Introduction to fire safety

1. INTRODUCTION

1.1 Fire losses

An international survey of fire losses conducted 20 years ago gives the following values:

Human fatalities:      4 to 34 fire fatalities per million head of population

Financial losses:        1.6 to 5.9 0/00 of the Gross National Product per year

In order to obtain an overall perspective of the risk of fire fatalities in buildings, it is interesting to compare it with other accidental causes.

Activity	Fatal accident rate per person and for an average lifetime of 70 years
Motor cycling (UK)   Scheduled flights (USA)   Average for disease (USA)   Travelling by car (USA)   Travelling by car (UK)   At home - average (excl, sickness)   At home - total able bodied persons   Fires in hotels (UK)   Fires in dwellings (UK)   Natural disasters (USA)	4.1 1.5 0.7 0.6 0.4 0.02 0.01 0.01 0.001 0.0001

Table 1 Comparison of fatality statistics from different accidental causes (Sources [1] [2] [3])

 Although the risk of life loss in fire is quite low in comparison with other causes of death, there is a tendency for an accident involving multiple fatalities, over about 5 deaths, to attract a high level of news coverage. In this sense, building fires tend to be regarded in the same high profile way as air crashes or earthquakes. Nonetheless, it is important that the causes of fire fatalities should be examined with a view to public safety.

A breakdown of fatal casualties by fire location in Europe and the USA shows that approximately 80 to 85% of all fatalities occur in domestic buildings (dwellings, flats) and only 10% occur in public buildings. On the other hand, about 95% of all deaths in buildings are due mostly to smoke or, in very few cases, to heat.

A survey on non-domestic fires in the Netherlands and France shows that the financial loss of the building content outweighs the cost of building damage [4].

Losses to building content	43%
	+
Consequential losses	36%	=>	4/5
Losses to building	21%	=>	1/5

The indication is that damage to content and consequential losses are more significant financial factors than damage to buildings.

The global cost of fire in Europe includes the following items:

(in 0/00)

direct fire losses on building and content	2 - 5
consequential losses	0.2 - 3
human fatalities	0.3 - 2
fire brigade costs	1 - 3
administration costs of insurers	1 - 3
education costs, cost of research	0.1 - 0.5
cost of fire safety measures in buildings	2 - 5

and varies between 1.3 to 2% of the Gross National Product. The last item, i.e. the cost of fire safety measures in buildings, represents about 1 to 3% as an average of the total building costs. In most countries, a high investment in fire safety in buildings brings a reduction on direct, indirect, and human losses. Still, it is very important to analyse the cost-effectiveness or, in other words, the return of investment for each detailed fire precaution measure (see Section 1.5).

1.2 Fire risk

The usual way to measure the risk of fire is expressed by

R_fire = P x L_ext

where 

R_fire = fire risk

P = the probability of occurrence of a fire

L_ext = probable extent of total losses.

R_fire < R_accepted

R_accepted represents the target risk which varies from country to country.

 

The risk R can never be zero, and we have to accept a certain level of risk for every type of building and/or occupancy. This level will depend on the number of persons, their ability to escape, and the value of content exposed to fire.

Table 2 gives some indications of the occurrence of fire in different types of building:

Type of building occupancy	Source	Number of fires per million m2 floor area per year
INDUSTRIAL BUILDINGS	United Kingdom [5] Germany [6] CIB W14 [7]	2 2 2
OFFICES	United Kingdom [5] USA [8] CIB W14 [7]	1 1 0.5 + 5
DWELLINGS	United Kingdom [5] Canada [9] CIB W14 [7]	2 5 0.05 + 2

Table 2 Occurrence of fire

The probable extent of losses varies for different occupancies and is a function of the degree of compartmentation, type of building, extent of automatic detection and extinguishing devices (Sprinkler/CO₂/Halon), structural fire resistance, and of the involved fire brigade.

The probability of fires getting out of control is strongly related to the type of active measures available, as indicated in the table below (reference CIB W14 Workshop 'Structural Fire Safety' [7]).

Type of active measures	Probability of fires getting out of control
Public fire bridage   Sprinkler   High standard residential fire brigade combined with alarm system   Sprinkler and high standard residential fire brigade	100/1000 20/1000 ≥10/1000 : 1/1000 ≥1/10 000

Table 3

1.3 Objectives of fire safety

Fire safety in buildings is concerned with achieving two fundamental objectives:

1. to reduce the loss of life in, or in the neighbourhood of, building fires
2. to reduce the property or financial loss in, or in the neighbourhood of, building fires

In most countries, the responsibility for achieving these objectives is divided between government or civic authorities, who have responsibility for life safety via building regulations, and insurance companies who are concerned with property loss through their fire insurance policies.

Often, the two objectives are thought to be incompatible, even occasionally conflicting. For example, sprinklers and automatic detection devices tend to be regarded as property protectors rather than life protectors and insurance companies will commonly offer substantial premium discounts when they are used. They do not figure highly in most national building regulations, yet the evidence that is available suggests that they are extremely effective in preserving life.

In fact, the actions required to achieve life and property preservations are very similar.

Figures 1a and 1b use a systematic approach to identify the major options to reduce losses. They show that practically all options reduce the risk of human losses as well as the risk of financial direct and consequential losses. In fact, we must realise that global fire safety must ultimately be answered by adequate fire safety concepts.

1.4 Fire safety concepts

Fire safety concepts, also called fire engineering, are defined as optimal packages of integrated structural, technical, and organisational fire precaution measures which allow well-defined objectives agreed by the owner, the fire authority, and the designer to be fulfilled.

In order to develop possible fire safety concepts, it is essential to look at the typical development of an uncontrolled fire, as shown in Figure 2.

 

Another very similar presentation given as Figure 3 allows the reasons for success or failure of well-defined fire precaution measures to be visualised.

 

Analysing this figure, we realise that we will be able to overcome the fire risk through three basic concepts, which are:

• a structural concept accepting the occurrence of flash-over in a limited number of fire compartments
• a monitoring concept avoiding the occurrence of flash-over
• an extinguishing concept avoiding the occurrence of flash-over

1.4.1 Structural fire safety concept - Fire resistance

A structural concept comprises compartmentation combined with an adequate fire resistant structure. This may be the best choice as long as the normal (cold-design) use of the building allows compartmentation by fire resistant floors and walls.

It is admitted that the fire may reach flashover conditions before fire fighting action begins.

The necessary time of fire resistance should be determined by the condition that the fire should not spread outside the fire compartment. Hence, the separating and (possibly) load-bearing function of the relevant building components should be maintained during the anticipated duration of the fire.

Whenever possible, fire spread should be limited by fireproof partition walls and floors. Combustible building components should be designed or treated to prevent fire spread by smouldering, eg. in two layer built-up roofs the combustible layer should be covered by a non-combustible one. The design of the facade should prevent flames climbing into an upper storey.

It is important to underline that all partition elements like walls, decks, ceilings, and roofs (in some cases) must fulfil three criteria to be classified in a fire class (30/60/90...):

• a load bearing criterion proving the stability of the element - R
• an insulation criterion proving the insulation capacity of the element - E
• an integrity criterion proving that no flames and no smoke goes through the element - I

The load bearing structural elements with no partitioning function only have to fulfil the first criterion R.

On the contrary, a composite floor or a partitioning wall should fulfil the three criteria REI.

Fire resistance of the building components is usually prescribed in the building codes, where it is normally expressed in units of time.

The required time for fire resistance is usually expressed in terms of multiples of 30 minutes: for example 30, 60, 90, or 120 minutes, related to ISO Standard fire. This means that a component is able to fulfil its function during the required time under a temperature exposure according to ISO.

The time-temperature relationship in the standard fire may significantly differ from that in a real fire, but modern fire design procedures allow fire resistance to be determined for natural fires, as will be shown in section 1.5. The time criterion should not be interpreted as an escape time for occupants or an intervention time for the fire brigade.

For structures and their occupancies, it is often more effective to use alternative concepts based on the avoidance of flash-over by means of non-structural active fire precaution measures. Active measures are based on a monitoring or an extinguishing concept.

1.4.2 Monitoring concept

The monitoring concept is based on automatic detection devices and automatic alarm transmission to an adequate fire brigade (around the clock) - preferably to an on-site fire brigade.

A monitoring concept (shown in Figure 4), which involves limited or no structural fire resistance, may represent the best choice when the normal (cold-design) use of a building calls for a minimum of compartmentation.

This is most applicable for occupancies with reduced fire load densities, for low to medium-rise buildings in which fires may be expected to develop slowly, and where an effective and quick-responding fire brigade is available.

• Fire detection

Automatic alarm systems are activated by smoke, heat, or flames. They work mechanically or by electric or electronic systems. Preference is given to smoke detection since this is, in general, by far the most effective method. When detectors begin to operate, an alarm is automatically set off. For maximum effectiveness, the alarm should be transmitted day and night to a nearby fire brigade station. Alarm systems with sound generating sirens are almost the only means against deliberate fires.

Sprinklers act as extinguishing devices and as a 'slow' alarm system (heat detectors).

• Fire fighting

The effectiveness of fire fighting mainly depends on the time of arrival of the fire brigade and access to the fire.

The easiest means is the use of hand fire extinguishers if there are people who detect the fire and who are skilled enough to use an extinguisher.

Fire fighting services may be either public fire brigades or works (on-site) fire brigades. Works fire brigades have the advantage of being acquainted with the locality and having shorter distances to reach the fire, but for all fire brigades it is essential to have access routes for their vehicles. For sprinklers as well as for fire brigades, a sufficient water supply is necessary and special precautions may be necessary in wintertime.

1.4.3 Extinguishing concept

The extinguishing concept is based on automatic extinguishing devices such as sprinklers, CO_2, or Halon systems with automatic alarm transmission to an adequate fire brigade and the owner (see Figure 5).

 The extinguishing concept with limited or even no structural fire resistance may represent the best choice when the normal (cold design) use of a building calls for a minimum of compartmentation. It is most applicable for occupancies with medium or high fire load densities and fast developing fires.

Building owners often are afraid of the damage that these systems may cause by the water poured on the stored material or the manufacturing machines. But sprinklers open their valves only at the spot where temperature reaches a critical limit of 70° to 140°C. It has to be noted that 75% of all fires in premises with sprinklers devices are controlled by 1 to maximum 4 sprinkler heads. This represents approximately 50 m² watered by opened sprinkler heads. By means of an automatic alarm transmission system, they inform the owner and fire brigade at once. It is important to know that automatic detection and extinguishing systems have to be maintained once or twice a year by specialists.

The concept of fire engineering is now accepted in most countries and even applied in local regulations and design codes.

1.5 Cost-effectiveness

The type of occupancy and the choice of the structural 'cold-design' are the main variables governing the amount of fire protection measures necessary and thus the cost of the total fire engineering concept. The cold-design concept and the fire engineering concept should be integrated from the beginning in order to obtain an optimum safety level with a minimum of investment. This aim can only be reached through a dialogue between the designers of a building and the fire authority at a very early stage of the planning.

An outline cost-benefit analysis indicates that the return on investment in fire precautions is variable.

Figure 6 shows that, as the chosen expenditure level (and therefore also the level of safety precautions) is higher, the loss expectation due to fire will decrease. This relation is indicated schematically by the broken line. The loss-expenditure curve has a hyperbolic shape which means that, beyond a certain point, there is little benefit in increasing the level of protection.

 

From the relation between expenditure and loss expectation it is possible to deduce the relation between expenditure and overall cost due to fire (= loss expectation + expenditure).

See the solid curve, the minimum of which corresponds to the optimum solution.

In this context, it should be pointed out that, in general, the expenditure must not fall below a certain minimum, regarding the requirements of life safety and/or the minimum level of acceptability for purposes of insurance. These aspects are also indicated in the figure.

Finally, attention must be drawn to the criteria by which the behaviour of the structure under fire conditions will have to be judged. In applying measures with a view to improving the fire safety of a building, it will certainly be necessary to consider what the ultimate effect of such measures will be. It is known from experience that major building fires may damage the structure to such an extent that demolition of the building becomes necessary even though it has not collapsed. The money spent on protecting it from collapse will then have to be regarded as lost. In such a case, it would be better either to limit the precautions merely to a level where escape of the occupants in the event of a fire is ensured or to choose an alternative fire safety concept.

For a detailed cost-benefit-analysis, a differentiated approach is necessary by calculating the annual costs of fire safety and trying to optimise them by comparison of different fire safety concepts. The basic formula is the following one:

Annual costs of fire safety = 

[Sum of all investments for fire safety].[the mortgage rate in %] 

+ [The repetitive maintenance costs per year] 

+ [The annual premiums for the chosen fire safety concept (fire, acts of God, liability, business interruption)]

In most cases, alternative concepts will show more cost-effective rather than structural concepts.

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