View source for Eurocode classification of sections in fire

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The [[Member_design#Classification_of_cross_sections|classification of steel sections]] for ambient temperature design is described in BS EN 1993-1-1<ref name="No1">BS EN 1993-1-1:2005+A1:2014. Eurocode 3. Design of steel structures. General rules and rules for buildings. BSI</ref>, Section 5.6 and is a function of the geometry and the [[Steel_material_properties#Hot_rolled_steels|yield strength]],f<sub>y</sub>, which is introduced into the determination of class via a factor &epsilon; where:

&epsilon; = (235/f<sub>y</sub>)<sup>1/2</sup>

For a [[Steel_construction_products#Plate_girders|fabricated]] or [[Steel_construction_products#Standard_open_sections|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. 
<br>
For fabricated sections or [[Steel_construction_products#Structural_hollow_sections|hollow sections]], the section Class depends on the length:thickness ratio of flat elements between corners or welds.
 
==Classification of sections in fire according to the [[Design_using_structural_fire_standards#Fire_Eurocodes|Eurocodes]]==

In the fire design condition, BS EN 1993-1-2<ref name="No2">BS EN 1993-1-2:2005. Eurocode 3. Design of steel structures. General rules. Structural fire design. BSI</ref>, 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 &epsilon;'':

&epsilon; = 0.85*(235/ f<sub>y</sub>)<sup>1/2</sup>

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<ref name="No2"></ref>, 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 &theta;<sub>a</sub> at all cross-sections is not more than &theta;<sub>crit</sub>''.
It goes on to say that: ''The limit &theta;<sub>crit</sub> may be chosen in the National Annex. The value &theta;<sub>crit</sub> = 350&deg;C is recommended''. 

The value in 350&deg;C is confirmed in the National Annex<ref name="No3">NA to BS EN 1993-1-2:2005. UK National Annex to Eurocode 3. Design of steel structures. General rules. Structural fire design. BSI</ref>. It is very difficult and expensive to [[Fire_protecting_structural_steelwork|fire protect]] a steel section for a [[Design_using_structural_fire_standards#Effect_of_temperature_profile|limiting temperature]] as low as 350&deg;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 &epsilon; may also have minor consequences for [[#UKBs_in_bending|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 [[Design_using_structural_fire_standards#Effect_of_temperature_profile|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<ref name="No2"></ref>. This may enable the engineer to demonstrate that the [[Design_using_structural_fire_standards#Effect_of_temperature_profile|limiting temperature]] is greater than 350&deg;C. However, it is still likely to be significantly lower than for a Class 1, 2 or 3 section.

==Fabricated sections==

Special consideration needs to be given to [[Steel_construction_products#Fabricated_products|fabricated sections]] as they may be more susceptible than non-fabricated sections to changes in section classification in fire. For [[Steel_construction_products#Fabricated_products|fabricated I and H sections]] this can occur for two possible reasons:
<ol>
<li>The value of c as defined in Table 5.2 of BS EN 1993-1-1<ref name="No1"></ref>  is likely to be greater for [[Steel_construction_products#Fabricated_products|fabricated sections]] because the weld sizes for fabricated members are likely to be smaller than the root radii of equivalent rolled sections.</li>
<li>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. [[Steel_construction_products#Fabricated_products|Fabricated sections]] are not so constrained and can be designed more efficiently. </li> 
</ol>
<br>
It should be pointed out that fabricated sections in this context do not include [[Steel_construction_products#Cellular_beams|cellular beams]]. They are a special case and information on these can be found [[Fire_protecting_structural_steelwork#Protecting_cellular_beams|here]]. 
<br>

==[[Steel_construction_products#Standard_open_sections|UBs]] acting as columns==

In ambient design, the majority of [[Steel_construction_products#Standard_open_sections|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 [[Steel_material_properties#Yield strength|Grade S355]] sections :

{|class="wikitable"  width=300
|-
|1016x305x437||533x312x182||533x210x138
|-
|457x191x106||406x178x85||305x165x54 
|-
|305x127x48||305x127x42||254x146x43 
|-
|203x133x25||203x102x23||178x102x19
|-
|}

...and for [[Steel_material_properties#Yield strength|Grade S275]] sections:

{|class="wikitable"  width=400
|-
|1016x305x393||914x419x388||610x305x179||533x312x150
|-
|533x210x122||457x191x98||457x191x89||406x178x74 
|-
|356x171x67||356x171x57||305x165x54||305x127x42
|-
|305x127x37||254x146x37||254x146x31||254x102x28
|-
|254x102x25
|}
<br>
Whilst S275 UBs are no longer widely produced, they do exist in many buildings that may become available for reuse in the future.

==[[Steel_construction_products#Standard_open_sections|UBs]] in bending==

All [[Steel_construction_products#Standard_open_sections|UBs]] in bending are Class 1 in ambient design. No [[Steel_construction_products#Standard_open_sections|UBs]] in bending become Class 4 at elevated temperature. A small number of [[Steel_material_properties#Yield strength|Grade S355]] [[Steel_construction_products#Standard_open_sections|UB]] become Class 3 at elevated temperature:
*762x267x134
*356x171x45
*305x165x40
*203x133x25
<br>
This will result in a small decrease in [[Design_using_structural_fire_standards#Effect_of_temperature_profile|limiting temperature]], typically about 20-30&deg;C.

==[[Steel_construction_products#Standard_open_sections|UCs]] in compression==

[[Steel_construction_products#Standard_open_sections|UCs]] are Class 1, 2 or 3 in compression. No [[Steel_construction_products#Standard_open_sections|UCs]] are Class 4 at elevated temperature.

==[[Steel_construction_products#Structural_hollow_sections|Hot formed/finished hollow sections]]==
{{#image_template:image=File:Bankside.png|caption=Hollow sections at Bankside|align=right|wrap=true|width=300}}
In ambient temperature design, Class 4 [[Steel_construction_products#Structural_hollow_sections|square hollow sections]] are designed according to BS EN 1993-1-5<ref name="No4">BS EN 1993-1-5:2006+A2:2019. Eurocode 3. Design of steel structures. Plated structural elements. BSI</ref>. A further four sections (listed below) become Class 4 at elevated temperature. 

In ambient temperature design, Class 4 [[Steel_construction_products#Structural_hollow_sections|rectangular hollow sections]] are designed according to BS EN 1993-1-5<ref name="No4"></ref>. There are eight of these. A further three sections (listed below) become Class 4 at elevated temperature.

In ambient temperature design, Class 4 [[Steel_construction_products#Structural_hollow_sections|circular hollow sections]] are designed according to BS EN 1993-1-6<ref name="No5">BS EN 1993-1-6:2007+A1:2017. Eurocode 3. Design of steel structures. Strength and Stability of Shell Structures. BSI </ref>. There are five of these. A further 16 sections (listed below) become Class 4 at elevated temperature.

{|class="wikitable"  width=600 style="text-align: center"
|-
|'''SHS'''||'''RHS'''||colspan="4"|'''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 
|-
|}

==References==
<references/>

==Resources==
*[[Media:Steel_construction_-_Fire_Protection.pdf|Steel construction - Fire Protection supplement, 2013]]

==See also==
*[[Design_using_structural_fire_standards|Design using structural fire standards]]
*[[Steel_construction_products|Steel construction products]]
*[[Member_design|Member design]]

==CPD==
*[[Continuing Professional Development#Design for fire|Design for fire]]
*[[Continuing_Professional_Development#Fire_Engineering_Seminars_2013|Fire engineering seminars 2013]]


[[Category:Fire and steel construction]]
[[Category:Index of main articles]]