Fire Resistance of Galvanized Steel Sections
Galvanizing is commonly used to provide protection against corrosion for a wide variety of steel components, ranging in size from nuts and bolts to large structural sections.
In addition to this, recent research has shown that the zinc protective layer also provides a reduction in the surface emissivity of the steel component, which influences the rate at which the temperature of a steel section increases when exposed to a source of heat.
How Can You Calculate Fire Resistance?
The process of designing a steel member in fire is made complicated primarily due to the need to know the temperature of the member at the time of interest. This is in essence an iterative process which requires solving an equation(s) hundreds of times.
Galvanizers Association with the aid of Steel Construction Institute (SCI) has recently published a design guide which greatly simplifies the design of galvanized steel members in fire, avoiding any need for complex calculations.
Design guide for galvanized steel in fire
The publication includes:
- Design tables to calculate fire resistance and maximum fire exposure periods for galvanized steel beams, composite beams, columns, and plates in tension, according to the Eurocodes and the UK and Irish National Annexes
- Design tables in accordance with BS 5950 are also provided
- Worked examples are also provided to illustrate the use of the tables.
The design tables clearly show where the use of galvanized steel leads to an increase in fire resistance or fire exposure compared to non-galvanized steel.
Benefit of Using Galvanized Steel in Fire
The benefit of utilizing galvanized steel members for fire resistance is apparent in structures that require short fire resistance periods, that is, 15 or 30 minutes of fire exposure, where the temperature reached by the galvanized steel members is around 500°C.
Examples of structures that require such fire resistance periods include car parks and single-storey residential/office industrial buildings. There may also be benefit in using galvanized steel for other types of structures, such as single storey industrial buildings or some multi-storey office buildings, where the use of sprinklers may enable a reduction of the minimum fire period to 30 minutes.
Performance
of Galvanized Steel in Fire
Due to the reduction in surface emissivity of the zinc coating , galvanized steel members may experience a smaller temperature increase than equivalent non-galvanized steel members.
The Benefit of the Reduced Emissivity
The benefit of the reduced emissivity is most significant for structures that require shorter fire resistance periods, such as car parks or single storey industrial buildings, where the temperature reached by the galvanized steel members is around 500°C. The reasons for this are:
- for temperatures lower than 500°C , the rate at which the temperature increases in a galvanized steel member is given by a surface emissivity of 0.35, while for non-galvanised steel member this is given by a surface emissivity of 0.70.
- the steel strength is most sensitive to temperature at around 500°C and so at this temperature range, small differences in temperature have a pronounced effect on strength loss.
How Can You Calculate Fire Resistance?
An important factor that affects the rate at which the temperature in a steel member increases is the section factor.
In EN 1993-1-2 the section factor is defined as the surface area of the member exposed to a fire per unit length, Am, divided by the volume per unit length, V. Therefore, a beam exposed to a fire on four sides has a higher section factor than an equivalent one exposed on three sides.
This factor has the same effect irrespective of whether the section is galvanized or non-galvanized, as it only depends on the geometric proportions of the cross-section. That is, the larger the section factor, the faster the temperature in the member increases. As a result of this, the largest benefit of using galvanized steel over non-galvanized steel can occur in different cross-sections at different fire exposure times, depending on the section factor of the cross-section.
Temperature Rise of Galvanized and Non-Galvanized Steel Sections Subject
The figure 1 , opposite compares the rise in steel temperature of galvanized and non-galvanised steel beams for three different Universal Beam sections. The beams are exposed to fire from three sides with section factors ksh [Am⁄V]m of 75 m-1 109 m-1 and 170 m-1, respectively.
The figure indicates that for a fire exposure period of 15 minutes, the galvanized steel sections can achieve 3.5 minutes, 3.5 minutes, and 2.0 minutes longer fire resistance period compared to the equivalent non-galvanized steel sections, respectively.
For a fire resistance period of 30 minutes, the galvanized steel section with a section factor of 170 m--1 (UB 254 x 146 x 43) performs very similarly to the equivalent non-galvanized steel section. This is because at 30 minutes fire exposure, the temperature in the galvanized section is 820°C, which is significantly higher than 500°C. For the section with a section factor of 75m-1 (UB 533 x 210 x 122), however, at 30 minutes fire exposure, there is still a noticeable gain in fire exposure time of 3 minutes.
Figure 2: Maximum utilisation for galvanized and non-galvanized steel beams exposed to fire on three sides
The Resistance Versus Time Behaviour for the Galvanized and Non-Galvanized Steel Beams
If the gains in fire exposure time using galvanized steel are translated into increased resistance, the advantages are more pronounced.
In figure 2 , the resistance is represented by the maximum utilization that can be achieved by the member, calculated as the ratio of the cross-sectional resistance of the beam in the fire situation to the cross-sectional resistance at room temperature. A degree of utilization of 0.7 is indicated by a horizontal line which corresponds to the largest practical value for which a laterally restrained beam can be designed in the fire situation. This is because beams designed in fire for a degree of utilization larger than 0.7 are likely to fail at room temperature.
At 15 minutes Fire Exposure
Increase in Bearing Capacity
For sections UB 533 x 210 x 122 and UB 254 x 146 x 43, the maximum utilization that can be achieved by the non-galvanized steel sections decreases below 0.7 at noticeably shorter fire exposures than that of the galvanized sections. For example, for the steel section with the lowest section factor (UB 533 x 210 x 122), at 23 minutes fire exposure, when the maximum utilization of the galvanized section decreases to 0.70, it can carry 70 % more load than the non-galvanized section. At 30 minutes of fire exposure, even though the gain in fire resistance time is low for sections UB 533 x 210 x 122 and UB 254 x 146 x 43 (see Figure 1), they show a modest gain of 9% and 14% in load carrying capacity, respectively.
speaking
Research on fire protection in steel galvanizing
It was found that galvanized steel reaches lower temperatures due to the effect associated with the lower emissivity of the galvanized surface.
Laboratory research – full scale testing
The first tests
of galvanized steel
fire resistance
Figure 3: Test specimens suspended from the compartment ceiling
Measured temperature rise during fire tests
- Open and closed steel specimens, one metre in length, were hung from the ceilingof the compartment at the location of the highest expected gas temperature.
- The arrangement of the specimens ensured an even temperature distribution.
- Profiles were arranged in pairs (galvanized and non-galvanized).
- Specimens were isolated at both ends by mineral fibre wool such that the sample simulated an infinite length element and that heat transfer occurred only at the outer surface.
The temperature of each specimen was measured by one 2 mm diameter thermocouple, which was placed at the mid-point of each specimen. The gas temperature in the fire compartment was measured by twenty 3 mm thermocouples and seven plate thermocouples. That table shows the temperature rise of the specimens in the compartment.
The measured temperature rise was compared against the predicted rise assuming
The measured temperature rise was compared against the predicted rise assuming a value for the surface emissivity of 0.32, which had been measured previously in horizontal furnace fire tests. The figure shows the good correlation between the measured and predicted results, especially in the first 20 minutes of the fire.
Proposed revision
of EN 1993-1-2
(as of June 2020)
| Type of steel | Carbon steel | HDG steel |
|---|---|---|
| εm (≤ 500 °C) | 0.7 | 0.35 |
| εm (> 500 °C) | 0.7 | 0.7 |
More recently, Gaigl4 carried out two full-scale tests and 147 laboratory tests to measure the emissivity of galvanized steel. In small-scale laboratory tests, different types of galvanized specimens were heated and the emissivity was deduced by measuring the temperature of the specimens with thermocouples and infrared sensors.
These studies demonstrated consistent emissivity values for a range of galvanized steels with different silicon content.
Silicon content is important because its presence can influence the thickness and structure of the galvanized coating.
The ranges of silicon content associated with consistent emissivity values of 0.35 were those normally encountered in modern structural steels and are defined by Category A and B of EN ISO 14713-2.
Further fire tests on galvanized steel
France
Further supporting studies have been carried out in France. Three sets of standardfire tests were performed at Effectis, France in a joint project by CTICM, Galvazinc and EGGA . In 2016, fire tests were carried out on I and H profile steel columns exposed on four sides. In 2017, further tests were carried out on I and H profile beams exposed on three sides and I profile and hollow section columns exposed on four sides.
- SALA, A. Radiant Properties of Materials: Tables of Radiant Values for Black Body and Real Materials, PWN and Elsevier, 1986
- HEINISUO, M. Effect of zinc coating to the resistance of WQ-beam bottom flange in fire, Tampere University of Technology, 2014
- JIRKU, J. and WALD, F. Influence of Zinc Coating to a Temperature of Steel Members in Fire, Journal of Structural Fire Engineering, Vol 6, 2015
- GAIGL, C. and MENSINGER, M. Fire resistance of galvanized structures, 25th International Galvanizing Conference (Intergalva 2018), Berlin, Germany, 2018
- EN ISO 14713-2: 2020 Zinc coatings. Guidelines and recommendations for the protection against corrosion of iron and steel in structures. Hot dip galvanizing, CEN, 2020
- EN 1993-1-2: 2005 Eurocode 3: Design of steel structures – Part 1-2: General rules -Structural fire design, CEN, 2005
- BIHINA, G. Temperature assessment of galvanized steel profiles: validation of proposed emissivity values for bath galvanized steel, Technical report A 30.4.2018), Centre Technique Industriel de la Construction Métallique for the European General Galvanizers Association, 2017
- RASZKOVA, S. Emissivity of hot dip galvanized steel, MS thesis, Czech Technical University in Prague, 2019
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