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Unpredictable Fires

Written by: Alan Baker 14th June, 2017

Fires are unpredictable things, and the recent fire at Grenfell Tower shows how even the experts can be caught out.

Basically, fires burn upwards since the heat they generate causes the air above the fire to expand, lowering its density and therefore floating upwards like a helium balloon.  Fresh air is then drawn in at the base of the fire to replace the hot air which is rising: this air contains the oxygen essential to support combustion so the fire continues to burn with ever increasing intensity as the hot gases pre-heat the material, so making it more likely to ignite.  Of course, if the fire is channelled in a chimney like structure the route of the hot gases are dictated by the sides of the chimney, so they rise more rapidly, with more of the heat retained in a smaller area than a free burning fire.  The “chimney effect” therefore causes a more intense fire, with a more rapid spread, in its vicinity.  Domestic or industrial chimneys are designed to capitalise on this effect to ensure both better burning characteristics and the ability of expelling the smoke and toxic gases higher into the atmosphere to reduce pollution.  However sometimes a “chimney” is incorporated into a building unintentionally, and it only becomes apparent when an uncontrolled fire occurs in a area where it is not intended.  Lift shafts are such an area, but the risks are known and mitigating steps are often taken.  Other less obvious areas are between walls, especially where false walls have been constructed within buildings to either cover defects or provide insulation.  Fire stops can be incorporated in curtain walling or similar, but the problem with them is that they have to provide a complete barrier between two vertical surfaces which, by their very nature and intended function, cannot be viewed to check integrity during and after construction.

To complicate matters, whilst fire travels vertically, when it hits a horizontal surface, such as a ceiling the flames, smoke and toxic gases rapidly travel across the ceiling, radiating heat back down towards the floor.  If the horizontal area is large this means that areas remote from the original fire become heated: this may result in ignition of materials at a distance from the source, and hot air being drawn into the flames, allowing them to burn at a higher temperature, so accelerating the damaging effects of the fire.  The classic example of this is the fire in a domestic sitting room, when the sofa is the item first ignited.  The initial flaming appears benign, but as the room heats up and the flames intensify the whole room suddenly ignites with explosive force.

The aftermath of the fire at Kings Cross Underground Station in the 1980s threw up another factor, known as the trench effect.  Initially the investigators were surprised at how the fire spread up the escalators into the ticket hall, rather than burning vertically up to the ceiling.  The rapid movement of the flames up the escalators caused many of the fatalities, especially since even firefighters thought that they had sufficient time to escape.  The trench effect is basically explained by the “stickiness” of gases (and liquids) as they are move against a solid surface (watch the movement of water along a gutter) and this attraction will overcome the expected vertical movement.  However it is unusual to have a long “trench” in a building, so the effect is normally insignificant in a typical fire (if such a thing exists)