Structural Fire Design According to EN 1993-1-2 (Heating Behavior)

General Heating Behavior of Steel

Steel consists of a crystal grid with the individual crystals moving around a resting point at normal temperature. This movement decreases when reaching the absolute zero temperature of -273°C and increases when heating up. Due to this movement of the crystals around the resting point, steel's ductility increases with rising temperature. Meanwhile, the steel strength decreases. Due to the strength loss, it is quite difficult to protect the unprotected components against fire effects without additional measures, since steel has already lost 50% of its strength at a temperature of 600 °C. Therefore, it is usually overloaded, as plastic reserves of the structure regarding the material are taken into account in modern conventional steel construction. For example, if the cold-worked or heat-treated steel is subjected to thermal stress, it has already lost its strengthening at 400 °C from the design method mentioned above. Furthermore, steel has the disadvantage that thermal expansion applies when increasing the temperature, which is very high compared to other building materials. This can lead to effects due to restraint in the component that were not present in the design at normal temperature.

Steel has poor thermal properties for civil engineering, especially in terms of fire resistance. The temperature rise depends on the massiveness of the steel component. This means: The more massive the component, the more energy it can absorb. If the surface is even and the volume is increased, the result is lower temperatures in the structural component. This component property is called section factor A/V. It is the relation of the surface area to the volume per length unit of the component. In DIN 4102, this factor was still referred to as the U/A ratio and was related to the relation of the perimeter to the area, although this is the same if the cross-section relating to the length does not change. Eurocode [5] provides tables to facilitate the calculation of this section factor.

Section Factor Am/V for Unprotected Steel Components (Source: [5])

Heating Behavior of Protected Steel Components

In the case of fire-resistant steel structures, the heating behavior changes towards the positive as the poor temperature properties of the steel are absorbed or compensated by the fire-protection system. The fire-protection systems are usually composed of materials with a low thermal conductivity. Moreover, such materials usually have a high specific heat capacity (storage capacity). The fire resistance duration can be increased significantly by using passive fire‑protection systems. These materials are often very heavy and should, therefore, be considered in the structural analysis. However, EN 1993‑1‑2 includes no information about the material properties of encasements or claddings, as these depend on their producer. For this reason, the values important for the structural analysis of fire-resistant components are missing, but were submitted meanwhile in the National Application Document for building materials approved according to DIN 4102‑4. The section factor of such structural components is composed as follows.

Hollow encasement: A hollow encasement is usually best suited, because the encasement of the cross‑section reduces the perimeter of the structural component while the area remains the same. Thus, the section factor becomes smaller, which increases the massiveness of the structural component. Gypsum plaster fire protection boards or calcium silicate boards are often used as a cladding. All major manufacturers of such boards also provide suitable board systems, including the fire resistance properties.

Contour encasement: If the appearance of the steel support should be preserved, we recommend using the contour encasement or cladding, or applying a plaster system. The disadvantage of contour encasements is the section factor of the cross-section, because this does not change. Cladding types are usually plaster systems or board claddings. To apply plaster systems, wire mesh is usually attached to the beams, to which the plaster system then adheres. Cladding with board systems is similar to cladding with hollow encasement, but it takes more work effort, as the board systems have to be cut to size. Another alternative is intumescent, a coating that is applied to the component and is a type of contour encasement. This intumescent swells when heated and acts as an insulating layer between the structural component and the surrounding area. However, there is still no calculation method for this type of cladding, as the high-temperature properties of this substance are usually not precisely known or vary too widely.