Fire safety systems. Behaviour of fires

Transcription

1 Fire safety systems The objective of the BCA Housing Provisions (Volume 2) with respect to Class 1 buildings (residential) and fire, is to safeguard occupants from illness or injury, to avoid the spread of fire, and to protect a building from the effects of a bushfire. Go to NCC Online, and read Section 2 Performance Provisions, Part 2.3 Fire Safety. In order to know a little about the different construction methods required to satisfy fire safety requirements of the BCA, it is important to first understand something about the behaviour of fires. Behaviour of fires The term compartment can be used to describe any confined space in a building. It includes rooms, corridors, stair wells, shafts and ducts. For the purposes of this topic, we are referring to compartments as enclosures, and not fire-rated compartments as referred to in the BCA. A fire in an enclosed compartment can develop in a multitude of different ways, depending on the enclosure geometry, ventilation and fuel type. Development of a fire in an enclosed compartment A number of stages can be identified in an enclosed fire. These are: Pre-ignition Smouldering and/or flaming ignition Growth of flaming Topic 1.5 Enclosing systems CPCCBS6001 Ed 1 1

2 Fully developed fire Decay Pre-ignition In the pre-ignition stage an ignition source and a material are in contact. Depending on the conditions, smouldering or flaming ignition may or may not occur. Ignition If smouldering ignition occurs, it will then propagate until: all materials is consumed, all the oxygen in the enclosure is depleted, or transition to flaming combustion occurs. If flaming combustion occurs, the fire will grow until it reaches a maximum rate of burning and the fuel is depleted without flashover being achieved, all the oxygen in the enclosure is depleted without flashover being achieved, or flashover occurs. Growth phase During the initial part of the growth phase, the fire grows independently of the size, shape and material of the enclosure. The temperature rises are localised and escape is relatively easy. If there is sufficient fuel and oxygen, the fire will continue to grow causing an increase in temperature, particularly in the upper part of the enclosure. Flashover Flashover is the transition from the growth stage to the fully developed stage. It occurs over a relatively short period of time and is characterised by the rapid involvement of all the combustible materials within the enclosure and a rapid rise in temperature. The time to flashover is of critical importance to life safety because it generally represents the maximum escape time from that enclosure. Fully developed Once flashover has occurred, the fire is said to be fully developed. During this stage, the rate of heat release is at its maximum. Generally the fire is ventilated controlled (i.e. more fuel is pyrolised than can be combusted with the available oxygen and unburnt fuel which escapes the enclosure may burn outside where further oxygen is available). Such ventilated controlled fires produce large amounts of carbon monoxide and hence conditions can be quite dangerous to occupants. During this stage the fire represents a threat to adjacent enclosures spreading flame and combustion products internally (through open doors and ducts) and externally (through open windows, etc.). In addition, the heat form the fire is sufficient to cause structural damage. Decay The decay stage is considered to commence when the average temperature in the enclosure has fallen to 80% of its peak value. The decay stage may last for an extended 2 Topic 1.5 Enclosing systems CPCCBS6001 Ed 1

3 period with fire reverting to a fuel controlled fire as the amount of pyrolisate produced decreases relative to the oxygen supply. Ultimately, the fire may cease flaming and become a smouldering mass of glowing embers producing high local temperatures and hazardous atmospheres of unburnt pyrolisates for some time. The decay phase is not of great significance as far as the safety of occupants and damage to the building is concerned, although it may influence fire-fighting activities. Description of the temperature variation with time in an enclosure Fires in partially enclosed compartments There may be cases where an enclosure is only partially enclosed, such as when there may be windows, doors or other openings. This will in turn affect the flow of fire and smoke. The mass flows will depend on the pressure differences across the openings. This can be demonstrated by indicating the various stages of fires in partially enclosed compartments. Stage 1 - In this stage, the pressure inside is higher than the pressure outside the compartment. This is due to the expansion of the hot gases, which have a greater volume than cold gases. If the opening is not at ceiling height, the cold gases will be forced out through the opening due to the hot gas expansion. As a result, the pressure difference across the opening is positive and there will be no inflow through the opening, only outflow of cold gases. See figure A below. Stage 2 - This stage only lasts a few seconds. The smoke layer has just reached the top of the opening and hot gases have started to flow out. The pressure inside is still higher than the pressure outside and both hot and cold gases flow out through the opening. There is no mass flow into the compartment. See figure B. Stage 3- In this stage, the hot gases flow out through the top of the opening and the mass balance demands that cold gases of equal mass flow in through the lower part of the opening. This stage can last for a considerable time, until the room is filled entirely with smoke or until flashover occurs. Stages 1, 2 and 3 are associated with the growth stage. See figure C. Topic 1.5 Enclosing systems CPCCBS6001 Ed 1 3

4 Stage 4- This stage is often termed the well mixed stage, where the compartment is filled with smoke that is assumed to be well mixed. This stage is associated with the fully developed stage. See figure D. Flashover usually occurs between stages 3 and 4. Pressure profile across the opening as the fire in a partially enclosed compartment develops Influencing factors The factors that influence the development of a fire in an enclosure can be divided into two main categories, being those that have to do with the enclosure itself and those that have to do with the fuel. These factors are: the size and location of the ignition source, the type, amount, position, spacing, orientation and surface area of the fuel, the geometry of the enclosure, the size and location of the compartment openings, and the material properties of the enclosure boundaries. Additional resources View the following video clips for a more visual understanding of the development of and destructiveness of fire in buildings: Compartment fire behaviour video: Compartment fire behaviour training video: A video showing flashover: Smoke production Unless combustion is complete, smoke will be produced and thus virtually all real fires will generate smoke. Smoke is defined as "the airborne solid and liquid particles and gas evolved when a material undergoes combustion" (NFPA 1997). Smoke is generated both during smouldering and flaming combustion but the chemical and physical nature of smoke differs. Smoke from smouldering fires mainly consists of relatively high molecular weight materials which condense to form an aerosol or "mist" of liquid droplets. Such aerosols are typically light in colour and tend to deposit on a cool surface to give oily residues. Smoke from flaming combustion generally consists of solid particles formed at high temperatures in the gas phase. Particulate matter, including elementary carbon, may be formed regardless of whether the fuel is gas, liquid or solid. 4 Topic 1.5 Enclosing systems CPCCBS6001 Ed 1

5 Most house fire deaths are not caused by burns but by smoke inhalation. The content of smoke in the form of toxic gases and superheated gases often incapacitates people so quickly that they are overcome and cannot make it to an otherwise accessible exit. In addition to producing smoke, fire can incapacitate or kill by reducing oxygen levels. Fire protection systems (passive and active) Passive protection Passive fire protection measures are those which are associated with the way the building itself is built as opposed to the services which are installed within the building (like fire extinguishers), and includes any required fire resistance level of walls, floors, columns, ceilings, and roofs. Passive protection uses the principle of compartmentalisation to stop or slow the spread of flames and smoke in the event of fire. This form of protection is essential for fire personnel safety during the extinguishment of a fire. If a building does not have compartmentalisation its structural elements may collapse prematurely, thereby hindering the safe evacuation of occupants. Compartmentalisation for residential development is largely to prevent the spread of fire from between dwellings and between a dwelling and adjacent properties. Passive protection can be achieved by: 1. protection of structural elements: o o encasement of steel beams and columns which would otherwise be exposed to fire adequate cover over reinforcement steel in concrete elements and adequate bulk size of timber elements, to delay deterioration. (Fire provisions are included in the structural design codes, such as those for steel, concrete, timber and masonry.) 2. fire-proof materials: appropriate fit-out and furnishing materials that will not promote fire development or the spread of fire. 3. use of ceilings/floors as partitions: systems that maintain the integrity of fire rated ceilings. 4. fire-resistant glazing: fully insulated fire integrity systems which inhibit fire growth. 5. penetration seals: various commercially available materials to seal penetration openings in walls, floors and ceilings within fire-rated elements of a building. Active protection This involves the use of firefighting techniques to attack the source of the fire and extinguish it. It utilises such devices as automatic sprinklers, hydrants, extinguishers and various alarm systems. Active fire-control services in buildings most commonly consist of: sprinkler installation (an automatic fire-sprinkler system) to keep the fire under control, enable safe evacuation of occupants, and ideally prevent further property damage until the firebrigade arrives and/or the fire is brought under control hose installations (a network of hydrants with hoses and control valves) Topic 1.5 Enclosing systems CPCCBS6001 Ed 1 5

6 hand extinguishers (independent of water main). smoke detectors and alarms Ceiling mounted smoke detector Ceiling mounted automatic sprinkler head In residential buildings, smoke alarms are a requirement of the BCA, but most other active fire protection systems are employed only in Class 2 to 9 buildings. Acceptable construction practices for fire safety (Classes 1 & 10) The BCA Volume 2 addresses construction practices relating to fire safety in Class 1 and 10 buildings in Part 3.7 Fire Safety. Note that: Part describes the DTS acceptable construction practices with respect to fire separation, including the permissible materials that can be used for separation, Part addresses the requirements for the installation of smoke alarms and of lighting to assist in evacuation, Part is about heating appliances, both built-in and free-standing, and the requirements for flues, Part refers to AS 3959 Construction of buildings in bushfire-prone areas and the NASH Standard Steel Framed Construction in Bushfire Areas as Acceptable Construction Manuals for construction in bushfire areas, and there are several state and territory variations, and 6 Topic 1.5 Enclosing systems CPCCBS6001 Ed 1

7 Part details specific requirements that apply in alpine areas due to the difficulties of access (by fire-fighting crews) and egress (evacuation) in snow conditions. You should make yourself familiar with these sections of the BCA. Before we look at each of these in a little more detail, there are some terms which you will need to understand. Fire-rated elements and Fire Resistance Level (FRL) FRL is a defined term in the BCA meaning the graded period in minutes determined in accordance with Specification A2.3 (Vol. 1 of the BCA), for the following criteria: structural adequacy integrity insulation and expressed in that order. Structural adequacy, in relation to an FRL means the ability to maintain suitable and adequate loadbearing capacity under AS Fire-resistance test of elements of construction. Integrity, in relation to a FRL, means the ability to resist the passage of flames and hot gases specified in AS Insulation, in relation to a FRL, means the ability to maintain a temperature on the surface not exposed to the fire below the limits specified in AS Table - fire resistance level examples FRLs comments 1 60/60/60 60 minutes is required for each of the three criteria above also known as 1 hour FRL because 60 minutes is required for each of the three criteria 2 90/60/30 90 minutes is required for structural adequacy, 60 minutes for integrity, and 30 minutes for insulation this cannot be referred to as a 1 ½ hour FRL because the values for integrity and insulation are each less than 90 minutes 3 90/90/90 90 minutes is required for each of the three criteria can be referred to as a 1 ½ hour FRL because 90 minutes is required for each of the three criteria this FRL is higher than that for example /-/- 60 minutes is required for structural adequacy only 5 -/-/- no FRL is required Topic 1.5 Enclosing systems CPCCBS6001 Ed 1 7

8 You will have noticed the reference above to Specification A2.3 (Vol. 1) for the determination of fire resistance levels. This Specification (towards the end of Section A) is referred to within both Volumes 1 and 2 of the BCA. Resistance to the incipient spread of fire This means, in relation to a ceiling membrane, the ability of the membrane to insulate the space between the ceiling and roof, or ceiling and floor above, so as to limit the temperature rise of materials in this space to a level which will not permit the rapid and general spread of fire throughout the space. This concept is represented by the following diagram: In this diagram the idea of resistance to the incipient spread of fire is to prevent the spread of fire from one unit (whether in units, townhouses, villas or a dual occupancy) into the roofspace where it could spread to the adjoining unit(s) and/or cause collapse of the roof. To address this problem the BCA requires provision of a ceiling with a resistance to the incipient spread of fire to the roof-space of 60 minutes. This typically includes two layers of 16 mm thick fire protective plasterboard. Alternatively, the wall between each unit that is required to have an FRL may extend to the underside of the roof. The former choice is designed to provide an FRL from underneath or within the unit, whereas an FRL for a building element such as a wall includes the performance of the material in a fire that considers a fire from either or both sides. Volume 2 Housing Provisions The requirement for an FRL of 60/60/60 for an external wall may be achieved by brick veneer or masonry construction (of specified thickness) as explained in Part 3.7. Look at the acceptable construction requirements for the height and construction of separating walls between dwellings, gaps, and type of masonry veneer. You will note that separating walls to have a 60/60/60 FRL must extend to the underside of a non-combustible roof, although roof battens (only) may cross the top of the wall. There are additional requirements for construction with gaps, or combustible roofs. Fire separation We have already looked at the requirements for fire separation for separating walls in townhouses. Another requirement is for a fire resistance level of 60/60/60 for the walls of a dwelling that is located less than 900 mm to the allotment boundary. Where a portion only of a wall falls into this category then only that portion needs be protected in this way. There is also a requirement where class 10a buildings are to be located between a class 1 building and the boundary. Another situation which requires protective measures is that of class 10a 8 Topic 1.5 Enclosing systems CPCCBS6001 Ed 1

9 buildings located between class 1 buildings. An FRL of 60/60/60 applies to all of these examples (unless a greater physical separation is provided, when no FRL is required). The requirement for an FRL of 60/60/60 for an external wall may be achieved by brick veneer or by masonry construction (of specified thickness) as explained in Part 3.7. Look at the acceptable construction requirements for the height and construction of separating walls between dwellings, gaps, and type of masonry veneer. You will note that separating walls to have a 60/60/60 FRL must extend to the underside of a non-combustible roof, although roof battens (only) may cross the top of the wall. There are additional requirements for construction with gaps, or with combustible roofs. Read Part of the BCA Volume 1. Observe your local neighbourhood and compare what you see with the many illustrations provided in the BCA regarding carports and adjoining structures. Do you see any non-compliances? Smoke alarms Read Part of the BCA Volume 2 and answer the following questions: 1 How does a smoke alarm work? 2 True or false: In a class 1a dwelling, smoke alarms need not be installed in a level which doesn't contain bedrooms. 3 True or false: Smoke alarms must be installed on the ceiling. 4 When complying with the prescriptive BCA requirements for smoke alarms in a Class 1a dwelling, what is the number of the Australian Standard which must be complied with? 5 True or false: In class 1b buildings, smoke alarms must activate lighting to assist evacuation. (Answers at the end of this topic) Clearances and protection from heating appliances Heating appliances are addressed in Volume 2 of the BCA in Part Read all of this section to learn about open fireplace construction, chimney construction, insert fireplaces and flues, and free standing heating appliances. Find the acceptable construction requirements for the installation of free standing heating appliances (part ). Examine the requirements and the associated schedule of referenced documents, and answer the following questions: 1. What is the number and title of the Australian Standard that applies to heating appliances? 2. True or false: A free standing heating appliance must be located not less than 1.2 m from adjoining walls (other than a masonry wall). 3. What must a heat shield be constructed of? 4. Find a definition of hearth. 5. Find a definition of flue. 6. True or false: Acceptable construction requires for free standing heating appliances, the installation in all cases of a hearth. 7. Is there another BCA clause relating to the installation of hearths? Topic 1.5 Enclosing systems CPCCBS6001 Ed 1 9

10 8. What happens if someone wants to propose a flue installation in a different way to those shown in figures in the BCA. 9. True or false: The requirements of figures in the BCA are advisory only and need not be taken into account. 10. Where do you find out what height a flue must project above a roof? (Answers at the end of this topic) Read also Acceptable Standards of Construction Class 1 & 10 Buildings Section 13. Building in bushfire areas Australia s mostly hot, dry climate is conducive to bushfires which are frequent events during the hotter months. The 1983 Ash Wednesday bushfires and the Black Saturday fires of 2009 recorded a loss of life of 75 and 173 people respectively. In addition large numbers of homes and properties have been lost or damaged due to bushfires. New planning guidelines have been introduced in recent years to reduce the risk to life and property by addressing fire risk management at all stages of the planning process. Every state has guidelines and/or legislation for the management of planning and development in bushfire-prone areas. The list below may help you to locate information for your own state. Note that pdf documents may need to be located via a search rather than as a direct link. Familiarise yourself with the requirements in your state data/assets/pdf_file/0009/289584/revised_bran_ _building_in_bushfire_prone_areas_-_july_2014.pdf ll/2014/bushfire-prone-areas-identified-in-the-act BushfireManagementAndMitigation.pdf The BCA Housing Provisions deals with bushfire areas in Part It refers to Australian Standard AS 3959 Construction of buildings in bushfire-prone areas as one of the Acceptable Construction Manuals. Obtain access to this standard and answer the following questions: 1. What does BAL stand for and what is it? 2. What is BAL-FZ? 3. How is the BAL determined? 4. What factors influence the Bushfire Attack Level that is arrived at? 5. Where can the construction requirements be found that are appropriate to a dwelling in a bushfire-prone area determined to be of BAL 40? 10 Topic 1.5 Enclosing systems CPCCBS6001 Ed 1

11 (Answers at the end of this topic) Read also Acceptable Standards of Construction Class 1 & 10 Buildings Section 24. Answers to questions within this topic Answers to questions on smoke alarms: 1 There are two types of smoke alarm. A photoelectric smoke alarm has a light and a photocell. When there is smoke the photocell does not detect the light and an alarm is activated. An ionisation smoke alarm uses a small amount of radioactive material to create an electric current through ionised air. Smoke impedes the current causing an alarm to be activated. 2 False. Alternative methods to those within an Australian Standard may be adopted when they can be proved to the approval authority's satisfaction to be no less effective than those in a Standard called up by a BCA DTS Provision 3 False. 4 AS 3786 Smoke alarms 5 True Answers to questions on free standing heating appliances: 1 AS/NZS 2918 Domestic solid-fuel burning appliances - installation 2 False, paragraph (a) provides for three options (i), (ii) or (iii). A lesser distance is possible with a heat shield. 3 90mm thick masonry constructed in accordance with Part A hearth is the floor or base of a fireplace, usually extending into a room and paved with brick, stone or cement. 5 A flue is a channel, pipe or duct for the passage of smoke and waste gases produced by a fire or heater. 6 True. 7 Yes, for open fireplaces. 8 You would need to assess the proposal against the performance requirements or those of AS False, as it is a part of the BCA text and is not commentary. 10 By referring to the requirements (and figure) for chimney construction. Answers to questions on building in bushfire areas: 1. BAL stands for Bushfire Attack Level and is a means of measuring the severity of a building s potential exposure to ember attack, radiant heat and direct flame contact. 2. BAL-FZ is the highest level of bushfire attack as a consequence of direct exposure to flames from the fire front in addition to heat flux and ember attack. FZ stands for Flame Zone. 3. The BAL can be determined by a simplified method as described in Clause 2.2 of AS 3959 or by a more detailed procedure described in Appendix B of the standard. 4. The BAL depends on a. The state and region b. The vegetation type(s) c. The distance of the site from the vegetation type(s) d. The effective slope(s) from the vegetation type(s) 5. Section 8 of AS 3959 provides details of construction requirements for buildings constructed in an area nominated as BAL-40. Topic 1.5 Enclosing systems CPCCBS6001 Ed 1 11