Materials In Action

Wood, concrete, and steel have an environmental impact on building construction, operation and end of life
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Fire Resistance

Cross-section taken from 3-ply CLT panel protected by two layers of gypsum board and exposed to the standard fire exposure (CAN/ULC S101) for 1 hour and 15 minutes.

Photo courtesy of FPInnovations

According to the National Fire Protection Association, property loss from fire was estimated at $11.7 billion in 2011.4 While no building is completely fireproof, construction materials and systems can improve a building’s fire safety. Concrete, and especially Insulating Concrete Form (ICF), is a good fire-resistant material. Unlike wood, concrete cannot burn; and unlike steel, it won’t soften or bend. Since it doesn’t burn, concrete doesn’t ignite, nor does it release toxic fumes or smoke, nor melt when it is exposed. Concrete will only break down at temperatures of thousands of degrees Fahrenheit. Concrete’s thermal mass properties—slow absorption and release of heat—help to mitigate fire risk, and it is able to achieve fire-resistance ratings without additional fireproofing. However, concrete can be subject to severe spalling, particularly if it has an elevated moisture content. Fireproofing is available for concrete but this is typically not used in buildings. Instead, it is used in traffic tunnels and locations where a hydrocarbon fire is likely to break out.

Structural steel requires fireproofing to prevent the steel from weakening in the event of a fire. When heated, steel expands and softens, eventually losing its structural integrity and, under extreme conditions, melting. According to the National Institute of Standards and Technology, when exposed to fire, steel loses its strength and stiffness much faster than high-strength concrete. With a lower thermal conductivity, high-strength concrete will maintain its structural integrity for a longer period of time in a fire situation.

Although seemingly counter-intuitive, wood can be an excellent performer under fire conditions. According to the Southern Forest Products Association, wood outperforms non-combustible materials in direct comparison fire tests. A 2x4 timber tie maintained more of its original strength under higher temperatures and for a longer period than did aluminum alloy or mild steel. This is because of wood’s unique charring properties. When wood burns, a layer of char is created which helps to maintain the strength and structural integrity of the wood beneath—a scenario that enables an exposed heavy timber system to achieve a fire-resistance rating of up to 90 minutes.5 Properly designed wood-frame walls, floors and roofs using conventional wood framing, wood trusses and wood I-joists, can also provide fire resistance ratings for up to two hours.

Another test comparing the performance of a glulam beam to a steel beam conducted at the Southwest Research Institute demonstrated the fire performance superiority of the glulam beam when both members were directly exposed in an ASTM E119 fire test.6

Seismic Considerations

Wood-frame house from the earthquake stricken Sichuan Dujiangyan area in China still stands.

Photo courtesy of Forestry Innovation Investment China

While seismic design is a complex undertaking subject to many variables, certain general material considerations can be stated. To withstand earthquakes, buildings are designed to be flexible and move without breaking. This ability to yield and deform without fracturing is called ductility. According to Timber Engineering Europe, the type of construction that causes the most fatal injuries in earthquakes is unreinforced brick, stone, or concrete buildings that tend not to be flexible and to collapse when shaken. Metal can be formed to flex and bend without breaking, allowing the building to sway and reducing the stress on the building.

Both the elastic limit and ultimate strength of wood are higher under short periods than under longer times, permitting higher working stresses under short-term live loads, such as heavy winds and seismic loads. In fact, one of the most earthquake-resistant building types is considered to be a low wooden structure anchored to its foundation and sheathed with plywood. Such low-rise wood-frame structures with correct wall bracing, connectivity and anchorage provide safety during seismic activity. The many nailed joints in wood-frame structures make them inherently more ductile, which enables them to dissipate energy from the sudden shock of an earthquake. Numerous load paths, such as those found in wood buildings, help avoid collapse in the event that some connections fail. Because structures constructed of other materials have relatively few structural members and connections, the failure of one load can lead to overloading of adjacent members or joints. Also, since forces in an earthquake are proportional to the weight of a structure, lightweight structures fare better. Concrete walls are seven times heavier than typical wood-frame walls.

Seismic simulation testing in 2009 on a high-capacity mid-rise structure registered a major earthquake with minimal damage.

 

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Originally published in Architectural Record
Originally published in November 2013

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