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.

Compartmentation

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

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.

Smoke movement

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 41.4).

Figure 41.4 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:

where
ΔPso	= the pressure difference from the shaft to the outside
g	= acceleration of gravity
Patm	= absolute atmospheric pressure
R	= gas constant of air
To	= absolute temperature of outside air
Ts	= 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.