Fire from First Principles

A Design Guide to International Building Fire Safety

Paul Stollard

• Publisher: Routledge
• Year: 2014
• ISBN:
  9781317919032
• Edition: 4
• Topic: Architektur
• Subtopic: Architektur Methoden & Materialien

Fire safety is a fundamental requirement of any building, and is of concern to several professions which contribute to the construction process. Following on from the success of the previous three editions, Paul Stollard has returned to update and expand this classic introduction to the theoretical basis of fire-safety engineering and risk assessment.

Avoiding complex calculations and specifications, Fire From First Principles is written with architects, building control officers and other construction professionals without fire engineering backgrounds in mind. By tackling an overview of the factors which contribute to fire risk, and how building design can limit these, the reader will gain a fuller understanding of the science behind fire regulations, safe design, and construction solutions.

All regulations content is fully updated, and has been expanded to cover the USA and China as well as the UK. Ideal for students of architecture and construction subjects, as well as practitioners from all built environment fields learning about fire safety for the first time.

Chapter 1

Theory

Invariably there is too little time in the design process for architects to become fully involved in the technicalities of the combustion process and it just is not necessary. Architects do not normally want, and they do not need, to become fire scientists. However, to ensure an acceptable standard of fire safety without allowing it to dominate the design, it is necessary to be aware of what happens in a fire. Architects need to know what their design objectives ought to be, and how these can be achieved. Therefore this chapter briefly sketches in the few essentials that the designer should know, and places them in the context of the design process. It should not be necessary for architects to become involved in calculations, formulae and chemical symbols, and all of these have been avoided here. The references in the final chapters provide boundless technical data, but if architects become involved in the fine detail of chemical reactions or the structure of flames, then they have gone beyond the stage where specialist advice is essential. This chapter, then, is for the “ordinary” architect, who does not have the time or inclination to become a fire safety specialist.

1.1 Fire science

This first section is intended to provide an outline of the key stages in ignition and fire growth and to outline the products of combustion. Some of the most common technical terms are explained, so that the designer will be able to follow manufactures’ literature and to discuss their designs with the legislative authorities. A full glossary of fire terms is included at the end of the book, as is an index. Terms in the glossary are highlighted in bold on their first use in the text.

1.1.1 Ignition

Combustion is a series of very rapid chemical reactions between a fuel and oxygen (usually from the air), releasing heat and light. For combustion to occur, oxygen, heat and a fuel source must all be present and the removal of any one of these will terminate the reaction. These three ingredients of fire are so essential that they are referred to as the triangle of fire. Removal of any of the three (heat, fuel or oxygen) will terminate the reaction and put out the fire.

Flames are the visible manifestation of this reaction between a gaseous fuel and oxygen. If the fuel is already a gas and already mixed with oxygen, then this is described as a pre-mixed flame; if the fuel is a solid or liquid and the mixing occurs only during combustion as the fuel gives off flammable vapours, then the flames are described as diffusion flames. The gasification of a solid or liquid fuel occurs as it is heated and chemically degrades to produce flammable volatiles. Simply heating a suitable fuel does not necessarily lead to combustion, this only occurs when the vapours given off by the fuel ignite, or are ignited.

The temperature to which a fuel has to be heated for the gases given off to flash when an ignition source is applied is known as the fuel’s flash point, while the temperature to which a fuel is heated for vapours given off by the fuel to sustain ignition is described as the fire point. If these vapours will ignite spontaneously without the application of an external flame, then it is said to have reached its spontaneous ignition temperature.

Therefore it is not the fuel itself which burns, but the vapours given off as the fuel is heated. Once ignition has begun and the vapours are ignited, these flames will in turn further heat the fuel and increase the rate of production of flammable vapours. For the flames to exist at the surface of the fuel, the combustion process must be self-sustaining and capable of supplying the necessary energy to maintain the flow of flammable vapours from the fuel.

In diffusion flames the rate of burning is determined by the rate of mixing of the fuel and oxygen and this is normally controlled by the degree of ventilation, the amount of fuel and the configuration of the room – all factors which the architect can influence. However, no such restrictions exist with pre-mixed flames and therefore the rate of burning can be very much faster. A common example of a pre-mixed flame is the laboratory Bunsen burner. A pre-mixture of fuel and oxygen in a confined space will lead to an explosion risk. Although a gaseous fuel can be mixed in different proportions with air, not all such mixtures are flammable, and it is possible to establish upper and lower limits of flammability outside of which flame cannot travel through the mixture.

Throughout this section it is intended to refer to recent fire statistics, showing the practical applications in buildings of the theoretical issues of fire ignition, growth and products. In 20011/12 there were a total of 272,000 fires attended by public fire services in Great Britain; however less than 30 percent of these fires occurred in occupied buildings. In very general terms, these figures are very encouraging as they show the continuing downward trend in the number of fires and consequential injuries, which has been the case for a significant number of years.

It is occupied buildings which are obviously of concern to the architect. In 2011/12 there were 312 fatalities in occupied buildings, with the number of other injuries over 10,000. It is important to distinguish between dwellings and other occupied buildings. Over 60 percent of fires in occupied buildings occurred in dwellings, but they accounted for around 90 percent of the deaths and non-fatal injuries (Table 1.1).

Table 1.1 Fire statistics, Great Britain 2011/12

	Fires	Fatal injuries	Non-fatal injuries
Dwellings	44,000	287	8,900
Non-dwellings	27,000	25	1,200
All occupied buildings	71,000	312	10,100
Outdoor fires	193,000
Chimneys	8,000
Total	272,000

Source: Department for Communities and Local Government, Fire Statistics, Great Britain 2011 to 2012, derived from tables 1.1, 2.1 and section 3.1.

1.1.2 Fire growth

The three basic mechanisms of heat transfer are conduction, convection and radiation; and all three are common in building fires. Conduction is the mode of heat transfer within solids, and although it occurs in liquids and gases, it is normally masked by convection. Convection involves the movement of the medium and therefore is restricted to liquids and gases. Radiation is a form of heat transfer which does require an intervening medium between the source and the receiver (Figure 1.1).

1.1 Forms of heat transfer

Fires within enclosures behave differently and with different rates of burning from those in the open. It is important to understand the stages in the development of an enclosed fire as they will be the most common (Figure 1.2). The presence of a “ceiling” over the fire has the immediate effect of increasing the radiant heat returned to the fuel, and the presence of the walls will increase this effect, provided there is sufficient ventilation. With sufficient fuel and ventilation, an enclosed fire will pass through a series of stages after ignition: a period of growth, one of stability and then a period of cooling. The plotting of temperature against time from ignition will give a fire growth curve, and as these will vary to reflect the conditions of the fire, they are extremely useful to fire scientists considering the consequences of changing the conditions.

1.2 Standard compartment fire

The growth period lasts from the moment of ignition to the time when all combustibles materials within the enclosure are alight (Figure 1.3). At first, the vapours given off by the fuel will be burning near the surface from which they are generated; the ventilation is normally more than enough to supply oxygen for this, and the rate of burning is controlled by the surface area of the fuel. The duration of the growth period depends on many factors, but a critical moment is reached when the flames reach the ceiling. As they spread out under the ceiling, the surface area greatly increases. Consequently, the radiant heat transfer back to the surface of the fuel is dramatically increased. This will probably occur (in a domestic sized room with typical furnishings) when the temperature at the ceiling has reached approximately 550°C. The remaining combustible materials will now rapidly reach their fire points and ignite within 3–4 seconds. This sudden transition is known as flashover and represents the start of the stable phase of the fire.

1.3 Standard fire growth curve

If there is inadequate ventilation available to the fire during the growth period, then the fire may fail to flashover due to oxygen starvation. The fire may die out completely, or it may continue to smoulder; and such a smouldering fire can be extremely hazardous as the enclosure fills with flammable vapours. If this is then mixed with a new supply of oxygen (e.g. by a door being opened), it may ignite with an eruption of flame, this effect being known as backdraught. This can be highly dangerous for firefighters attempting to enter rooms to search for survivors, and they have to ensure sealed or semi-sealed spaces are provided with some ventilation at high level before attempting entry.

During the stable phase of an enclosed fire the flaming is no longer localised, but occurs throughout the enclosure. The volatiles are mixing with the incoming air, and the rate of burning will be determined by the level of ventilation and the amount of fuel present. It is this stage of the fire which is of the greatest significance to the architect because maximum temperatures will be attained. The fire resistance of the elements will have to take into account both the maximum temperatures which will be reached and the length of time for which they are likely to be sustained. The final cooling period sees the decay of the fire, once all available fuel has been consumed.

Combustion can only occur if oxygen is present; many extinguishing agents operate by limiting the amount of oxygen available to the fire (e.g. carbon dioxide, foam, sand). The most common extinguishing agent, water, works by cooling the materials involved in the reaction. Without heat the reaction cannot start and if the materials are suddenly cooled the reaction will cease. The third method of extinguishing a fire is by interrupting the reaction itself, and dry powder does this by slowing down the reaction until it ceases to be self-sustaining.

Looking at the statistics for fires in occupied buildings, it can be seen that between 33 and 43 percent are confined to the first material ignited and only 9 to 13 percent extend beyond the room of origin, with the greater fire spread occurring in the occupied building other than dwellings. Over 90 percent of fires in dwellings do not extend beyond the room where they start. These figures are based on the spread of fire damage, and may not fully take account of smoke spread in advance of the fire (Table 1.2).

Table 1.2 Fire spread statistics, United Kingdom 1992 (percentages)

	Percentages of fires in dwellings	Percentages of fires in other occupied buildings
Confined to first material ignited	43	33
Confined to room of origin	48	54
Confined to building or origin	8	8
Spread beyond building of origin	1	5

Source: Home Office, Fire Statistics: United Kingdom, 1992 (1994), derived from table 56.

(Note: this data is not included in the more recent statistics for Great Britain.)

The major products of combustion are heat, light and smoke. ...