CHAPTER 2: FIRE SPREAD
Chapter 2: Fire spread
The chapter discusses aspects of fire spread so as to understand spread of fire, fire transfer, and how fire moves or escapes.
Most fires spread quickly in buildings because of the presence of combustible materials like furniture, and flammable gases. Controlling of fire spread is easy in the first stage also referred to as the insipient fire. Nevertheless, fire can move swiftly and even reach higher speed limits in the presence of assistant factors found in gaps or penetration inside a building. Internally, spread of fire is common in obscured spaces, small wall gaps, and attic or roof spaces. Externally, fire spreads along the outside part of a building,
Research by the Building Research Establishment (BRE) on fire spreads inside buildings concluded that several factors resulted in separation of fire. Furthermore, it was also discovered that approximately eight and a half thousands fire that have separated from the room of origin (J S Hopkinson 1984). The specific percentage of a fire spread in hospitals where it is limited in the room of origin is 86%, where the fire spread beyond the room of origin is 10%, and 4% unrecorded spread (See appendix 1).
The three modes of transfer where fire spreads are convection, conduction, and radiation. However, the most dangerous transfer mode is convection because it has been proven that it results in most casualties and deaths. When fire starts in an enclosure, the smoke rises vertically towards the ceiling. The smoke produces a mass of toxic gases that will be trapped at the ceiling. Afterwards, the smoke spreads to various directions penetrating gaps found in floors, walls, or ceilings so as to escape to other locations either within the building or in buildings nearby.
Some materials like metal transfer heat quickly and this is through conduction method. This occurrence—conduction—can result in heat being transferred to combustible materials inside a building, which result in a fire. Similarly, radiation is like conduction but through the air, where a heated electric bar in a room can transmit heat towards a combustible material resulting in a fire.
2.2 Fire spread on internal linings
The spread of fire inside the building can be inhabited through making sure internal linings: adequately resisting spread of the flame over their surface, and if ignited, have a heat release rate or fire growth rate that is reasonable in the circumstances. In this case, internal linings signify, “materials or products used in lining any partition, wall, ceiling or other internal structure” (ADB 2006, p. 62). The above graph is mentioned in The Approved Document B2, where it briefly describes the requirements that need to be considered in this topic of area.
Spread of fire and its growth has a high chance of being affected by the type of material used in making the internal structure—wall, ceiling, and many more. Rounded areas inside buildings are significant for linings to spread fire, and also where a strong spread might inhibit occupants from escaping. Therefore, the lining material can enhance internal fire spread, by accelerating the ease of ignition, which the lining material gives off heat when burning.
Hospitals and other healthcare enclosures are designed and constructed in a way that it provides acceptable fire resistance, and the building to have the ability to remain stable obtaining time for escape and extinguishment. Furthermore, the buildings are to have the capacity to prevent smoke and fire from spreading to adjacent buildings and other rooms inside the burning building.
Therefore, there are several ways fire separates in such insensitive building, where different factors assist the process of fire spread. Six ways are listed and include:
Fire spread via doorways: This is where fire spreads from origin room of corridors to occupied areas through exposed doors. Fire can also spread through fire resistant doors if they are held with fire debris, wedge, or lack of door closer.
Fire spread via walls and partitions: Fire Penetrates through non-loading partitions, and through unstopped ceiling voids.
Vertical spread, via stairways: Fire can be spread upward if a door has a direct opening with the staircase from the room of origin.
Via ceilings: Early stages of failure of lath and plaster ceiling giving spread of fire to locations above (room and roof) because of the damage of ceiling materials.
Via wall cavities: Ignition of combustible materials within and external wall during installation and maintenance process enabling fire spread through cavities to upper floors or roofs.
Via furniture (curtains): Fire ignites along the lower edge by bed covers or beddings. It could penetrate to combustible lining of ceiling void.
Structural elements in a building
The major structures are:
A column, beam, or other member forming part of the structure.
A load bearing wall.
The above listed elements should be protected from early failure so that such elements have a minimum time of fire resistance to maintain itself and not to collapse, or the capacity of the load-bearing not to fail. Resistance to fire penetration and resistance to heat transfer should be considered, so that such elements have the ability tackle fire hazards and meet the demanded consequences. The minimum fire resistance period provided by the element of the structure should meet the provisions that are mention as in table 2. (See appendix 2).
2.3 Fire spread on external walls
In hospitals it is crucial that the external wall is protected from fire. This is because external element could significantly influence the separation of fire in minutes or it could be in seconds in some circumstances through roof and wall penetrations to adjacent buildings or parts within the indicated building. The minimum time of fire resistance for external walls for a building with a height to the top floor not exceeding 5 meters should be 30 minutes fire resistance, and where the height exceeds 5 meters should be 60 minutes fire resistance. Hospitals or health care buildings having above 18 meters should have an external wall that limits combustion.
2.4 Fire spread though cavities
Hidden spaces and cavities in buildings affect fire spread and their growth. Fire and smoke moves rapidly and continuously from where the fire sets through these cavities. Therefore, it is important to set up fire resisting barriers in these cavities control their sizes (Department of Health – Estates & Facilities Division 2007; HTM 05-02 2007. p.29). Also, concealed spaces and cavities should be subdivided to restrict fires and smokes separation. The Approved Document B (ADB), section 9 (concealed spaces) has provisions and requirements for subdividing in concealed cavities (see appendix 3).
Edges of cavities can be closed by cavity barriers or around the openings. It should be also between the junction of an external and internal wall and every compartment floor or wall. Compartment walls should be constructed to reach the ceiling or roof cavity and run up the full height of the storey to the compartment floor to have the required fire resistance. The minimum fire resistance period of a cavity barrier should be not less than 30 minutes when constructed. Dimensions of thickness of cavity barriers in a wall, partition, or around openings are listed below:
Not less than 0.5 mm thick for steel
Not less than 38 mm thick for timber
Not less than 12 mm thick for polythene sleeved mineral wool, mineral slab, calcium silicate, cement based or gypsum.
Cavity barriers should not be affected by the movement of building due to the subsidence or due to the temperature change or other factors such as failure when subject to fire to obtain best performance (see appendix 4).
Compartmentation refers to subdivisions of spaces provided inside a building separated from each other by either walls or floors of fire resisting construction. By this method, separation of fire is avoided and large fires are prevented from occurring. Therefore, all floors in health care buildings should be constructed as a compartment floor to prevent the listed threats (ADB 2006, p71; HTM 2007, p25).
The planning for the horizontal evacuation (PHE) concept is to sub-divide the locations provided for the residents by compartment walls and compartment floors to obtain the required protection in the event of fire. This is made to give horizontal escape into aligned protected areas. The aim of this process is to evacuate in a short period of time to the ultimate safety (place of safety). In health care buildings, at least three protected areas by compartment walls must be provided in each storey and all floors should be compartment floors. The following provisions need to be considered for such premises, as mention in Approved Document B:
In any protected area, it should exceed 10 beds.
Protected areas should be sufficient in its area and large enough to accommodate its own occupants and the occupants escaping and arriving from aligned protected areas.
Construction of compartmentation (healthcare premises)
In hospitals, the maximum size of compartment can be determined depending on its usage. 2000 m² is required in a multi storey building, and 3000 m² in a single storey building should not be exceeded in these limitations. Minimum period of fire resistance by a compartment wall should be:
30 minutes for single healthcare premises.
30 minutes for not exceeding 12 m above ground level for sprinklered healthcare premises.
60 minutes for all other healthcare premises.
There are certain requirements that should be considered to obtain an effective compartment. If a floor area in such premises is more than 750m², or if 30 patients will enter a department like the operation room at the same time, or a sleeping accommodation area contain more than 30 patients, then in these cases subdivision of compartment would be required. The compartments should be enclosed by walls with a minimum of 30 minutes of fire resistance and should terminate at the underside of a compartment floor, a roof, or a ceiling that is non-demountable and imperforate with a minimum period of fire. However, all openings in sub compartment walls should be protected and with a fire resistance of 30 minutes minimum.
Openings in floors and compartment walls
All cavities in buildings must have an equal fire resistance in floors and compartment walls. In maintaining the safety of the cavities in such compartmentation it is recommended that:
Doors in these compartments should not be less than the compartment fire resistance time.
Pipes that contain within the compartment with a diameter of not more than 160 mm, when subject to high temperatures of 800 °C should not fail when fire and heat transfer passes through the wall of the pipe.
Ventilation ducts should meets the requirements given in BS 5588-9 standards
Doors are significant features that can separate and trap fires in a place within a reasonable period of time if the appropriate fire door and the required fire resistance are selected. Normally, such doors contain self-closing devises unless in areas such as cupboards or service risers. However, the doors without locks should always be kept locked. Protection of escape routes, protection of occupant, fire fighters and the building structure are that main function of a fire doors. Fire doors should have an acceptable fire resistance that is mentioned in table 32 in The British Standards 9999 and in accordance with BS 476-22 or BS EN 1634-1 (See appendix 5).
Junction of compartment wall and compartment floor with other walls
Where walls and floors compartments join with any other compartment, the junction at this point should have the ability to resistance fire by fire stopping methods which will be discussed in detail in chapter 3. The same principles apply at the junction of compartment walls with roofs and should be protected with fire stopping to prevent and reduce the risk of penetration separation in other parts of the building or compartments.
Openings in subdivision of compartments
1. Ductwork passing through sub compartment walls and does not need to be provided with automatic fire shutters because: the sub compartment is the only one served by the duct; the ductwork has a minimum of 30 minutes fire resistance in accordance with the BS 476.
2. Transfer grills not to be installed in sub-compartment walls to decrease the probability of smoke movement between them, unless they are supported with fire and smoke dumpers set to be activated at 74°C.
3. Un-insulated fire resisting glazed panels can be installed in sub-compartment walls if: the glazing resists for a minimum time of 30 minutes against fire; there is a maximum of 1000 mm² of glazing in any room.
If there are critical areas located inside the premises like special care baby units, then they should be divided into two sub-compartments to separate the nursing area from the utility area. For instant:
Sub-compartment (1) staff base:
This area can contain the bed area, clean and dirty utility, linen store, and status laboratory.
Sub-compartment (2) entrance area:
The area can contain rooms for Staff changing, rest, cleaners’ store, and the main equipment.
Therefore, through identifying the methods of fire spread and how it can move and transfer through factors mentioned in this chapter, one can allocate a method to prevent and avoid separation of fire. Fire stopping will be discussed in details in the following chapter (3), where it will be indicating the technical methods used in the present for health care premises.
Chapter 4- Case Studies
4.1 The Great Ormond Street Hospital Fire 29 September 2008 (see appendix 9)
The hospital-The Great Ormond Street Hospital—is located in UK and specializes in children’s health. It is internationally known and located in Bloomsbury in central London. The hospital provides nearly 335 inpatient bed in addition to family and parent accommodation.
On 29th September, 2008 a fire occurred on the fifth floor on a Monday morning in the cardiac annex. The smoke started to transfer and spread to other parts of the building throughout the corridors. Ten minutes after the fire, an oxygen cylinder ignited and resulted in an explosion, which affected and damaged the ceiling resulting to its collapse. Members of the staff took immediate action and followed the evacuation plan provided for such event and moved the patient (children’s) to a safe environment (ultimate safety) allocated for emergency purposes (situation) within the building. Most patients faced difficulties and problems in the evacuation process where they were having medical equipments attached to them making their movement slow.
The whole building was not affected and the fire was controlled rapidly by The London Fire Brigade (LFB). Occupants and member of staff existing in the building and other floors were not aware of the situation because the fire was managed. However, it was decided that it was necessary to inform other staff to evacuate the place. The Great Ormond Street Hospital used a communication system to contact each other by the BBM, which is the blackberry service on their mobile phones. The staff had accessed their email from their mobiles by this service to pass details and information with others outside the building.
The event was well managed and controlled by the medical staff by following the procedures required. This step or action assists preventing further possible problems. The control room was unreachable because of the fire and explosion that occurred. Therefore, the room had been relocated. It was planned that there was no possible chance of accessing the hospital from other entrances by visitors or other patients ensuring their safety in such. These means was controlled and planned effectively to maintain the safety measures and avoiding any further hazards that could occur.
Both horizontal and vertical evacuation process had been used in the hospital plan. It was recommended for patients that were affected by the fire, smoke, and water or were in a hazard situation, to evacuate horizontally from the risk area to other safer locations. Other members of the staff managed to stay few minutes to solve the children’s medical equipments where parents assist the staff by carrying children and escaping vertically through stairways. This cooperation of staff and occupants (parents) helped ease the movement process as well as reducing the risk of injuries or deaths.
The Great Ormond Street Hospital has an acceptable evacuation plan and they have dealt with it in an appropriate method. The staff of the hospital has responded urgently with serious action and followed the evacuation plan that is provided as required.
This could mean that the hospital has included fire resistance structure and separating elements by separating the building with both fire resistance compartment walls and compartment floors. The ceiling could be an essential factor that could support the fire spread when it collapsed. However, from a point of view in a sensitive building such as a hospital, fire stooping system could be installed in the origin room where the fire started; otherwise the fire will spread throughout the ceiling and transfer rapidly toward the upper and the lower floors.
Evidence on that, the hospital has an acceptable means of fire stopping or fire protection systems in terms of separation of flames and smokes to other location where staff, patients, parents and visitors exist. Furthermore, other members of the hospital staff who were in other floors or areas in the premise were not aware of the incident; hence the fire prevention system was up to date.