The load-bearing structures
the load-bearing structures
Wood is a combustible material, but it is also a poor conductor of heat; for this reason, it is often used in the construction of load-bearing structures – not only because it is aesthetically pleasing and economical, but above all because, in the event of a fire, it offers greater mechanical strength, which helps to increase the time available to evacuate any occupants of the premises to safety.
STEEL-CONCRETE COMPOSITE
A rise in temperature reduces the mechanical properties of materials but leaves the cross-sectional geometry unchanged
WOOD
A rise in temperature leaves the mechanical properties of materials unchanged (or almost unchanged) but reduces the cross-sectional area
WOOD
The fire spreads at an average rate of 0.7–0.9 mm/minute, creating a charred layer that limits the advance of the flame. The temperature just a few centimetres inside the timber beam is only 40°C, compared with nearly 1,000°C on the outside.
Fire and heat do not alter the mechanical strength. Collapse occurs when combustion reduces the cross-sectional area to such an extent that the beam can no longer fulfil its load-bearing function. The beam’s load-bearing capacity is proportional to its thickness; therefore, high loads and strength can only be maintained by oversizing the beam without the need for treatments such as intumescent or fire-retardant paints, plaster or calcium silicate boards.
STEEL
Steel is considered a non-combustible material, and this is absolutely true, but it is also an excellent conductor of heat. When subjected to a drastic rise in temperature, as in the case of a fire, steel rapidly loses its stability.
After just 5 minutes of exposure to fire, steel reaches a temperature of around 500°C. This represents the so-called critical temperature for this material. After 10 minutes of fire, the steel structure yields and collapses in on itself like a house of cards. Steel expands by approximately 0.012 mm for every metre of length and degree of temperature. This means, at a temperature of 500°C, an elongation of 6 mm per metre. The use of intumescent paints or other types of protection is mandatory.
CONCRETE
Due to its composition, reinforced concrete or simply cement is also classified as a non-flammable building material. The thermal conductivity of concrete is 2.1 W/mk, compared with 60 W/mK for steel – a factor that should not be overlooked when dealing with reinforced concrete components and, in particular, prestressed concrete.
Limestone aggregates do not undergo any appreciable reduction in mechanical strength until temperatures exceed 750°C, at which point the thermal decomposition of limestone into lime and carbon dioxide begins. The problem lies in the steel content within the structure, and the key factor is the thermal protection of the concrete cover. Indeed, in reinforced concrete structures, high temperatures are of particular significance, especially with regard to the steel, which loses most of its properties at temperatures above 500°C.
These weak points become preferential pathways for heat flow, capable of causing a localised rise in temperature that can exceed 500°C in a very short time. In this case too, the fire behaviour is similar to that of a steel beam, so it is mandatory to use intumescent paints or other types of protection.