Confining Fires by Compartmentation
Building and site planning
Fire safety engineering work should begin early in the design phase because the fire safety requirements influence the layout and design of the building considerably. In this way, the designer can incorporate fire safety features into the building much better and more economically. The overall approach includes consideration of both interior building functions and layout, as well as exterior site planning. Prescriptive code requirements are more and more replaced by functionally based requirements, which means there is an increased demand for experts in this field. From the beginning of the construction project, the building designer therefore should contact fire experts to elucidate the following actions:
- to describe the fire problem specific to the building
- to describe different alternatives to obtain the required fire safety level
- to analyse system choice regarding technical solutions and economy
- to create presumptions for technical optimized system choices.
The architect must utilize a given site in designing the building and adapt the functional and engineering considerations to the particular site conditions that are present. In a similar manner, the architect should consider site features in arriving at decisions on fire protection. A particular set of site characteristics may significantly influence the type of active and passive protection suggested by the fire consultant. Design features should consider the local fire-fighting resources that are available and the time to reach the building. The fire service cannot and should not be expected to provide complete protection for building occupants and property; it must be assisted by both active and passive building fire defences, to provide reasonable safety from the effects of fire. Briefly, the operations may be broadly grouped as rescue, fire control and property conservation. The first priority of any fire-fighting operation is to ensure that all occupants are out of the building before critical conditions occur.
Structural design based on classification or calculation
A well-established means of codifying fire protection and fire safety requirements for buildings is to classify them by types of construction, based upon the materials used for the structural elements and the degree of fire resistance afforded by each element. Classification can be based on furnace tests in accordance with ISO 834 (fire exposure is characterized by the standard temperature-time curve), combination of test and calculation or by calculation. These procedures will identify the standard fire resistance (the ability to fulfil required functions during 30, 60, 90 minutes, etc.) of a structural load-bearing and/or separating member. Classification (especially when based on tests) is a simplified and conservative method and is more and more replaced by functionally based calculation methods taking into account the effect of fully developed natural fires. However, fire tests will always be required, but they can be designed in a more optimal way and be combined with computer simulations. In that procedure, the number of tests can be reduced considerably. Usually, in the fire test procedures, load-bearing structural elements are loaded to 100% of the design load, but in real life the load utilization factor is most often less than that. Acceptance criteria are specific for the construction or element tested. Standard fire resistance is the measured time the member can withstand the fire without failure.
Optimum fire engineering design, balanced against anticipated fire severity, is the objective of structural and fire protection requirements in modern performance-based codes. These have opened the way for fire engineering design by calculation with prediction of the temperature and structural effect due to a complete fire process (heating and subsequent cooling is considered) in a compartment. Calculations based on natural fires mean that the structural elements (important for the stability of the building) and the whole structure are not allowed to collapse during the entire fire process, including cool down.
Comprehensive research has been performed during the past 30 years. Various computer models have been developed. These models utilize basic research on mechanical and thermal properties of materials at elevated temperatures. Some computer models are validated against a vast number of experimental data, and a good prediction of structural behaviour in fire is obtained.
A fire compartment is a space within a building extending over one or several floors which is enclosed by separating members such that the fire spread beyond the compartment is prevented during the relevant fire exposure. Compartmentation is important in preventing the fire to spread into too large spaces or into the whole building. People and property outside the fire compartment can be protected by the fact that the fire is extinguished or burns out by itself or by the delaying effect of the separating members on the spread of fire and smoke until the occupants are rescued to a place of safety.
The fire resistance required by a compartment depends upon its intended purpose and on the expected fire. Either the separating members enclosing the compartment shall resist the maximum expected fire or contain the fire until occupants are evacuated. The load-bearing elements in the compartment must always resist the complete fire process or be classified to a certain resistance measured in terms of periods of time, which is equal or longer than the requirement of the separating members.
Structural integrity during a fire
The requirement for maintaining structural integrity during a fire is the avoidance of structural collapse and the ability of the separating members to prevent ignition and flame spread into adjacent spaces. There are different approaches to provide the design for fire resistance. They are classifications based on standard fire-resistance test as in ISO 834, combination of test and calculation or solely calculation and the performance-based procedure computer prediction based on real fire exposure.
Interior finish is the material that forms the exposed interior surface of walls, ceilings and floor. There are many types of interior finish materials such as plaster, gypsum, wood and plastics. They serve several functions. Some functions of the interior material are acoustical and insulational, as well as protective against wear and abrasion.
Interior finish is related to fire in four different ways. It can affect the rate of fire build-up to flashover conditions, contribute to fire extension by flame spread, increase the heat release by adding fuel and produce smoke and toxic gases. Materials that exhibit high rates of flame spread, contribute fuel to a fire or produce hazardous quantities of smoke and toxic gases would be undesirable.
In building fires, smoke often moves to locations remote from the fire space. Stairwells and elevator shafts can become smoke-logged, thereby blocking evacuation and inhibiting fire-fighting. Today, smoke is recognized as the major killer in fire situations (see figure 1).
Figure 1. The production of smoke from a fire.
The driving forces of smoke movement include naturally occurring stack effect, buoyancy of combustion gases, the wind effect, fan-powered ventilation systems and the elevator piston effect.
When it is cold outside, there is an upward movement of air within building shafts. Air in the building has a buoyant force because it is warmer and therefore less dense than outside air. The buoyant force causes air to rise within building shafts. This phenomenon is known as the stack effect. The pressure difference from the shaft to the outside, which causes smoke movement, is illustrated below:
= the pressure difference from the shaft to the outside
g = acceleration of gravity
= absolute atmospheric pressure
R = gas constant of air
= absolute temperature of outside air
= absolute temperature of air inside the shaft
z = elevation
High-temperature smoke from a fire has a buoyancy force due to its reduced density. The equation for buoyancy of combustion gases is similar to the equation for the stack effect.
In addition to buoyancy, the energy released by a fire can cause smoke movement due to expansion. Air will flow into the fire compartment, and hot smoke will be distributed in the compartment. Neglecting the added mass of the fuel, the ratio of volumetric flows can simply be expressed as a ratio of absolute temperature.
Wind has a pronounced effect on smoke movement. The elevator piston effect should not be neglected. When an elevator car moves in a shaft, transient pressures are produced.
Heating, ventilating and air conditioning (HVAC) systems transport smoke during building fires. When a fire starts in an unoccupied portion of a building, the HVAC system can transport smoke to another occupied space. The HVAC system should be designed so that either the fans are shut down or the system transfers into a special smoke control mode operation.
Smoke movement can be managed by use of one or more of the following mechanisms: compartmentation, dilution, air flow, pressurization or buoyancy.
Evacuation of Occupants
Figure 2. Principles of exit safety.
People evacuating from a burning building are influenced by a number of impressions during their escape. The occupants have to make several decisions during the escape in order to make the right choices in each situation. These reactions can differ widely, depending upon the physical and mental capabilities and conditions of building occupants.
The building will also influence the decisions made by the occupants by its escape routes, guidance signs and other installed safety systems. The spread of fire and smoke will have the strongest impact on how the occupants make their decisions. The smoke will limit the visibility in the building and create a non-tenable environment to the evacuating persons. Radiation from fire and flames creates large spaces that cannot be used for evacuation, which increases the risk.
In designing means of egress one first needs a familiarity with the reaction of people in fire emergencies. Patterns of movement of people must be understood.
The three stages of evacuation time are notification time, reaction time and time to evacuate. The notification time is related to whether there is a fire alarm system in the building or if the occupant is able to understand the situation or how the building is divided into compartments. The reaction time depends on the occupant’s ability to make decisions, the properties of the fire (such as the amount of heat and smoke) and how the building’s egress system is planned. Finally, the time to evacuate depends on where in the building crowds are formed and how people move in various situations.
In specific buildings with mobile occupants, for example, studies have shown certain reproducible flow characteristics from persons exiting the buildings. These predictable flow characteristics have fostered computer simulations and modelling to aid the egress design process.
The evacuation travel distances are related to the fire hazard of the contents. The higher the hazard, the shorter the travel distance to an exit.
A safe exit from a building requires a safe path of escape from the fire environment. Hence, there must be a number of properly designed means of egress of adequate capacity. There should be at least one alternative means of egress considering that fire, smoke and the characteristics of occupants and so on may prevent use of one means of egress. The means of egress must be protected against fire, heat and smoke during the egress time. Thus, it is necessary to have building codes that consider the passive protection, according to evacuation and of course to fire protection. A building must manage the critical situations, which are given in the codes concerning evacuation. For example, in the Swedish Building Codes, the smoke layer must not reach below
1.6 + 0.1H (H is the total compartment height), maximum radiation 10 kW/m2 of short duration, and the temperature in the breathing air must not exceed 80 °C.
An effective evacuation can take place if a fire is discovered early and the occupants are alerted promptly with a detection and alarm system. A proper mark of the means of egress surely facilitates the evacuation. There is also a need for organization and drill of evacuation procedures.
Human behaviour during fires
How one reacts during a fire is related to the role assumed, previous experience, education and personality; the perceived threat of the fire situation; the physical characteristics and means of egress available within the structure; and the actions of others who are sharing the experience. Detailed interviews and studies over 30 years have established that instances of non-adaptive, or panic, behaviour are rare events that occur under specific conditions. Most behaviour in fires is determined by information analysis, resulting in cooperative and altruistic actions.
Human behaviour is found to pass through a number of identified stages, with the possibility of various routes from one stage to the next. In summary, the fire is seen as having three general stages:
- The individual receives initial cues and investigates or misinterprets these initial cues.
- Once the fire is apparent, the individual will try to obtain further information, contact others or leave.
- The individual will thereafter deal with the fire, interact with others or escape.
Pre-fire activity is an important factor. If a person is engaged in a well-known activity, for example eating a meal in a restaurant, the implications for subsequent behaviour are considerable.
Cue reception may be a function of pre-fire activity. There is a tendency for gender differences, with females more likely to be recipient of noises and odours, though the effect is only slight. There are role differences in initial responses to the cue. In domestic fires, if the female receives the cue and investigates, the male, when told, is likely to “have a look” and delay further actions. In larger establishments, the cue may be an alarm warning. Information may come from others and has been found to be inadequate for effective behaviour.
Individuals may or may not have realized that there is a fire. An understanding of their behaviour must take account of whether they have defined their situation correctly.
When the fire has been defined, the “prepare” stage occurs. The particular type of occupancy is likely to have a great influence on exactly how this stage develops. The “prepare” stage includes in chronological order “instruct”, “explore” and “withdraw”.
The “act” stage, which is the final stage, depends upon role, occupancy, and earlier behaviour and experience. It may be possible for early evacuation or effective fire-fighting to occur.
Building transportation systems
Building transportation systems must be considered during the design stage and should be integrated with the whole building’s fire protection system. The hazards associated with these systems must be included in any pre-fire planning and fire protection survey.
Building transportation systems, such as elevators and escalators, make high-rise buildings feasible. Elevator shafts can contribute to the spread of smoke and fire. On the other hand, an elevator is a necessary tool for fire-fighting operations in high-rise buildings.
Transportation systems may contribute to dangerous and complicated fire safety problems because an enclosed elevator shaft acts as a chimney or flue because of the stack effect of hot smoke and gases from fire. This generally results in the movement of smoke and combustion products from lower to upper levels of the building.
High-rise buildings present new and different problems to fire-suppression forces, including the use of elevators during emergencies. Elevators are unsafe in a fire for several reasons:
- Persons may push a corridor button and have to wait for an elevator that may never respond, losing valuable escape time.
- Elevators do not prioritize car and corridor calls, and one of the calls may be at the fire floor.
- Elevators cannot start until the lift and shaft doors are closed, and panic could lead to overcrowding of an elevator and the blockage of the doors, which would thus prevent closing.
- The power can fail during a fire at any time, thus leading to entrapment. (See figure 3)
Figure 3. An example of a pictographic warning message for elevator use.
Fire drills and occupant training
A proper mark of the means of egress facilitates the evacuation, but it does not ensure life safety during fire. Exit drills are necessary to make an orderly escape. They are specially required in schools, board and care facilities and industries with high hazard. Employee drills are required, for example, in hotel and large business occupancies. Exit drills should be conducted to avoid confusion and ensure the evacuation of all occupants.
All employees should be assigned to check for availability, to count occupants when they are outside the fire area, to search for stragglers and to control re-entry. They should also recognize the evacuation signal and know the exit route they are to follow. Primary and alternative routes should be established, and all employees should be trained to use either route. After each exit drill, a meeting of responsible managers should be held to evaluate the success of the drill and to solve any kind of problem that could have occurred.