Combustible Construction
Article and photos by William Shouldis
Part 1 of this series, “The Struggle for Safety,” provided cues for an effective roadmap for public safety organizations when confronting with suppression, search, and rescue operations. Part 2, “Noncombustible Construction,” examined noncombustible construction features to provide incident commanders and incident safety officers a guideline for making operational decisions in structures with fire-resistive characteristics.
Part 3 analyzes the effects of fire, smoke, and toxic gases in a structure with combustible components. The International Building Code (IBC) has a strict requirement on the flame spread of insulation placed between two layers of combustible materials without any intervening airspace.
Ordinary Construction (Type III) has a combination of combustible and noncombustible features. Because of its popularity, this architectural style has been termed “Main Street USA.” The exterior walls are protected by a noncombustible material and the fire resistance of the interior depends on building’s age. It is common to see parapet walls between adjoining properties, decorative cornices, and even marquees in such structures in business districts. Type III buildings can be divided into older and newer construction features. Older buildings have floors that may consist of tongue-and-groove boards supported by solid lumber joists. The flooring in newer construction may consist of plywood, supported by lightweight wooden trusses, and wooden I-beams. Combustible walls and ceilings are protected with plaster, plasterboard, or drywall. These materials will help contain a fire for a reasonable period of time. However, once flames or high heat enter the interconnecting combustible voids behind a partition, companies must open walls and ceilings to ensure complete extinguishment. Using thermal imaging cameras (TICs) can assist firefighters in finding areas of hidden fire that must be opened to ensure complete extinguishment.
The structural floor and roof members in older construction consist of solid joists and rafters that can be three inches wide and 10 inches deep. At times, columns would be added to provide additional support. The size of rooms is limited by the span of the supporting element. Floor joists commonly have a fire cut on each end, an approximately 30° cut on each end of the solid beam joist. The longer end of the cut is placed downward and rests in the bearing wall. In a floor collapse, the fire cut allows the floor joist to fall down into the building so that the joist in will not push the masonry wall outward. The danger of a fire cut to fire companies and crews operating on the building interior is the increased risk of a collapse.
Older buildings have combustible floors that may consist of tongue-and-groove boards supported by solid timber joints whereas the flooring in newer construction may consist of plywood, oriented strand board (OSB), laminated veneer lumber (LVL), and parallel strand lumber (PSL), supported by lightweight wooden trusses and wooden I-beams. These supports are more likely to collapse sooner than the larger solid lumber joists used formerly.
In Type III construction, older and newer construction methods are also used in the roof assembly. In an older building, 1- to 1¼-inch thick wooden planks were commonly used as roofing material. Contemporary construction use lightweight materials and evolving methods. Roof coverings such as tar and gravel, asphalt shingles, rolled asphalt, and rubber are likely to be found with this type of construction. Peaked roof structures commonly have an accessible attic area of variable dimensions.
The flat roof building has a smaller, inaccessible void that is normally free of storage. This concealed space, the cockloft, is an open path for fire and smoke spread.
(1) Preincident planning and size-up allow for better overall operations. Building construction can produce many designs and shapes. Each has unique operational challenges for first responders. Decorative cast iron components can rapidly fail. Any column failure leads to a catastrophic collapse and warrants immediate evacuation of interior crews. Click to enlarge
A drawback of Type III construction is that the structural elements will burn while the exterior load-bearing and nonload-bearing walls, made of fire-resistive materials, will not. Firefighters must be trained to carefully recognize and fully evaluate the presence of void spaces which is typical of this construction type. Over the decades, experience has shown that these buildings usually have been modernized extensively. Often, interior walls have been altered to create an open contemporary appearance and suspended ceilings have been added. Channels for new HVAC equipment and shafts for plumbing fixtures can rapidly spread the fire. Numerous voids and open spaces can be found in these structures, creating a deadly ambush for unsuspecting firefighters.
Heavy timber (Type IV) construction is also known as mill construction because of the substantial size of the wooden structural elements. These buildings are found in many parts of the United States. This type of structure, when properly maintained, is not prone to early collapse. The large-cross-sectional lumber used in walls, ceilings, floors, and roof assemblies makes it very sturdy. The masonry exterior walls can range up to 60 to 80 feet in height. Depending on the structure’s height, the extra thick walls can be 36 inches thick at the foundation.
Often, the large interior is divided by fire walls and self-closing fire doors. The fire wall is distinguishable by its thickness. It is wider than an ordinary wall and does not bear the weight of the building. The large open floors of solid wood plank were built to handle heavy loads; they are usually supported by solid timber trusses or wooden columns that are at least eight inches thick.
(2) Heavy Timber buildings have been modified because changes in occupancy. As a community changes from a manufacturing sector to retail businesses, some of the inherent construction characteristics are eliminated. Modern plumbing systems, elevator hoistways, and electrical features can create vertical shafts. On arrival at an emergency scene, it is best to have a 360-degree view of the situation. Fire protection systems may help in you pinpoint and contain problems in these large structures. Sprinkler system will not extinguish fires in concealed spaces.
Interior modifications to these structures may change some of the inherent qualities of the original construction. In Type IV construction, the interior walls and ceiling are not finished; masonry walls are exposed and there are relatively few concealed spaces. Recently, factories and warehouse occupancies have been converted into retail or wholesale outlets. Other heavy timber buildings have been converted into energy-efficient, multiuse occupancies such as restaurants, small shops, galleries, vertical malls, apartments, and condominiums.
Historically, fires that get beyond the incipient stage generate the potential for a tremendous amount of radiant heat and require several operational periods to contain and control. For this reason, the International Building Code (IBC) requires Type IV buildings to have at least a 20-foot separation from external exposures. First-arriving responders must anticipate rapid fire spread and recognize hazards created by the many original openings for freight elevators, stairs, beltways and utility shafts. Key strategic concerns include proper apparatus placement and quickly identifying all available water supplies with reliable pressure to supply numerous master streams. A fire department’s ability to dispatch and deploy sufficient resources can mean the difference between containment and conflagration.
Wood-frame construction (Type V) has a support system that consists of wood or a similar material, and is the most prevalent type of construction used today. Model building codes are frequently adjusted and new energy-efficient construction techniques are always being used for residential and commercial properties. These changes either can greatly assist or seriously impair responders in their quest to contain and control a small fire.
To prevent weather-related damage, the wooden exterior framing system is commonly covered and treated with a fire-retardant material. In older construction, wood siding, hard asbestos shingles, or flexible asphalt wrapping are common. In newer construction, combustible coverings such as aluminum and vinyl are used and add to the fuel load. Stone, stucco, and brick veneer are noncombustible materials that can be used to protect the structural components without adding any fire load.
Structural elements in this type of construction vary. Older wood-frame construction was assembled from solid lumber with nail connectors whereas newer construction relies on preengineered lumber and lightweight fasteners. Since the bearing walls are made of combustibles, there is a significant loss in the load-carrying capacity under fire conditions.
Different styles of wood-frame construction were used in different eras.
Balloon-frame was very popular when long structural materials were readily available from nearby lumber mills (until about the mid-1900s). In this framing system, the exterior walls are assembled with wood studs that present a continuous inner void from the basement into the roof area. This type of framing usually lacks compartmentation and adequate fire-stopping, which is an operational concern. The resulting open flue creates the potential for rapid fire extension.
Platform-frame is an open construction style that has been popular since the late 1940s. The side walls are erected on the floor deck. After the wall studs are in place, the floor joist and flooring for the next level are set onto the studs of the floor below or the sill plate of the new floor. This construction method prevents the creation of uninterrupted voids between floors. It does not allow fire to extend to upper levels because of the amount of extensive materials that must burn.
Post-and-beam frame is a less common framing system that uses posts as the vertical supporting members and beams as the horizontal members. This technique uses joists that are used to tightly connect the building. A modern version of post-and-beam frame can be found where large pressure-treated poles are firmly set into the ground and the framework of the structure is hung from the supporting elements.
Log frame uses wooden logs that interlocked by notches on each end. In this style of construction, the exterior walls will supply an abundant amount of fuel to a fire. This log cabin design can be found in several regions of the country.
William Shouldis retired as deputy chief of the Philadelphia (PA) Fire Department, where he served in line and staff positions for more than 34 years. His assignments included working directly for the chief on labor relation and accountability issues and serving as field commander for one-half of the city, department safety officer, director of training, and hazardous material task force leader. He is an instructor at the Graduate School at St. Joseph’s University in Philadelphia (PA), the National Fire Academy, and the Emergency Management Institute. He has a master’s degree in public. He can be contacted via e-mail at WShouldis@gmail.com.
Subjects: Building construction for firefighters, fireground decision-making.
