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Wood, like all materials used in building, has two possible approaches in relation to fire, as summarised in the Technical Building Code:  reaction to fire and resistance to fire.  The treatment of the two aspects is completely different, as much in trials as in results and calculations.

Defines the reaction of a material in contact with fire.  It consists of trying to establish whether a material will collaborate or not in the extension of a fire, if it will contribute calories and smoke or gases.  The result is a classification, not a magnitude, which summarises the behaviour with regard to some conventionally established variables and limits.

It is a functional and structural concept, the time during which an element maintains the functions that were assigned to it within a building.  Although a classification tends to be established, there is a measurable magnitude which is time.

Depending on the use made of the wood either one or the other concept is more important.  Reaction to fire, even though it is always of interest for the material we use to bring little energy to a fire, is decisive in the case of panelling, as it will condition the probability of a fire and the behaviour of same if one is produced.  Currently, only the European UNE EN 13501 guidelines are valid, with classifications from A to D, from best to worst behaviour, with the Spanish guidelines that classified from M0 to M4, abandoned.   Although the variables that are used for the classifications are similar and are to do with the flames and smoke produced and the energy brought to the fire, the current trial is more representative and not comparable (it measures different variables), and therefore the old classifications cannot be accommodated.  The Technical Code requires determined classifications for some elements.  How to know if a material complies with this?  It is necessary to carry out the trials in a laboratory, in accordance with UNE EN 13823 guidelines (trial occasionally known as the SBI) and to obtain a certificate for the product.  It is indispensable to request the certificate in order to accept the use and insertion of the product in the building work.

The closure and separation elements to which containment of the fire is assigned, such as doors, walls or facades, must maintain their function during the time established, function that includes them maintaining their position (structural stability), not to let flames or smoke through, and to not increase in temperature beyond certain levels on the side exposed to the fire.  The denomination is RF XX where XX is the resistance to fire, the time during which they remain functional, for example RF 90.  Reaction to fire is not required although evidently to have an RF the reaction to fire must be good.  The Technical Code requires RF in many situations, and the way to guarantee it is by trials with European guidelines in the laboratory, and the certificate should accompany the documentation of the element in order to allow its installation.  During the trial the element or the system is tested, so that which corresponds to what is to be installed must also be checked.

Structural elements, beams, footings, columns, do not have requirements in terms of reaction to fire, although evidently the better their performance, the remaining conditions being equal, the more favourable the use of a material.  But without doubt the critical point is the time during which the structure maintains its function and does not collapse, in other words, the stability.  Classifications have been established with the nomenclature EF YY where YY is the time the structure must be maintained in the event of a fire, for example EF 180 indicates that the structure must support the building for at least 180 minutes.  The Technical Code establishes different requirements for different types of buildings.  There are trials, also with European guidelines, to establish the stability in the face of fire of specific building systems, for slabs for example.  The corresponding certificate from the laboratory guarantees the EF of the system, but is only valid when exactly the same materials are installed and with exactly the same disposition.  Structures, combining materials, sections and distances are practically infinite, and it is therefore impossible to test them all.  If we add that as the only variable is time, and that you can predict what the behaviour of each material will be, the stability of a structure in the event of fire is calculated on the basis of the material employed and the section used.

In the photos on the right iron bars can be seen after a fire, supported by a wood beam.

Wood burns and is classified as a D in its reaction to fire, in the same way as do its derivatives, fibre boards, chipboard, etc.  This is nothing unusual, many overlays and paints burn, and generally with worse behaviour in the emission of smoke than wood.  If used as for panelling, for the most safety-demanding purposes, or if low loads of combustible are required, it is necessary to improve the behaviour of wood.  The only reasonable way in economic, technical and aesthetic terms is in-depth fireproofing.  The use of fireproofing products applied in autoclave would solve the reaction to fire, permitting the use of wood in almost any circumstances.  Insist in this case that the corresponding trials must be required in accordance with UNE EN 13823 guidelines, together with the certificate guaranteeing this.

For structural elements, reaction to fire is not decisive, although it always an improvement if the classification increases.  Wood behaves extraordinarily well in the event of fire, as despite its apparent disadvantage, that it burns, it is remarkable for the stability of its properties at high temperatures and the nil deformations or explosions.   To this is added a lineal behaviour that permits an easy and exact calculation, in comparison to the rest of the structural materials.  Therefore, despite simplification after simplification being frequently made, always erring on the side of safety, wood designs provide excellent stability in the face of fire with both economy and safety, without uncontrolled contingencies or variables.  The calculation is very simple: calculate the speed at which wood carbonizes and therefore the time it remains stable with loads established, in such a way that the required limit is exceeded, EF YY minutes.  Therefore the key element for the calculation, and for the behaviour, is the carbonisation speed of the wood.   If we improve this speed, we improve stability in the face of fire and the greatest requirements of the Technical Code can be met with slim pieces.  Trials can be carried out for specific building systems, and thereby obtaining a specific EF qualification, but this can only be an endorsement of what has been calculated.

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