Eurocode classification of sections in fire
The classification of steel sections for ambient temperature design is described in BS EN 1993-1-1, Section 5.6 and is a function of the geometry and the yield strength,fy, which is introduced into the determination of class via a factor ε where:
ε = (235/fy)1/2
- the web depth between the root or weld radius divided by the web thickness and/or
- the value of the width of the flange outstand beyond the root radius divided by the flange thickness.
For fabricated sections or hollow sections, the section Class depends on the length:thickness ratio of flat elements between corners or welds.
In the fire design condition, BS EN 1993-1-2, Section 4.2.2 states that: For the purpose of these simplified rules the cross-sections may be classified as for normal temperature design with a reduced value for ε:
ε = 0.85*(235/ fy)1/2
As a consequence of this, it is possible that in certain situations the section classification of members changes between the ambient (normal) and fire design cases. This can be significant if the section classification changes to Class 4. This is because BS EN 1993-1-2, Section 18.104.22.168(1) states that for members ..... with class 4 cross-sections other than tension members it may be assumed that .....the steel temperature θa at all cross-sections is not more than θcrit. It goes on to say that: The limit θcrit may be chosen in the National Annex. The value θcrit = 350°C is recommended.
The value in 350°C is confirmed in the National Annex. It is very difficult and expensive to fire protect a steel section for a limiting temperature as low as 350°C and therefore it is recommended that checks are carried out during the ambient temperature design to ensure that the section dimensions are not such that the section is, or becomes, Class 4 in fire.
The modification of ε may also have minor consequences for hot rolled beams in bending, where it is possible for the section classification to change from Class 1 or 2 to 3.
A method to calculate the limiting temperature for class 4 sections using effective cross section areas and section moduli is presented in Appendix E of BS EN 1993-1-2. This may enable the engineer to demonstrate that the limiting temperature is greater than 350°C. However, it is still likely to be significantly lower than for a Class 1, 2 or 3 section.
Special consideration needs to be given to fabricated sections as they may be more susceptible than non-fabricated sections to changes in section classification in fire. For fabricated I and H sections this can occur for two possible reasons:
- The value of c as defined in Table 5.2 of BS EN 1993-1-1 is likely to be greater for fabricated sections because the weld sizes for fabricated members are likely to be smaller than the root radii of equivalent rolled sections.
- Webs are likely to be thinner in fabricated than in non-fabricated sections. This is because the relationship between web and flange thickness in hot rolled sections is largely determined by the need to prevent warping when cooling. This means that the web is often thicker than is necessary for structural reasons alone. Fabricated sections are not so constrained and can be designed more efficiently.
In ambient design, the majority of UB sections are Class 4 in pure compression and so are unlikely to be used in this way. A small number are not Class 4 in ambient temperature design in pure compression but become Class 4 at elevated temperatures. The list of these is shown for Grade S355 sections :
...and for Grade S275 sections:
Whilst S275 UBs are no longer widely produced, they do exist in many buildings and thus may become available for reuse in the future.
This will result in a small decrease in limiting temperature, typically about 20-30°C.
In ambient temperature design, Class 4 rectangular hollow sections are designed according to BS EN 1993-1-5. There are eight of these. A further three sections (listed below) become Class 4 at elevated temperature.
In ambient temperature design, Class 4 circular hollow sections are designed according to BS EN 1993-1-6. There are five of these. A further 16 sections (listed below) become Class 4 at elevated temperature.
|550 x 550 x 16||550x350x16||139.7 x 3.2||244.5 x 5.6||323.9 x 5.6||406.4 x 8.0|
|650 x 650 x 19||650x450x19||219.1 x 4.5||273.0 x 5.0||323.9 x 6.3||457.0 x 8.0|
|700 x 700 x 19||759x500x22||219.1 x 5.0||273.0 x 5.6||355.6 x 6.3||457.0 x 10.0|
|750 x 750 x 22||244.5 x 5.0||273.0 x 6.3||355.6 x 8.0||508.0 x 10.0|
- BS EN 1993-1-1:2005+A1:2014. Eurocode 3. Design of steel structures. General rules and rules for buildings. BSI
- BS EN 1993-1-2:2005. Eurocode 3. Design of steel structures. General rules. Structural fire design. BSI
- NA to BS EN 1993-1-2:2005. UK National Annex to Eurocode 3. Design of steel structures. General rules. Structural fire design. BSI
- BS EN 1993-1-5:2006+A2:2019. Eurocode 3. Design of steel structures. Plated structural elements. BSI
- BS EN 1993-1-6:2007+A1:2017. Eurocode 3. Design of steel structures. Strength and Stability of Shell Structures. BSI