Changing Residential Fire Dynamics and Its Implications on Firefighter Operational Timeframes

Changing Residential Fire Dynamics

There has been a steady change in the residential fire environment over the past several decades. These changes include larger homes, different home geometries, increased synthetic fuel loads, and changing construction materials. Several experiments were conducted to compare the impact of changing fuel loads in residential houses. These experiments show living room fires have flashover times of less than 5 min when they used to be on the order of 30 min. Other experiments demonstrate the failure time of wall linings, windows and interior doors have decreased over time which also impact fire growth and firefighter tactics. Each of these changes alone may not be significant but the all-encompassing effect of these components on residential fire behavior has changed the incidents that the fire service is responding to. This analysis examines this change in fire dynamics and the impact on firefighter response times and operational timeframes.

Introduction

There is a continued tragic loss of firefighters’ and civilian lives, as shown by fire statistics [1, 2]. One significant contributing factor is the lack of understanding of fire behavior in residential structures resulting from the changes that have taken place in several components of residential fire dynamics. The changing dynamics of residential fires as a result of the changes in home size, geometry, contents, and construction materials over the past 50 years add complexity to the fire behavior (Figure 1). NFPA estimates [3] that from 2003 to 2006, US fire departments responded to an average of 378,600 residential fires annually. These fires caused an estimated annual average of 2,850 civilian deaths and 13,090 civilian injuries. More than 70% of the reported home fires and 84% of the fatal home fire injuries occurred in one- or two- family dwellings, with the remainder in apartments or similar properties. For the 2001–2004 period, there were an estimated annual average 38,500 firefighter fire ground injuries in the US [4]. The rate for traumatic firefighter deaths when occurring outside structures or from cardiac arrest has declined, while at the same time, firefighter deaths’ occurring inside structures has continued to climb over the past 30 years [5]. Additionally, on average firefighters in the United States receive less than 1% of their training on the subject of fire behavior [6]. The changes in the residential fire environment combined with the lack of fire behavior training are significant factors that are contributing to the continued climb in firefighter traumatic deaths and injuries. As homes become more energy efficient and fuel loads increase fires will become ventilation limited making the introduction of air during a house fire extremely important. If ventilation is increased, either through tactical action of firefighters or unplanned ventilation resulting from effects of the fire (e.g., failure of a window) or human action (e.g., door opened by a neighbor) heat release will increase, potentially resulting in flashover conditions. These ventilation induced fire conditions are sometimes unexpectedly swift providing little time for firefighters to react and respond.

Background

While the physics of fire development has not changed over time, the fire environment or more specifically the single family home has evolved. Several factors including home size, geometry, contents and construction materials have changed significantly over the past 50 or more years. Each of these factors will be examined in detail as they pertain to the safety of occupants and the responding fire service.

Home Size

Many contemporary homes are larger than older homes built before 1980. Based on United States Census data [7] homes have increased in average area from approximately 144 m2 in 1973 to over 232.3 m2 in 2008. Twenty-six percent of homes constructed in 2008 were larger than 278.7 m2 (Figure 2). In addition to increased area more homes are being built with two stories. In 1973 23% of homes were two-story and that has increased to 56% by 2008. The percentage of single story homes has decreased from 67% to 44% in the same time period (Figure 3). The larger the home is the more air available to sustain and grow a fire in that home. Additionally, the larger the home the greater the potential to have a larger fire, and the greater the potential hazard to the responding fire service resources if the proper tactics aren’t utilized. While the average home size has increased 56%, the fire service resources available to respond have not increased proportionally in many areas of the United States. This is emphasized in suburban areas where larger homes are being built but fewer fire service resources are available [8]. The increase in the number of homes with a second story means a potential for more volume above the fire which allows the smoke layer to remain above the fire and allows a longer time for the fire to grow. It also means more above ground areas for the fire service to access for civilian rescue and egress, potentially increasing the chance of injury.

Home Geometry

Newer homes tend to incorporate features such as taller ceilings, open floor plans, two-story foyers and great rooms [9]. All of these features remove compartmentation, add volume and can contribute to rapid smoke and fire spread. Commercial building codes require fire and smoke separations to limit the impact of the fire on occupants, there are minimal codes requiring compartmentation in single family homes [10]. A trend in new homes is to incorporate taller ceilings and two-story spaces or great rooms [11]. Much like the impact of having a two-story home, taller ceilings create a longer smoke filling time that allow for more oxygen to be available to the fire for it to grow before being surrounded by smoke filled, oxygen deficient air. The heat release rate of a fire slows down significantly once the oxygen content of the air decreases. Newer homes are being constructed with ceilings taller than the traditional 2.4 m, upwards of 4.3 m to 6.1 m [9]. It is also common for great rooms and open foyers to directly connect the living spaces to the sleeping spaces allowing for smoke generated in the living spaces to rapidly trap potential sleeping occupants. Another trend in homes is to remove walls to open up the floor plan of the home [12]. As these walls are removed the compartmentation is lessened allowing for easier smoke and fire communication to much of the home. In the living spaces doors are often replaced with open archways creating large open spaces where there were traditionally individual rooms. Combining of rooms and taller ceiling heights creates large volume spaces which when involved in a fire require more water and resources to extinguish. These fires are more difficult to contain because of the lack of compartmentation. Water from a hose stream becomes increasingly more effective when steam conversion assists in extinguishment, without compartmentation this effect is reduced. The simple tactic of closing a door to confine a fire is no longer possible in newer home geometries.

Home Contents

The challenge of rapid fire spread is exacerbated by the use of building contents that have changed significantly in recent years, contributing to the decrease in time to untenable (life threatening) conditions. Changes include: (a) the increased use of more flammable synthetic materials such as plastics and textiles (b) the increased quantity of combustible materials (c) the use of goods with unknown composition and uncertain flammability behavior Over time home contents have transitioned from being compromised of natural materials to dominated by synthetic materials [13, 14]. Synthetic materials such as polyurethane foam have replaced cotton as the padding found in upholstered furniture. Today more than 95 million kilograms of flexible polyurethane foam are produced in the US, enough to make 140 million sofas [15]. This difference was examined in the early 1980s when oxygen consumption calorimetry was utilized to measure the heat release rate of furniture. A study led by Babrauskas [16] compared different constructions of upholstered chairs. The cotton padded chair covered in cotton fabric produced a peak heat release rate of 370 kW at 910 s after ignition. The foam padded chair covered in polyolefin fabric produced a peak heat release of 1,990 kW at 260 s after ignition. Both chairs had a very similar total heat released 425 MJ for the natural chair and 419 MJ for the synthetic chair.

Home Construction Materials

Another change that has taken place over the last several decades is the continual introduction of new construction materials into homes [17]. The construction industry is continually introducing new engineered products that provide better structural stability, allow for faster construction time and are more cost effective. Additionally, the market for green or environmentally sustainable building materials experienced a growth rate of 23% through 2006 and is expected to continue growing at a rate of 17% through 2011 according to Green Building Materials in the US [18]. The increased market demand for environmentally sustainable products is driving engineered lumber products to further reduce material mass that could potentially result in even further concern for fire safety in building construction today and in the future. Environmentally sustainable products take into account resource efficiency, indoor air quality, energy efficiency, water conservation and affordability [19]. Life and fire safety are not part of the material selection criteria, while using less material and being more affordable are. Many home construction materials have changed significantly for numerous reasons such as lack of supply, ease of manufacturing, cost, improved structural or energy efficiency performance, and many other reasons [20]. Home wall linings, structural components, windows and doors are some of the construction materials that have evolved. Table 1 shows some iterations of the evolution. Evolutions in building materials create changes in the fire environment. How all of these changes compound to impact fire behavior and firefighting tactics is not well understood.

Construction Material Evolutions