Structural Fire Design According to EN 1993-1-2 (Heating Behaviour)
Using RF-/STEEL EC3, you can apply nominal temperature-time curves in RFEM or RSTAB. For this, the standard time-temperature curve (ETK), the external fire curve, and the hydrocarbon fire curve are implemented in the program. Based on these diagrams, the add‑on module can calculate the temperature in the steel cross‑section and thus perform the fire design. This article explains the behaviour of protected and unprotected steel cross‑sections.
General Heating Behaviour of Steel
Steel consists of a crystal grid with the individual crystals moving around a resting point at a 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, the steel ductility increases with the rising temperature. On the other hand, 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; plastic reserves of the structure regarding the material are taken into account in the conventional steel construction nowadays. For example, if the cold‑worked or heat‑treated steel is subjected to thermal stress, it will have 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 may lead to the effects due to restraint in the component, which were not present in the design at normal temperature.
For civil engineering, steel has poor thermal properties, especially in terms of fire resistance. The temperature rise depends on the massiveness of the steel component; the more massive is 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 a 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. For the calculation of this section factor, Eurocode  provides tables to facilitate the calculation.
Heating Behaviour of Protected Steel Components
In the case of fire‑resistant steel structures, the heating behaviour 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). By using passive fire‑protection systems, it is possible to significantly increase the fire resistance duration. 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.
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. As a cladding, gypsum plaster fire protection boards or calcium silicate boards are often used. All major manufacturers of such boards also provide suitable board systems, including the fire resistance properties.
If the appearance of the steel support should be preserved, it is recommended to use the contour encasement or cladding, or to apply 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. In order to apply the plaster systems, wire grids are usually attached to the beams, holding the plaster system. The cladding with board systems is similar to the cladding with hollow encasement, but it takes more effort as the board systems have to be cut to size.
As an alternative, insulation intumescent can be used, which is a type of contour encasements and is applied to the structural component. This intumescent swells when heated and acts as an insulating layer between the structural component and the surrounding area. However, there is no calculation method developed for this cladding type since the high‑temperature properties of this material are usually not exactly known or too widely scattered.
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Design of steel members according to Eurocode 3
Design of steel members according to Eurocode 3