Fire resistance class (part 2)

Which standards are important for the fire resistance class?

In the first part of the post, we talked about some of the basics of fire resistance class. In this article we delve deeper into the topic.

The model list of technical building regulations from the German Institute for Building Technology (DIBt) regulates which standards are valid in Germany . The current list as of 2014 includes both the "old" national standards and the current European standards for fire protection:

Experts brought the DIN 4102 up to date in 2016. So that was after the building inspectorate introduced the Eurocodes in 2012. They also discussed whether to withdraw the national standard. The practice was against it and so the committee decided to keep the norm. He made sure that DIN 4102 supplemented the Eurocodes. It contains constructive details that are missing in the Eurocodes. However, there are no duplicate regulations because this would not be permitted by the building authorities. How long have the national standard DIN 4102 and the European standards coexisted? That is so far unclear.

For my own work, I use the fire protection parts of the Eurocodes to design fire-exposed members (e.g. EN 1993-1-2 for verification of a steel column).

Is every material well suited to achieve a fire rating?

Which material properties are important for this?

You can achieve a fire resistance rating with any material. Some materials are inherently highly fire resistant, while others are much more sensitive to heat. Let's have a quick look at the most important properties of the materials in case of fire.

The thermal conductivity at high temperatures is crucial for the fire resistance class. If a building material conducts heat poorly, parts of the interior of the cross-section remain "cool" and stable, depending on how long the fire has lasted. In contrast, the outer heated portions quickly lose strength and stiffness.

The specific heat capacity also plays a major role. You know it from the kitchen. How do I mean that? When you heat a pot of water on the stove, it takes a while for it to boil. So you first have to add energy to the water to heat it up. The more energy it takes to raise a substance by one degree Celsius, the higher its heat capacity. Let's take another example. Imagine it's a sweltering hot summer day. If you happen to be nearby, a historic church, for example, is a great way to beat the heat. Why? This is because the thick walls can absorb a lot of heat and warm up with a delay. This is not only a great feature in summer, but also in the event of a fire.

There are other useful properties of materials in case of fire. Since these apply to individual materials, we will discuss them in the following sections for each material.

Steel members

Let's start right away with the mimosa among the materials in case of fire 😉 As strong and elastic as steel is at room temperature, it is unfortunately very sensitive in a fire. This is due to its high thermal conductivity. Steel conducts heat very well. This means that even short fires heat up steel cross-sections very quickly. As a result, steel quickly loses strength and rigidity.

Contrary to what is often claimed, steel only melts at temperatures around 1500°C. Incidentally, at temperatures of around 785°C, it benefits from the fact that its crystal lattice changes. This process binds a lot of energy and somewhat delays further heating of the cross section.

There are also steels that behave somewhat more favorably in fires. In the event of a fire, stainless steel has lower thermal conductivity. In addition, stainless steel reflects the heat better - firefighters speak of the emissivity of a component here.

All in all, it (unfortunately) stays the same. Unprotected members made of steel usually do not achieve a fire resistance class. Don't worry, but there are constructive solutions to also equip steel members for fires.

Reinforced concrete members

To make it short: components made of reinforced concrete are much more suitable in the event of a fire. Concrete benefits from its low thermal conductivity. This is about 20 times smaller than that of mild steel! No wonder, then, that components made of reinforced concrete slowly heat up.

You have to be careful with the reinforcement. It's also made of steel. To protect them from the heat, provide a sufficient concrete cover. How many centimeters is that? Take a look at the example of concrete cover in the event of fire.

Concrete can continue to score in the event of a fire. The free moisture in the concrete is between 2% and 10%, depending on the environmental conditions. In the event of a fire, the water heats up and evaporates at 100°C. This binds energy and delays the heating of the cross section.

However, be careful with high-strength concrete! It achieves its high strength through its dense structure. In the event of a fire, the steam is less able to escape because the high-strength concrete has fewer pores. A vapor pressure then builds up which quickly exceeds the tensile strength of the concrete. Large spalling then occurs, which can expose the reinforcement - in this case, a reinforced concrete column would quickly fail! The EN 1992-1-2 standard therefore specifies stricter rules for the reinforcement of high-strength members.

Wooden members

Can it get any better? Yes, with wood - at least as far as thermal conductivity is concerned. This is again significantly lower than the thermal conductivity of concrete. However, in the event of a fire, wood behaves differently than steel or concrete. The fire chars the outer layers of the wood. This so-called pyrolysis protects the interior of the cross-section. However, if the fire temperatures exceed approx. 500°C, shrinkage cracks appear in the wood. Therefore, we consider the thermal conductivity of wood more as an approximation.

Are you interested in this topic?

Are you interested in what influence the material has on fire protection? Then I recommend this article, which compares cross-sections of steel, reinforced concrete and wood in the event of a fire.

What do I do if I don't achieve a fire resistance class?

Have you exhausted all detection methods above? From level 1 to level 3? Then you still have an ace up your sleeve. You can think about using so-called natural fires instead of the ISO standard fire. For example, I sometimes provide evidence for steel components that do not require any fire protection. To do this, I use a hot design to prove that the construction resists the respective natural band. Feel free to contact me if you have a need here.