Galvanized steel is recognised for its corrosion protection properties, however recent research has shown that these benefits extend to improving the structural performance of steel sections in fire.
© Philip Durrant
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.
Laboratory and full-scale testing have demonstrated that below approximately 500°C, the zinc layer of a steel section galvanized to EN ISO 1461 remains stable, and its surface emissivity is around half of that for non-galvanized steel. A galvanized steel section will therefore heat up at a slower rate than an equivalent non-galvanized section.
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.
The 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 utilising 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.
Figure 1 | Fire resistance of galvanized and non-galvanized steel beams exposed to fire on three sides as a function of time
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.
Design guide summary
The design guide provides tables to calculate fire resistances and maximum fire exposure periods for galvanized steel beams, composite beams, columns, and plates in tension, according to the Eurocode design standards (EN 1993-1-2, EN 1994-1-2) and the UK and Irish National Annexes.
Figure 2 | Temperature rise of galvanized and non-galvanized steel sections subject to the standard nominal fire curve
The design tables are applicable for fire exposure periods of up to 30 minutes, and they clearly show the cases where the use of galvanized steel leads to an increase in fire resistance or fire exposure compared to nongalvanized steel. The Eurocode design tables are given in Appendix A of the design guide. Additionally, design tables in accordance with BS 5950 are given in Appendix B.
For steel sections with moderate section factors (105 m-1< ksh [Am⁄V]m < 182 m-1) the effect is more noticeable for a 15 minute fire resistance period. However, for steel sections with low section factors (33 m-1< ksh [Am⁄V]m < 64 m-1), the largest benefits are achieved at 30 minutes fire resistance. Sections with section factors falling within these ranges cover a large proportion of UB/UC/hollow sections.
Design examples are also provided to illustrate the use of the tables and the potential advantages of galvanized steel over nongalvanized steel in fire (e.g., increased load resistance or longer fire exposure period for a given load).
Click to view Fire Resistance of Steel Sections Galvanized to EN ISO 1461 and request a free copy.
Image: Philip Durrant