Characteristics of Building Materials in Bushfire Conditions
Fire effects common building construction materials in many different ways. In many cases, the material chosen for a structural application needs to meet fire resilience criteria. Regardless, the firefighter needs to understand how these materials react to fire. In the past, the fire service looked at the characteristics of four basic material types: wood, steel, concrete and masonry.
Perhaps the most common building material and traditionally seen as combustible. However, many timbers are bush fire resistant and may be used in construction in areas of up to BAL - 29. Timber is used in most residential and commercial buildings. Wood is relatively inexpensive, easy to manipulate and a replenishable natural resource. Wood has marginal resistance to forces compared to its weight, but it does the job for most residential and small commercial buildings. Engineered wood can react differently when exposed to heat from fire. Engineered wood includes a host of products that take many pieces of native wood and glue them together to make a sheet or longer beam’ Some newer wood products such as composites present safety concerns for all firefighters. Wood may ignite from radiant heat when the tolerance level is too great. Intense fires will see wood splitter and combust very quickly.
Steel is a mixture of carbon and iron ore heated and rolled into structural shapes to form elements for a building. Steel has excellent tensile, shear, and compressive strength. For this reason, steel is a popular choice for girders, lintels, cantilevered beams and columns. Additionally, steel has high factory control. It is easy to change its shape, increase its strength, and otherwise manipulate it during production. As it relates to fires, steel loses strength as temperatures increase. The specific range of temperatures depends on how the steel was manufactured. Cold drawn steel, like cables, bolts, rebar and lightweight fasteners, loses 55 percent of its strength at 430 degrees Celsius. Therefore, in relatively low-level heat, steel will quickly lose its shape and structural integrity. Steel will twist and buckle under a bush fire and will not provide long lasting protection on its own.
Extruded structural steel used for beams and columns loses 50 percent of its strength at 600 oC. Structural steel will also elongate or expand as temperatures rise. At 600 oC, a 30 metre beam will elongate 250mm. This may have significant impact on a building. If a beam is fixed at two ends, it will try to expand-and likely deform, buckle and collapse. If the beam sits in a pocket of a masonry wall, it will stretch outward and place a shear force on the wall-which was designed only for a compressive force. This could knock down the whole wall! Because steel is an excellent conductor of heat, it will carry heat of a fire to other combustibles. This can cause additional fire spread, sometimes a considerable distance from the original fire. Each of these materials can be found together or separately.
Concrete is an excellent material to use in the guard against bush fire attack. Concrete is a mixture of portland cement, sand, gravel and water. It has excellent compressive strength when cured. The curing process creates a chemical reaction that bonds the mixture to achieve strength. The final strength of concrete depends on the ratio of these materials, especially the ratio of water to portland cement. Because concrete has poor tensile and shear strength, steel is added as reinforcement. Steel can be added to concrete in many ways. Concrete can be poured over steel rebar and become part of the concrete mass when cured. Cables can be placed through the plane of concrete and can be tensioned, compressing the concrete to give it required strength. Extreme fires may weaken concrete and the reinforcement within it. Various tests are available to determine the suitability of concrete following a fire. However, during the fire, concrete is a good product to have for protection.
All concrete contains some moisture and continues to absorb moisture as it ages. When heated, this moisture content will expand, causing the concrete to crack or spall. Spalling refers to a large pocket of concrete that has basically crumbled into fine particles, taking away the mass of the concrete. Reinforcing steel that has been exposed to a fire can transmit heat within the concrete, causing catastrophic spalling and failure of the structure. Unlike steel, concrete is a heat sink and tends to absorb and retain heat rather than conduct it. This heat is not easily reduced. Concrete can stay hot long after the fire is out, causing additional thermal stress to firefighters performing overhaul
Masonry is a common term that refers to brick, concrete block and stone. Masonry is used to form load-bearing walls because of its compressive strength. Masonry can also be used to build a veneer wall. A veneer wall supports only its own weight and is most commonly used as a decorative finish. Masonry units (blocks, bricks and stone) are held together using mortar. Mortar mixes are varied but usually contain a mixture of lime, portland cement, water and sand. These mixes have little to no tensile or shear strength. They rely on compressive forces to give a masonry wall strength. A lateral force that exceeds the compressive forces within a masonry wall will cause quick collapse of the wall. Brick, concrete block and stone have excellent fire-resistive qualities when taken individually. Many masonry walls are typically still standing after a fire has ravaged the interior of the building. Unfortunately, the mortar used to bond the masonry is subject to spalling, age deterioration and washout. Whether from age, water or fire, the loss of bond will cause a masonry wall to be very unstable.
New material technologies have introduced some interesting challenges for the firefighting community. Composites are a combination of the four basic materials listed above as well as various plastics, glues and assembly techniques. Of particular interest are the many wood products that are widely used for structural elements.
Lightweight Wooden 'I' Beams (Joists)
These are wood chips that are press-glued together into the shape of an I beam. While structurally strong ( stronger than a comparable solid wood joist), the wooden I beam fails quickly when heated. Actually, no fire contact is required. Ambient heating causes the binding glue to fail, leading to a quick collapse. The bottom of a beam is under tensile forces. If the bottom of the beam falls off, due to glue failure, the beam will immediately snap and collapse.
New products, known as FiRP (fibre-reinforced products) are becoming common in the construction industry. FiRP can be plastic fibres mixed with wood to give the wood increased tensile strength. As with most plastics, fire exposure can cause quick failure as the plastic melts. The mixture of steel and wood as a structural element can cause rapid collapse because steel expands.