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...


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 timber construction contribute to the fuel load, while non-combustible and fire resistive construction do not.Slideshow Images:

As discussed in the previous article, fire behavior indicators can be grouped into five general categories: Building, Smoke, Air Track, Heat, and Flame (Figure 1). A simple mnemonic for remembering the categories is B-SAHF ("be safe").

This article focuses on the only set of factors that is available before the fire starts; the building and its contents.

Building Factors Overview

Within the building category, it is important to consider structural elements (i.e., floor and roof support systems), non-structural elements (i.e., compartmentation, interior finish), contents, ventilation profile (i.e., building openings; heating, ventilation, and air conditioning (HVAC) systems), and the size of the building. The interrelationship of these factors is graphically illustrated in Figure 2.

This article provides an overview and examines a number of important building factors to consider as part of pre-incident planning and incident size-up. Practice reading the building under non-fire conditions to identify critical characteristics that will influence fire behavior and structural stability if a fire does occur.

Construction

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 timber construction contribute to the fuel load, while non-combustible and fire resistive construction do not. It is important to note that construction classification (i.e., fire resistive, non-combustible) refers to structurally supporting materials, not interior finish and roofing material that may contribute substantially to fire load.

In examining the influence of construction on fire development and spread it is also important to consider the structural void spaces as a path of fire travel and as compartments which may present different fire conditions than encountered in adjacent spaces.

Older wood-frame structures may be of balloon-frame design. This type of wood-frame construction provides a ready path of travel for fire that breaches compartment walls or ceilings, or that originates in an unfinished basement. Newer wood-frame construction is generally of platform frame design, and has less potential for fire spread from floor to floor through structural voids. However, structural voids can still present a significant hazard as evidenced by the smoke explosion in a Wyoming apartment that resulted from accumulation of flammable products of combustion and pyrolysis products in the trussloft (Hartin, 2006; NIOSH, 2005). Figure 3 illustrates the same type of "room in attic truss" that was involved in this incident.

Even voids in non-combustible construction can significantly influence fire behavior and present a hazard to firefighters. In 2003, two Memphis, TN, firefighters died in a collapse subsequent to a ventilation induced flashover or backdraft in a non-combustible commercial building (NIOSH, 2004). Extremely hot flammable products of combustion and vapors from asphalt roofing material accumulated in a void space between the ceiling and metal deck roof (Figure 4). This hot fuel ignited when firefighters opened the suspended ceiling during firefighting operations.

It is more difficult to recognize the presence of void spaces during firefighting operations than before the incident. Studying buildings and identifying potentially problematic construction features during informal or formal preplanning is important to understanding fire development and spread.

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