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,f_y, which is introduced into the determination of class via a factor called ε where:

ε = (235/f_y)^1/2

For a fabricated or hot rolled I or H section, whether a section is a Class 1, 2, 3 or 4 is generally dependent on the value of:

• 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 or hot formed/finished hollow sections, it is a function of the inner dimension (between the root radii for rolled sections) and the wall thickness.

Classification of sections in fire according to the Eurocodes

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/ f_y)^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 4.2.3.6(1) states that for members ..... with class 4 cross-sections other than tension members it may be assumed that .....the steel temperature θ_a at all cross-sections is not more than θ_crit. It goes on to say that: The limit θ_crit may be chosen in the National Annex. The value θ_crit = 350°C is recommended.

The value in 350°C is confirmed in the National Annex.

This 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 exists to calculate the limiting temperature for class 4 sections using effective cross section areas and section moduli 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.

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 following general advice is provided.

Fabricated sections

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:

1. 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 root radii are smaller than for equivalent rolled sections.
2. 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.

It should be pointed out that fabricated sections in this context do not include cellular beams. They are a special case and information on these can be found by clicking here.

UBs acting as columns

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 design in pure compression but are so in fire. The list of these is shown for Grade S355 sections :

1016x305x437	533x312x182	533x210x138
457x191x106	406x178x85	305x165x54
305x127x48	305x127x42	254x146x43
203x133x25	203x102x23	178x102x19

...and for Grade S275 sections:

1016x305x393	914x419x388	610x305x179	533x312x150
533x210x122	457x191x98	457x191x89	406x178x74
356x171x67	356x171x57	305x165x54	305x127x42
305x127x37	254x146x37	254x146x31	254x102x28
254x102x25

UBs in bending

All UBs in bending are class 1 or 2 in ambient design. No UBs in bending are class 4 in ambient or hot design. A small number of 3 sided (non-composite) and four sided UB are class 3 in fire when they are class 1 or 2 in ambient design in Grade S355 sections:

• 762x267x134
• 356x171x45
• 305x165x40
• 203x133x25

This will result in a small decrease in limiting temperature, typically about 20-30°C.

UCs in compression

UCs are class 1, 2 or 3 in compression. No UCs are Class 4 in fire.

Hot formed/finished hollow sections

Hollow sections at Bankside

In ambient design, class 4 square hollow sections are designed according to BS EN 1993-1-5. There are seven of these. They and an additional four are also class 4 in fire. The additional 4 are listed below.

In ambient design, class 4 rectangular hollow sections are designed according to BS EN 1993-1-5. There are eight of these. They and an additional three are also class 4 in fire. The additional 3 are listed below.

In ambient design, class 4 circular hollow sections are designed according to BS EN 1993-1-6. There are five of these. They and an additional sixteen are also class 4 in fire. The additional sixteen are listed below:

SHS	RHS	CHS
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

Resources

• Steel construction - Fire Protection supplement, 2013

See also

• Design using structural fire standards
• Steel construction products
• Member design

CPD

• Design for fire
• Fire engineering seminars 2013