30-60 minutes, and the solution for increasing the fire resistance (if the wood cannot be protected by other means) is simply that of increasing the cross section. "Modern" wood, like composite beams, plywood, etc., may behave differently given the presence of glues/resins, etc. With steel the resistance is given by its tensile strength, that decreases very rapidly from around 300-400 C (easily reachable in a fire and roughly equivalent to the ignition temperature of wood): https://www.steelconstruction.info/Fire_damage_assessment_of_hot_rolled_structural_steelwork https://www.steelconstruction.info/Fire_damage_assessment_of...