The LWF Blog

Fire protection requirements for structural steel design – Part two

April 15, 2014 10:57 am

Last week’s blog on structural steel design and the fire protection requirements gave an overview of its use and looked at the protection of applied steel. This week we look in more depth at steel assessments, time equivalency and the potential benefits to the project should a steel assessment be undertaken.


Steel assessment methods

There are various factors that can contribute to the structural failure of a steel member:

  • Loads imposed on a steel member under fire conditions, as a proportion of the average capacity of the member
  • Size, shape and relative position of the steel, and the proportion of the cross section that is exposed to fire
  • Temperature to which the steel is exposed, and duration.
  • To assess the performance of steel in fire, loadings are calculated for steel members, which are then exposed to a design fire. 

Alternatively, a design fire can be used to estimate the total burnout of the building.

The design fire is specifically chosen so that the steel will be exposed to the highest temperatures likely to occur in a fire, in the building in question


The likely performance of steel members when exposed to elevated temperatures can be assessed according to several methods. These vary in their complexity and accuracy. However, the simpler methods typically have a greater degree of conservatism than the complex models. The methods likely to be adopted in an assessment are outlined below. Within each method there can be further changes in approach depending on the project. 

Time equivalency

This method is included in Eurocode 2001 documentation and considers means of ventilation, such as windows and skylights, as well as construction and fire load, to determine the likely time that an unsprinklered fire will continue in a given area. The result is a time-of-exposure equivalent to a standard ISO fire, producing the same maximum temperature in a protected structural steel member, given a complete burnout of the fire compartment with no fire fighting intervention (Buchanan 2001). 

This method is not applicable to unprotected steel, where steel temperatures closely follow the fire temperatures (Buchanan 2001). However it can be used to justify the degree of fire protection to be applied. There are limits to the use of this method depending on the geometry of the building or compartment.

Temperature assessments

This method estimates the temperatures to which the steel would be exposed in the compartment. The rise in temperature of unprotected steel closely follows that of the surrounding air, so the air temperature can be used to indicate the maximum temperatures in unprotected steel. Where steel is protected, a finite-element analysis uses the protection properties to calculate the expected heat transfer due to conduction. This has a theoretical basis but has been justified through research.

To complete the comparison, the temperatures at which specific steel members are expected to fail are calculated, based on the relative fire loads on individual members.

Structural assessment

A detailed load-distribution assessment examines the consequence of a structural element failing in terms of the extent to which the surrounding structure will maintain structural stability by redistributing the load. This method is based on containing a fire within a defined area to ensure that multiple failures do not occur. This form of assessment becomes increasingly complex as the structure itself becomes more elaborate.

This method needs to be carried out by a structural engineer, with input by a fire engineer where required. 

Possible outcomes 

The results from these assessment methods can be used to justify a lower level of protection for steel; or even not providing any protection at all. Depending on the method of assessment used, variations in fire rating to the structure may be appropriate, so those members that at greater risk are provided with greater degrees of fire rating.

Reducing a steel structure’s fire rating can be further justified with increased fire safety systems in the building, including an automatic fire sprinkler system, sophisticated fire alarm system, smoke control system or increases in compartmentation. 

The benefits of a steel assessment can be a combination of:

  • Cost savings due to lower requirements for protection
  • Achieving architectural features in the building
  • Ease of construction.

High temperatures can change the mechanical properties of steel to the extent of reducing the load that a member can support. This can be taken into account by a lower design load during fire conditions; but heating the steel members further will lead to eventual failure.

Prescriptive requirements for the protection to be applied to steel members exist in Approved Document B and the BS 9999 series. Alternatively, performance-based design can be applied, as permitted by Approved Document B.

The temperatures at which steel members can be expected to fail, and how long after exposure to fire that this might occur, can be assessed using specific structural steel assessments. These can predict the impact of a real fire on a steel structure and show that a lower fire rating will provide sufficient structural fire safety. 

The benefits of preparing a structural steel assessment can include significant cost savings to the project and greater architectural freedom to the design team.

It should be noted that, although the methods described have been verified through research and testing (by others), agreement with Building Control and the fire authorities must be reached. Approval cannot, however, be guaranteed where the local authorities consider that deviation from the prescriptive requirements is inappropriate. In addition, depending on the use, construction and layout of the building, it may not always be appropriate to alter the amount of fire rated protection applied to structural steel.

If you would like to know more about structural steel assessments or have a structural steel assessment undertaken on your project, please contact Peter Gyere on 0208 668 8663.

Lawrence Webster Forrest (LWF) is a  specialist fire engineering and fire risk management consultancy, which provides a wide range of consultancy services to professionals involved in the design, development and construction and operation of buildings.

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