Prevention of structure collapse

This covers the imposition of load-bearing capacity and integrity on the structure as a whole or in part during a fire.

Each of these can now be considered in greater depth.

1.1.1 Control of ignition

This needs considering under three subheadings; the first two are concerned with spread of flame and the third with management and maintenance of the structure. Ignition can occur through a variety of mechanisms. Generally, these are accidental, e.g. lighted cigarette ends, electrical faults or overheating of mechanical or electrical plant. However, deliberate actions or arson cannot be discounted.

1.1.1.1 Control of flammability

There have been too many cases where fire has spread rapidly owing to the unsuitable nature of the linings of a structure, thus any material used in the finishes on any part of the structure should be such that the spread of flame or flammability must be limited. This in general is controlled by the imposition of tests on flammability or flame spread by any relevant national or international standards, e.g. in the UK the relevant sections of the Fire Test Standard (BS 476: Parts 3, 6 and 7 or their equivalent European standards).

It is also essential to ensure that materials used in the contents of the structure should reduce any hazard. It is clearly impractical to insist that the contents of any structure make no contribution to the combustible fire load in a structure, but it is necessary to ensure that those contents produce as least a hazard as possible. This means that the surface coatings should not be easily ignitable, nor, as happened in recent cases in the UK with domestic fires involving foam-filled furniture, should certain foams which produce large quantities of highly toxic smoke on ignition be allowed. This latter has led to the use of such foams being controlled by legislation.

1.1.1.2 Control of growth of fire

One classic means of controlling fire spread is by the use of vertical or horizontal fire compartments. However, these compartments are only satisfactory if there is no possible route for smoke or flame through the compartment boundary. Fire spread can also occur within a room or to a compartment beyond its point of origin if the original fire boundary is incapable of containing it due to unsatisfactory closures to the room of origin (Hopkinson, 1984). A more recent case of fire spread, attributed to lack of fire stopping following replacement of the original façade, was the Torre Windsor Tower in Madrid (Dowling, 2005; Redfern, 2005; Pope, 2006). There were additional problems in this case, namely what appeared to be longer than normal for the fire brigade to actually start fighting the fire, and steel columns above the 17th floor was not fire protected (Arup, 2005).

An additional problem may arise where, although the compartment boundary is satisfactory when the civil (or structural) part of the construc-tion sequence is complete, the installaconstruc-tion of services may either destroy this fire break or not replace the fire break to a satisfactory standard.

This situation can also arise when subsequent modifications are made, forced either by changes to the use of the structure or by repairs to, or replacements of, existing services.

A further problem can occur due to failure to clear away accumulations of combustible rubbish which can either be ignited by fire as at Bradford (Anon, 1985, 1986) or can gradually cause flashover by very slow fire growth, i.e. smouldering (Anon, 1987, 1988).

Such problems can be reduced by ensuring that a fully effective fire safety management policy is in place.

1.1.1.3 Fire safety management

In single occupancies, it is relatively easy to set up procedures to ensure that, in the event of a fire, all personnel are aware of the proper proce-dures and that there are suitable people to act as marshals and direct the fire brigade as required. In multiple occupancies, especially where the occupancy changes frequently and there is a large transient population, such as shopping malls, this is more difficult and it is therefore essential that the owners, often corporate bodies, set up a fire safety management strategy and ensure that there is a responsible group of persons on duty at all times to take full control in the case of an outbreak of fire. Note that this function can be taken by the staff employed for normal day-to-day security provided, they are fully and properly trained. It is also essential that full records of the fire detection, fire control and fire-fighting systems are kept and that a full check is made on any occupancy to ensure that no action is allowed to be taken which will negate any part of those systems. It is essential that where a fire engineering approach to building design is approved and adopted, the measures contained in that design are retained at all times and that financial exigencies are not allowed to compromise fire safety.

1.1.2 Means of escape

There are generally statutory requirements for the provision of escape routing in all except the simplest single-storey structures. Such require-ments are based on the concept of the maximum length of escape route to a safe place, be it an external fire door or a protected fire-escape stairwell.

The maximum lengths are based on the type of occupancy and are also dependant on the method of escape, i.e. whether along a corridor or

through the fire compartment. For multi-storey structures, it may well be possible to make use of the concept of phased evacuation where initially only a reduced number of storey adjacent to the fire affected zone are cleared, with other floors being cleared subsequently if needed.

There will also be requirements on the total number of fire-escapes and the dimensions of escape routes which are normally functions of the building type, the number of people expected within the building at any one time and the potential mobility of such persons. The escape routes are sized to give complete evacuation from the fire compartment into either a protected area or the outside of the structure in some 2,5 min with a basic travel velocity on staircases of approximately 150 persons per minute per metre width of escape route. It should however be rec-ognized that staircases are built in discrete widths and that doubling the staircase width will not double the throughput as an individual person requires finite space, and that minimum widths also need to be specified.

The above design figures are for able-bodied persons and need modifi-cation when there is a likelihood of disabled persons being part of the building occupancy (Shields, 1993).

The historical background to the reasons for imposing requirements on escape routes and evacuation is given in Read and Morris (1993). This imposition followed a series of disastrous fires over a period of some 50 years from 1881, when a theatre fire in Vienna was responsible for some 450 people being killed, to a fire in Coventry in 1931. Much of the background to current legislation in the UK is given in a Ministry of Works Report (1952) which was based on then current international practice.

All escape routes must also be lined with non-flammable, non-toxic materials. It should be noted that the fire doors opening on to escape routes may have a lower fire resistance performance requirement than the structure itself as they are only required to be effective in the very early stages of the fire where the major concern is with evacuation rather than structural stability. It has to be pointed out that fire doors propped open, even by fire extinguishers, are totally ineffective!

It is regrettable that there have been too many cases where, although the requisite number of escape routes have been provided, the escape routes have not been kept clear as the fire doors at the end of the escape routes were inoperative due to their being locked and unable to be opened. Examples of this occurred at Summerland (Anon, 1973) and the Dublin (Stardust) Disco fire (Anon, 1983).

It is equally important that the occupants of the structure are educated to respond to the warnings of any fire. In domestic situations, where the occupants are in a familiar situation, response may be faster than that in an unknown situation. There is still a large amount to be learnt concerning human behaviour in a fire (Canter, 1985; Proulx, 1994). Any warning

system must, to use a colloquial phrase, be ‘user friendly’. It has still not been determined satisfactorily whether alarm bells or sirens should be implemented by broadcast instructions or graphical displays on the best manner of exit. It is, in any case, essential that all escape routes are fully illuminated with self-contained emergency lighting and all signs are also supplied by the emergency power supplies.

The number of stories, some, one suspects, apocryphal, whereby people have totally ignored warnings to continue whatever they were doing before the alarm are legion; for example, the restaurant user who insisted on continuing to eat the meal that had been paid for in spite of the large quantities of smoke gradually engulfing the individual concerned. Evidence suggests that individuals will carry on as long as possible behaving as if the fire did not exist or there were no warnings (Proulx, 1994).

The educational process must also extend to the owners and lessees of any structural complex. This process must form a part of any fire safety management policy adopted. For buildings where the occupancy is con-trolled, part of the educational process can take the form of fire drill pro-cedure. This, however, must be treated with caution as it is the author’s experience that more people who know when the drills are to take place, the more likely it is that the drill will be circumvented and its efficacy lost.

The author has even noted the individuals going in the opposite direction to the flow of evacuees to collect items from offices, and when questioned glibly respond with words to the effect that it is only a drill!

1.1.3 Detection and control of the fire

In order to ensure life safety through evacuation, it is necessary to ensure that means are available for detection and control of the fire. Control of the fire is needed both to reduce the production of smoke allowing more efficient evacuation and to keep temperatures down in the structure to reduce subsequent damage.

1.1.3.1 Fire detection

Systems installed for fire detection may be manual or automatic or a combination of these.