Behaviour during fire: Steel/Concrete/Wood

One of the major concerns in the construction is the resistance against fire damage. The main issue is the protection of the human life. Today, we will look at three most common materials used for construction and observe how they fail when subjected to fire.

Steel

Most materials tend to get weaker with increasing temperature, steel is no exception. When exposed to fire and higher temperatures, it begins to heat at its perimeter and slowly heats throughout its whole section. With increased temperature, its properties begin to change. It starts to lose its strength at around 300 degrees Celsius. The loss is quite fast and relatively steady. At 55 degrees of Celsius, the steel has already lost 40% of its initial yield strength and half of its stiffness.

This rapid change of properties can cause a fatal failure. Therefore it is essential to take this behaviour into consideration.

Concrete

Generally, the concrete performs better when exposed to high temperatures. It does not heat up as quickly as steel because it has lower thermal conductivity (app. 50 times lower than steel). Thus it takes a longer time to heat it though. However, one unexpected type of failure can happen. Spalling.  There are tiny bubbles of water trapped in concrete, essentially during its whole life-cycle. Those bubbles of water can heat up and evaporate. This evaporation followed by expansion of volume can cause dangerous explosions that tear the concrete apart, which can lower its strength quickly, without warning.

Wood

If the wood is a subject of the heat and fire, we have to first and foremost distinguish two types of wood construction. Frame construction consists of many smaller pieces of wood with a small sectional area. The other type is massive timber construction, where we have huge chunks of wood, either solid timber or engineered products, which have considerable sectional area.

The frame construction is very susceptible to fire. Small planks burn quickly and contribute to the magnitude of the fire.

On the other hand, massive timber is much safer during the fire. Similarly to when you throw a massive log into a campfire, it takes a considerable amount of time until it catches fire, and when it does, its perimeter charrs, which is a protective layer, that slows burning.  Both charred layer and the low thermal conductivity of wood protect the core, which looses its strength slowly.

Conclusion

Three most common building materials behave differently when exposed to high temperatures and fire. This has to be considered during design, better sooner than later and employ strategy that protects structural members from failure.

Cheers