Wood has a high strength-to-weight ratio, i.e., for the same strength required for a given structure, the weight of the timber material to be used can be as much as 16 times less than steel, or five times less than concrete. Weight for weight, wood can be designed to carry a heavier load than steel, i.e., one tonne of wood can carry a heavier load than one tonne of steel.
Modern engineered timber products like Glued Laminated Timber (Glulam) and Cross Laminated Timber (CLT) allow wood to be used in constructing high-rise buildings. Currently, the world's tallest timber building is a 14-storey apartment called Treet in Bergen, Norway. In the pipeline are plans to build a 34-storey timber building in Canada.
There is a great variety of timbers with a wide range of properties to suit various outdoor and internal applications, and for both aesthetic and structural purposes. While some timbers require treatment when used under harsh conditions, many species of timbers can naturally last for a very long time.
Timber can be treated to make it harder, termite resistant or weather-resistant. Treatment can be done by impregnating wood with certain chemicals which are effective yet safe for use by humans. This process is widely available and is affordable. Treated timber will often out-last naturally durable timbers.
Wood is an excellent insulating material because of the presence of the empty cell walls, which act as tiny air traps that resist the transfer of heat. This characteristic is deemed ideal for insulation materials. Compared with wood, other building materials such as bricks, steel and concrete are not good insulators.
Wood requires minimal external energy to keep a building within the thermal comfort zone of its inhabitants. Wood is six times better than brick as an insulator; eight times better than glass; 15 times better than concrete; 390 times better than steel and 1,700 times better than aluminium.
Performance in Fire
Research has shown that timber used as structures such as columns in large buildings, performs better during a fire compared to steel or concrete. This is because steel will buckle and concrete will crack and crumble very suddenly under high temperatures. Thick timber columns, on the other hand, will initially ignite but the charring of the outer layers of wood will cut off the oxygen supply and effectively slow down the burning of the deeper layers of the timber. The slow rate of burn is important because it gives the occupants enough time to evacuate during a fire. Timber columns have been found to be still standing and functioning after intense fires.
The speed and ease of ignition is dependent on the rate of accumulation of heat at the surface of the wood. Several factors influence this rate and they are the size of the piece of wood, the rate of heat loss from the surface, the presence of thin outstanding edges and the rate which heat is supplied to the surface of the wood. Small pieces with sharp projecting edges such as matches , ignite easily. On the other hand, large pieces, with round edges, like a round Glulam column catch fire at a much slower rate. In buildings with engineered timber panels, heat does not conduct from one side of the panel to the other.
Case Study: Timber's Performance in an Actual Fire
Shortly before midnight on 31 December 2012, a fire started in a strip mall in Salem, Oregon. The fire spread fast and destroyed everything, except for the Glulam beams the building was designed around. Post-fire, the Glulam beams were still so sound that crews had to use their backhoes and bulldozers to physically break the beams in half to get them down.
Glulam beams consistently outperform other leading materials in fire resistance tests. The average building-fire temperature ranges from 700° to 900° Celsius. Steel weakens dramatically as its temperature climbs above 230° Celsius, retaining only 10% of its strength at about 750° Celsius. Wood will not ignite until it reaches almost 260° Celsius. Once heavy timber ignites, it chars at a slow rate of 0.635 mm per minute*. The slowness of burn is due to the inherent property of wood to naturally insulate in a fire. Thus, in a 30-minute fire, only 19 mm of each exposed surface of the Glulam is lost to charring, leaving most of the original cross section intact.
Unprotected metals lose their strength quickly in a fire and often collapse suddenly due to their rapid loss of strength. Studies have shown that within 10 minutes of a fire starting, steel loses its structural properties by over 50%, while Glulam still holds over 80% of its strength. Actually, no building is fire-proof since most fires start with the structure's contents. The goal of fire-resistive construction is to provide occupants adequate time to evacuate the structure safely.
*under the American ASTM E-119 fire exposure.
Source: American Institute of Timber Construction.
Steel weakens dramatically as its temperature climbs above 230° Celsius, retaining only 10% of its strength at about 750° Celsius. Wood will not ignite until it reaches almost 260° Celsius.