Reading the Fire: Building Factors

First of the critical building features is construction method. Building construction influences both fire behavior and structural stability under fire conditions. Combustible construction such as wood-frame, ordinary (masonry and wood), and heavy...

For years, firefighters have learned that ventilation is "the planned and systematic removal of heat, smoke, and fire gases and their replacement with fresh air". However, from a building construction and indoor air quality perspective ventilation is simply the exchange of the atmosphere inside a building with the atmosphere on the outside to maintain a habitable and healthful environment.

Effective ventilation under normal (non-fire) conditions requires a regular exchange of air. Ventilation rates identified by building codes and related standards express the flow rate of outside air brought into the building and are typically expressed in terms of air changes per hour, floor area being ventilated (cubic feet per minute per square foot (cfm/ft2), or by the number of people being served (cfm/person). Ventilation may be accomplished using natural or mechanical means. Natural ventilation occurs primarily through open windows and doors and infiltration through cracks in the building envelope. On the other hand, mechanical ventilation involves delivery of outside air to the interior of a building through the use of fans and in many cases ductwork (e.g., heating, ventilation, and air conditioning (HVAC) system) (Lawrence Berkley National Laboratory, n.d.).

Ventilation profile is simply the existing ventilation and potential changes to ventilation that may occur due to fire effects or tactical action. Normal ventilation is designed to provide a healthful atmosphere for building occupants and is not sufficient to support fuel-controlled combustion, typically resulting in ventilation-controlled conditions at some point in fire development. Some building openings, such as windows may be more prone to failure under fire conditions than others. These potential changes are a critical building factor in predicting fire behavior. In addition, building construction has a significant influence on ventilation tactics. For example, consider the differences between wood, metal, and concrete roofs.

When a fire develops to the point where it becomes ventilation controlled, available ventilation will determine the speed and extent of fire development and in many cases the direction of fire travel. Under fire conditions current ventilation is based on the actual exchange of products of combustion inside the building or compartment with outside air. However, it is essential to recognize the potential for changing ventilation conditions during firefighting operations. Firefighters must consider the size, number, and arrangement of existing and potential ventilation openings (see Figure 8)

The photo series in Figure 8 illustrates changing ventilation profile due to fire effects. In the first photo the window is intact. The second photo illustrates increasing discharge of hot smoke. As the window begins to fail; hot, rich smoke ignites outside the window. After the window failed, the compartment flashed over, resulting in a fully developed fire in the compartment.

Fire Protection Systems

Fire detection systems such as smoke detectors increase the probability that firefighters will arrive early enough in fire development to encounter pre-flashover conditions (and that flashover may occur after the initiation of interior firefighting operations). While early detection and intervention is critical to occupant safety and an important step in reducing property loss, firefighters must recognize and mitigate the hazards presented by rapid fire development.

Automatic sprinkler systems have a tremendous impact on fire development and life safety. "When sprinklers are present, the chances of dying in a fire and property loss per fire are cut by one half to two thirds, compared to fires reported to fire departments where sprinklers are not present" (Hall & Cote, 2003, p. 2-21). The Station Nightclub fire in Rhode Island provides an excellent example of the impact of fire suppression systems. The National Institute for Standards and Technology (NIST) modeled this fire under two sets of conditions, first without automatic sprinklers (actual incident conditions) as previously illustrated in Figure 5. Second, fire development was modeled as if the building had been equipped with automatic sprinklers. Figure 9 illustrates temperatures five feet above the floor 90 seconds after ignition. Compare this to the temperatures modeled in the unsprinklered condition illustrated in Figure 5.