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Fire Safety Principles Analysis

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Published: Mon, 18 Sep 2017

The aim of this report is to survey existing means of fire escape in a number of local houses of multiple occupations with a view to give recommendations for the preparations of a guide for fire escape in houses of multiple occupations.

For that purpose, a comprehensive literature review of fire safety principles as well as fire escape issues is required. A set of survey criteria has to be drawn from the literature review to be able to evaluate the existing houses. If necessary, officers from the East Sussex Fire Services will be interviewed to prepare the criteria for evaluation.

The survey will be made once the criteria are developed. Each house will be visited and observed in regard to the criteria developed. An analysis of the observations will be made and conclusions drawn. Finally, recommendations will be made for the preparation of the guide. The recommendations will be based on the survey and the conclusions of the analysis. The actual preparation of the guide is beyond the scope of this project.

3.1 Fire and Combustion

3.1.1 Theory of Fire

Fire can be described as “a process of combustion characterised by the emission of heat accompanied by smoke and flame” (Malhotra, 2001). Combustion is a series of very rapid chemical reactions between a fuel and oxygen (usually from the air), releasing heat and light. (Stollard, 1991). For combustion to occur heat and a fuel source must all be present and the removal of one of these will terminate the reaction. Flames are the visible manifestation of this reaction between a gaseous fuel and oxygen. If the fuel is a solid or liquid, there is first a gasification process as it is heated. So, heating a solid does not necessarily lead to combustion. Combustion will only occur when the gas is 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. Once the ignition has begun and the vapours are ignited, these flames will in turn further heat 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.

3.1.2 Fire Development

Once a fire is started, there are 3 ways the heat is transferred:

  • Conduction – Conduction is the mode of heat transfer where the heat flows from one point (at higher temperature) to another (at lower temperature) by direct material contact. There is no flow of the material itself. This is the predominant more of heat transfer in solids.
  • Convection – In the convection mode of heat transfer, the particles of the material flow, carrying the heat with them. This is the predominant mode of heat transfer in liquids and gases.
  • Radiation – Radiation is a mode where there is no need of a material at all. The heat gets transferred from a hot surface by radiating in all directions.

The way a fire develops in the open is much different from the way it evolves in a closed space such as a room in a building. The existence of walls and a ceiling changes the way the heat transfer modes affect the growth of the fire. The development of a fire in a room has several stages.

Figure 1: Standard fire growth curve

The growth period starts at ignition and lasts until all the contents of the room are alight. Initially, the flame remains near the surface of the fuel, with excess oxygen supply from the air of the room. The flame provides more heat and the rate of growth, controlled by the amount of surface of fuel available, increases.

At one point, the flames reach the ceiling and spreads. The surface area being heated is suddenly increased considerably. Also, the ceiling then radiates the heat back towards the other contents of the room. The remaining of the room catches fire very quickly because of this sudden rise in temperature and the stage is called flash over.

Flash over is short-lived and marks the start of the stable stage, where all the contents are burning. In this stage, the rate of burning depends on the amount of fuel left and the flow of air to the room.

Eventually, all contents will burn out and there will be no more fuel to feed the fire. The fire will slow down and that is called the decay stage.

3.2 Principles of Fire Safety

3.2.1 Fire Hazard and Risk to Life

Statistics and surveys (Marchant, 1973) on fire have shown that most fires occur between 10 am and 11pm. This is the time when people are awake and active. Also, the greatest number of fires occurs in dwellings. There is a lot more fire in winter months than in summer months, clearly showing a link between fire and energy use. More than one third of all fires can be attributed to human error. Rubbish burning, children with matches and electrical appliances are the biggest culprit among the causes of fire.

Due to the hazardous nature of fire: involving flames, heat, smoke and toxic gases, 1000 people lose their lives in fire each year. Unsurprisingly, (Malhotra, 2001) 85% of those deaths are in occupied buildings. The heat generated in a fire is hazardous to the building structure and can lead to the collapse of the building. This presents a serious threat to the safety of fire fighters. But the main cause of death is neither the heat nor the flames. The burnt bodies found give the impression that the occupants were burnt to death. However, studies have shown that heat and flames account for only a small minority of deaths. Most of the deaths are related to smoke and toxic gases. More than half the deaths in fires are attributed to carbon monoxide poisoning. Smoke also significantly decrease visibility and people often cannot reach safety only because of lack of visibility.

3.2.2 Approach to Fire Safety

The prime objective of fire safety is “to reduce to within acceptable limits the potential for injury or death to the occupants of the building and for others who may become involved” (Stollard, 1991). In order to achieve these objectives, there are 5 fire safety tactics that can be employed.

  • Prevention
  • Communications
  • Escape
  • Containment
  • Extinguishment

They can be used together for best efficiency.

The traditional approach to fire safety in buildings has been to prescribe certain acceptable levels related to some components of fire safety. These components are (Stollard, 1991):

  • Travel distances and routes
  • Load bearing elements of the structure
  • Roof construction
  • Separating walls
  • Compartment walls and compartment floors
  • Protected shafts
  • Concealed spaces and fire stopping
  • Internal surfaces
  • Stairways.

There is a newer approach that consists of looking at the building as a complex system. Fire engineering goes beyond merely satisfying prescriptive criteria of the building regulations. The building is a complex system where fire safety interacts with all other systems and services.

3.3 Prevention

As explained in section 3.1, fire can only occur if all of the three requirements are present. These requirements are fuel source, oxygen and ignition. The absence of any one is sufficient to avoid a fire. However, it is impossible to exclude oxygen from a building as it is present in the air in more than sufficient proportions. Fire prevention is therefore all about avoiding the occurrence of the fire in the first instance by controlling fuel sources and ignition.

3.3.1 Fuel Limitation

Fire can be prevented or the risk of fire can be reduced by limiting the amount of fuel available. Reducing the quantity of potential fuel has two main advantages. Firstly, there is less fuel to burn (fire load) and therefore fires will grow at a slower rate and temperatures will be lower. Secondly, less fuel also means less smoke (smoke load)

Fuel limitation can be achieved by controlling the building fabric. B building fabric is meant the structural components and finishes of the building. It is very important that the structural components of a building are not potential fuel sources in case of fire. The structural integrity and stability of the building depends on it. Also, structural elements are essential for containing a fire. Interior finishes on walls and ceiling are also crucial to fire prevention. The outer surface of the finishes is as important as the substrate behind it. The King’s Cross fire in 1987 demonstrated that adding successive coats of paint on older ones can present a danger.

Fuel limitation can also be achieved by controlling the contents of the building. The type and amount of textile and furniture play a determinant role in fire behaviour. However, the contents of a building are likely to change over time.

3.3.2 Ignition Prevention

To prevent ignition, the four main classes of ignition have to be understood.

  • Natural Phenomena – The most common source of natural ignition is lightning. Lightning can be very destructive. In 1984, lightning struck the York Minster and the resulting fire was very destructive. Another source of fire is earthquake. Because of the very low probability, such occurrences can be neglected for the common dwellings.
  • Human Error – Human carelessness is the main cause of fire in dwellings. Cooking appliances and smoking material account for almost all fire-related accidents and deaths in dwellings. Some of the accidents can be avoided by simple house keeping and general vigilance. Kids playing with matches have also contributed significantly to the number of fires in houses.
  • Technological Failure – Electrical and cooking appliances are a major source of ignition, especially in non-residential buildings. Buildings services are not perfect and are likely to fail at point or another. In small domestic dwellings, the improper installation of services can lead failure leading to fire. Instead of counting this as a technological failure, this should be classed as human error.
  • Deliberate Fire – Arson can be for many reasons: insurance claim, concealing a crime, vandalism, terrorism, or to deliberately hurt somebody.

3.4 Communications

Once a fire is started, the response time has a great impact of the effectiveness of that response. As seen in Figure 1, fire growth is exponential. So, the sooner action is taken, the better. The response to a fire depends to a large extent to communications at that time. The location of the fire and the deployment of evacuation and fire fighting all depend on effective communication. There are four facets of communications that are important to fire safety.

3.4.1 Detection

How fast a fire is detected has a crucial impact on the response and the effectiveness of that response. As fire grows exponentially, an early detection gives a better chance to fight the fire and limit damage. The best method of detection remains the occupants of the building. Humans can detect and locate a fire by sight, sound and smell. There is no better detection system than human presence. Places where there is more movement are less liable to have an undetected fire because of constant human presence. It is very important therefore that fire escape routes are not kept exclusively for that purpose. That would lead to a fire in that zone being undetected. Also, an unused portion of a building can very quickly become ‘temporary’ storage spaces which always contain boxes and crates full of rubbish.

Other detection mechanisms include smoke detector that is fitted to most houses in UK. There are also heat detectors, flame (light) detectors and thermal turbulence detectors.

3.4.2 Analysis

Once a fire is detected, it has to be interpreted and analysed before coming to a conclusion. Here again, nothing beats the human brain. Detection and analysis sort of overlap each other when it is by the occupants. Otherwise, conventional systems consist of relaying the detection information to a panel where the data will be analysed and an alarm raised if need be. With use of micro processors, analysis has become better and addressable systems have become common.

3.4.3 Alarm

If the conclusion of the analysis is that there is a fire, an alarm has to be set off. This alarm can be in the form of a simple bell that marks the need to evacuate the building. This is the most common system used. Other systems are connected to the fire services and the latter are then notified of a fire automatically.

3.4.4 Signs

Signs are very important to occupants as well as fire fighters when they give clear and concise information. However, in a dwelling type building where the occupants are familiar with the exit routes and the surroundings, they are unnecessary.

3.5 Escape

Perhaps the most important part of fire safety, escape from a building once a fire breaks is the only measure that ensures the safety of the occupants. The occupants must be able to safety reach a place of safety without being hindered by smoke, fire or heat. It is therefore essential that they manage to escape before the fire spreads.

There are three main means of escape:

  • Egress – Egress simply means getting out of the building as soon as the alarm is heard.
  • Refuge – Sometimes in large buildings, the time to get out is too much or the way out is too complicated. Then, a fire-safe compartment is used to provide a safe place within the building. Evacuation can take place at a slower rate from that compartment.
  • Rescue – Rescue should be kept as a last resort but is still important, especially for old and disabled person. In an apartment with only one entrance, a fire at or near the entrance can prevent escape even if the rest of the apartment is not on fire. Rescue via ladders from outside is then crucial.

The effectiveness of escape depends on several factors.

  • Occupancy – the occupants and their behaviour is very important to how successful an escape can be. Buildings such as houses where people sleep are naturally more risky than industrial buildings and offices where people are only present during the day. The amount and density of occupation as well as the mobility of the occupants are also important factors to consider. One of the most important factors is familiarity to the place. A house owner is more likely to escape than a guest because he is more familiar with the place.
  • Travel Distances – The total distance to be travelled to finally reach a safe place depends on a lot of factors. The layout of the room itself may change the distance to be travelled. The number of storeys and the building layout generally controls the travel distance.
  • Escape Lighting – Escape during the night or when there is an electric fault are made possible by emergency escape lighting. This can make the difference between success and failure of an escape. The precious minutes during which the occupants find their way are crucial because fire can spread very fast.
  • Rescue – As said previously, rescue should be relied on as a last resort. However, is rescue is to be used; it must be possible for the rescuers to get access to the building from outside.

3.6 Containment

Containment is the ability of a building to contain a fire even in the event of every other tactic failing. It should be a built-in capability of the building. A fire should be contained to its compartment of origin to prevent it from spreading to other parts of the building. Containment is also responsible to preventing spread of smoke. Containment must ultimately limit the fire spread to the building only to prevent fire spread to neighbouring buildings.

The first step on containment is to design the structural elements to resist the fire. The amount of protection to be given to structural elements depends on the escape time required. Also, if fire fighters are to work inside the building, the structure must maintain its stability and integrity throughout the operations. The collapse of the World Trade Centre has shown how fire fighters can be at risk when entering a building on fire. If the structural elements are not capable of ensuring structural integrity on their own, it will be important to apply fire protection. Steel structures are usually protected with plaster boards, cement sprays of intumescent paints. Wooden structures need protection as well.

Compartmentation in a building can be compared to that on a ship. It involves physically incorporating fire and smoke tight barriers between different zones of the building. This gains time and limits the fire to one particular zone. Compartments can be rooms or storeys. The number of compartments into which to divide a given space is a function of the occupation and contents of that space. The higher the fire load, the smaller the compartments should be. Also, there must be a separate escape route from each individual compartment.

The exterior envelope of a building is used as the final barrier for the fire. It protects the building from external fires and fires to adjoining buildings. It also prevents the spreading of internal fires to spread to adjoining properties. Fire can spread by debris falling on the roof, flames spreading through openings and radiation through glazed areas.

The fire containment methods described up to now are passive methods. They are built-in characteristics of the building design. Active measures are those that operate only in the event of a fire. Pressurisation allow escape route to be clear of smoke even when the fire doors are opened to get access to the escape route. Venting is used to provide the smoke an easier way out of the building than by spreading to the rest of the building.

3.7 Extinguishment

Even after escape and containment are successful, a fire still needs to be extinguishment to limit the amount of property loss and to prevent spread to adjoining properties. It can be achieved by removing one of the three essential ingredients of fire: fuel, oxygen and ignition. When a fire is already started, it is self igniting and does not need additional ignition. Therefore, extinguishment can be achieved by cutting off the oxygen supply. Alternatively, the temperature can be brought down below that of self-ignition and thus killing the fire.

There are several material used in fire extinguishment:

  • Water – Water is the most commonly used material for fire fighting. It has the capacity of achieving both tactics of fire extinguishment discussed. It will drop the temperature down and will also cut off oxygen supply. However, the main problem with water is that it is a relatively good conductor of electricity. It cannot therefore be used on electrical appliances.
  • Foam – Foam is particularly good at extinguishing fires from liquids. They act mainly by smothering the fuel from the oxygen. They can also be high-expansion types which will fill the area and are generally used by fire fighters.
  • Carbon Dioxide – Carbon dioxide fire extinguishers provide both a cooling and smothering agent. Carbon dioxide is heavier than oxygen and will displace oxygen on and around the fuel. It is light and a large amount can be contained under high pressure in relatively light extinguishers. They can be used on electric fires but they are restricted to localised fire spots as concentrated carbon dioxide can be lethal.
  • Dry Powder – Dry powder act by quenching the chemical reaction of the fire. There are a number of possible substances available, some more appropriate than others on a given type of fire.

In section 3.5, fire escape has been introduced as one of the tactics of fire safety in buildings. In the context of the current research topic, the factors influencing escape in a fire are brought under the magnifying glass. Other key topics related to escape during a fire are discussed.

4.1 Occupancy

The nature and number of occupants as well as their likely behaviour pattern is a crucial factor in determining the speed and success of an escape from a fire. Five key characteristics of occupants that are most influential are:

  • Sleeping risk
  • Numbers
  • Mobility
  • Familiarity
  • Response to fire alarm

4.1.1 Sleeping Risk

Buildings where people sleep are more at risk of a fire than buildings with only day-time occupation. A building where people sleep is likely to be occupied for longer hours, combining day and night occupancy. Also, a fire start while people are sleeping is likely to be detected at a much advanced stage. Once detected, the response of people who are asleep is bound to be much slower than in day time. To understand the extent of the risk, consider the same people in the same building but in day time.

A cigarette butt or red ashes fall on the bed sheet and the latter immediately gets a hole in it, which slowly grows wider and wider as the red edge of the hole eats at the bed sheet. In day time with people fully awake, such a minor incident will be dealt with promptly. The ashes will be rapidly removed and the growth stopped by water and even by hand. At night, the same incident may have a completely different turn out of events. The unchecked growth would soon lead to the bed sheet catching fire and setting fire to the mattress. The surrounding furniture, curtains and carpet will soon be alight and that room will have a fully developed fire that can potentially spread over the whole buildings very fast if undetected.

Consider another scenario. A house wife has been cooking for breakfast before going to bed. She switches off the electric hob, turns off the lights of the kitchen, and goes upstairs to sleep. A kitchen towel has been left in contact with the hob by mistake. It does not catch fire immediately because there is no naked flame. It heats up, and eventually catches fire after a few minutes even though the hob is off. This fire can grow, away from the eyes of the sleeping occupants, in the kitchen and spread to the rest of the house. Had it been during the day, somebody will eventually walk into the kitchen and detect the towel starting the smoke and will simply put it in the sink and open the tap. Such is the difference between normal day time and at night, where people are asleep.

4.1.2 Numbers

The number of people in a building and their likely position within the building are important factors in designing an escape route. To a large extent, these factors depend on the purpose of the building. For houses of multiple occupations, the main purpose is obviously residential, but there can be more people at a given time than the actual number of residents. The maximum number of people that can be present can be estimated by the use of an ‘occupancy loaf factor’. The area of the building divided by the occupancy load factor gives and idea of the maximum number of people that can be present. A general guide is given in the form of Table 1.

Table 1: Building type and occupancy levels (Stollard, 1991)

Building Type

Occupancy

1

Houses

Five times bed spaces

2

Flats an maisonnettes

Five times bed spaces

3

Residential institutions (hospitals, prisons etc.)

Three times bed spaces

4

Hotels and boarding houses

Two times bed spaces

5

Offices, commercial and schools

Occupancy load factor = 6

6

Shops

Occupancy load factor = 2

7

Assembly and recreation

 
 

(a) bars

Occupancy load factor = 0.5

 

(b) dance halls, queuing areas

Occupancy load factor = 0.7

 

(c) meeting rooms, restaurants

Occupancy load factor = 1

8

Industrial

Occupancy load factor = 5

9

Storage

Occupancy load factor = 15

10

Car-parks

Two times parking places

     

For example, consider a small family house with two bed rooms. One bed is double and another is single.

Building type=House

Occupancy rate=Five times bed spaces

Number of bed spaces=2 + 1=3

Maximum number of people=5 x 3=15

This is only a guide but is quite useful. It may seem at first sight that with only 3 bed spaces, the occupancy should have been 3 instead of 15. However, the maximum number of people can occur during a party or while receiving guests.

Another factor to consider is the likely hood of concentration of people in a particular area. A concentrated number of people behave differently from individuals. They move at a much slower rate and therefore travel distances to escape routes should be kept shorter. The escape routes should also be wider than would normally be required to allow evacuation of a larger number of people at the same time.

4.1.3 Mobility

Different people move at different rates, depending on their age, fitness, state of mind, disability, any special requirements and several more factors. It has been found from experiments that a normal healthy person can move between 60 and 80 metres a minute. At the other extreme of the spectrum, very old and disabled persons may need assistance to move at all. A patient in intensive care in a hospital may have to be moved along with an array of bulky equipment to sustain his life. There are no hard rules about figures to use and a reasonable estimate has to be made from the information about the occupancy of the building.

The design of the escape route also has a big incidence on mobility. The layout of the escape route may include obstructions such as furniture. Changes in direction and use of stairs etc would normally reduce mobility. A qualitative assessment is the best that can be done.

4.1.4 Familiarity

If people are not familiar with a building, they will find more difficulty finding and reaching the escape route. In buildings such as normal houses, the occupants are likely to be residents and will normally be well familiar with the place. In an office with a regular staff, there is not likely to be problems related with familiarity. However, in buildings such as hotels, bed and breakfast etc, familiarity can be a problem. People will instinctively try to get out the same way they came in, and that may not b a protected fire escape route.

4.1.5 Response

The response to a fire or the sounding of a fire alarm is a very important factor to consider. In an office with a regular staff, there will be more discipline and the staff will be trained by means of evacuation drills. In a flat or other residential house, the response is most likely to be decided on the spot in the real fire. The response will also depend on the state of mind and attitude of the occupants. People who are asleep or drunk will be slow to react and may not react in a strictly rational way.

Very often, people do not immediately head for the evacuation route. Instead, they try to contact others in the building to confirm that there is fire and how serious it is.

4.2 Travel Distances

The maximum travel distance in the escape process is crucial for a successful escape. To quantify a travel distance, the steps involved in the escape process have to be considered one at a time.

4.2.1 Stage 1 – Escape from room of origin

Stage one of the escape process is getting out of the room of origin. The speed (and hence time) at which the room has to be evacuated depends of the rate of fire spread. However, the rate of fire spread is hard to quantify. It is therefore best to ensure early detection of the fire in the room. The room has to be evacuated as soon as possible. For small rooms with low occupancy, one exit is sufficient. For larger rooms or rooms with high number of people, two or more exits may have to be provided.

In some instances, there may be a smaller room inside a larger one. The escape from both the smaller room and the larger one has to be considered as stage 1. It is also crucial to make sure that the occupants of the inner room are aware of any fire incident in the larger room.

4.2.2 Stage 2 – Escape from compartment

Stage 2 involves the escape from the compartment where the fire started. This is usually via escape routes to the final exit, to a protected stairway or protected escape route, or an adjoining compartment that can be used as refuge. Compartments are usually designed for one hour protection and sub-compartments are designed for 30 minutes protection. This should give the occupants time to escape before being overwhelmed by the fire spread across compartments. The combined travel distances of stages 1 and 2 have to be maintained within a certain limit to allow the occupants to evacuate within that time. Table 2 below gives a list of types of buildings and suggested travel distances. This is based on the


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