The high cost of passive fire protection, usually with the help of different painting systems, is an economic disadvantage of steel and composite structures compared to simple concrete constructions. If the corrosion protection system also protect from fire it would be of great benefit. Steel components are non-flammable and do not contribute to fire loads, but the mechanical properties of the steel are temperature dependent.
The calculated yield strength (above 400 °C) and the modulus of elasticity (above 200°C) decrease with an increasing temperature. For example, if a steel member reaches a temperature of about 600°C, about 53% of the initial strength is lost. This must be taken into account when dimensioning steel structures in case of fire.
Fire consists of different phases:
- A slow start and smouldering phase at the beginning.
- Full fire phase with flashover and a corresponding increase in temperature.
- Afterwards, the fire subsides and the cooling phase starts, as the time progresses.
The time of the phases differ between different fires.
Temperature range
The zinc layer has a positive effect up to the steel reaching about 550 °C. That limit probably depend on the melting point of zinc and the intermetallic phases:
Pure zinc: 419.5 °C
Zeta phase: 530 °C
Delta phase: 530-670 °C
Several benefits with HDG in fire
The results of practical tests with fire showed that the hot-dip galvanized steel had several advantages in comparison with the uncoated:
-Lower emission factor
-Slower heating
-Longer time until the critical temperature is reached
One important thing for the infleuence of the emission factor is the colour of the coating. To get a low emission factor like in the table above the coating should be bright or light grey.
| Type of steel | Emission factor up to 500 C | Emission factor over 500 C |
| Carbon steel | 0,70 | 0,70 |
| Stainless steel (1) | 0,40 | 0,40 |
| HDG Steel (2) | 0,35 | 0,70 |
- According to Annex C
- Steel galvanized according to EN ISO 1461 and with steel composition according to Category A or B of EN ISO 14713-2, Table 1.
Table 1.Emission factors for
black steel, stainless steel and galvanized steel.
If the coating is dark grey the emission will be higher, so it is important to use the right type of steel. According to the foot note to the table above the steel should be Aluminum killed, or silicon killed with a silicon content of maximum 0.25 %. To be on the safe side it might be better to recommend a little lower maximum Si- content, depending on the construction.
| Fire resistant class | Fire resistant time (Minutes) | Symbol in the construction inspection rules |
| F 30 | ≥ 30 | Fire resistant |
| F 60 | ≥ 60 | Fire resistant |
| F 90 | ≥ 90 | Fire proof |
| F 120 | ≥ 120 | Fire proof |
| F 180 | ≥ 180 | Very fire proof |
Table 2. Fire resistance classes. Studies has shown that F30 is possible with galvanized steel without any further protection.
In Germany, Belgium and Netherlands this knowledge already are used on national level. Implementation in Eurocode 3 (Design of steel structures) has been suggested
Interesting business case
The large business interest for fire protection with galvanizing could easily be understand from the picture below. If protection with paint is used there is demand for maintenance on a regular basis. With galvanizing no maintenance is needed and the intital costs are on the same level.
Summary
Studies have shown that the zinc layer has a positive effect on the heat generation during fire exposure. In many cases is it possible to withstand F30 in fire resistance of a hot-dip galvanized steel structure, without any additional fire protection. This means that the hot-dip galvanized construction can withstand 30 minutes of fire exposure according to the standard EN 13501-2 (Fire classification of construction products and building elements).