Structural fire engineering

Benefits of structural fire engineering

Structural fire engineering as one component of a comprehensive performance-based approach to managing fire safety of building structures enables a range of benefits that can significantly and positively support the decision to utilise steel as the material of choice.

The potential benefits include: 

  • a reduction in the extent of fire protection necessary: in some cases elements may not need to have any fire protection. A typical example is secondary beams in composite floor systems

  • a reduction in the degree of fire protection necessary: this can, for example, facilitate the cost-effective use of intumescent paints to meet architectural or functional requirements, which might not otherwise be possible

  • a reduction in construction time and improved site logistics: compared to the Deemed-to-Satisfy solution

  • tighter construction clearances: with the omission of passive fire protection

  • facilitating the expression of the steel structure for architectural purposes: particularly common in entrance atria and similar

  • a net significant reduction in cost for fire protection: compared to the Deemed-to-Satisfy solution

  • a significant ROI: for the up-front cost of fire engineering based on the significant cost savings through elimination or reduction of fire protection. 

Requirements for minimum-level performance

The Australian National Construction Code (NCC) is a performance-based code, consisting of mandatory Performance Requirements and supporting General Requirements that specify the minimum level of performance for buildings. Compliance with the Performance Requirements can be achieved by:

  • complying with the Deemed-to-Satisfy (DtS) provisions; or
  • formulating a Performance Solution; or
  • a combination of the two.

Where a Performance Solution is developed, that Performance Solution uses one or more of the Assessment Methods in the NCC to demonstrate that the Performance Requirements have been met. Assessment Methods include Verification Methods, comparison with the DtS provisions, use of Expert Judgement or Evidence of Suitability.

A Verification Method is a test, inspection, calculation or other method that determines whether a solution complies with the relevant Performance Requirements. Structural fire engineering focuses on developing verification methods and documenting outcomes supporting a performance solution.

Further information on the process for development of performance solutions is available from the ABCB website.

The Structural Fire Engineering process

Structural Fire Engineering, which might also be termed ‘Structural design for fire’, supports performance-based approaches to limit risk to the individual and society, to directly exposed or neighbouring properties and to the environment from a fire event. 

In relation to the structural aspects of fire engineering, a generalised approach can be divided into three steps: 

  • Identify a design-basis fire, expressed as a relationship between fire temperature or heat flux as a function of time. This will be influenced by an analysis of fuel and compartment characteristics and the effects of any active fire protection systems present 

  • Determine the temperatures in structural members, components and systems through a heat transfer analysis 

  • Calculate the response of the structural system, taking into account the effect of elevated temperatures on strength and stiffness. 

This approach supports a range of detailed methodologies, ranging from simplified approaches through to advanced approaches involving fully nonlinear coupled thermal-structural analysis. The choice of approach is dictated by the complexity of the structural systems, the potential cost savings from reduced or eliminated fire protection and the expected return on investment (ROI) from a commitment to undertake the selected extent of fire engineering. Ultimately, a specialist structural fire engineer can provide a cost-benefit analysis for a specific project, but in the early stages of planning a steel building solution, ‘rules of thumb’ and general guidance are invaluable tools for decision makers to assess cost savings and ROI.

Deflection analysis under fire loading.

Simplified approaches

The simpler approaches would usually evaluate the performance of individual members when the member can be assumed to be subjected to uniform heat flux and to exhibit a uniform temperature distribution. The fire models selected may range from using a nominal time-temperature curve (prescriptive approach) to a parametric fire model or a specific fire simulation.

Advanced approaches

More advanced approaches assess the fire performance of the structure either as sub-assemblies or parts of the structure or the entire structure. The analysis includes all mechanical actions and appropriate boundary conditions and uses advanced calculation models.

Calculation of the mechanical behaviour of the structure exposed to fire may be performed in the time domain (fire resistance ≥ required fire resistance time, generally given in the National Regulation), the strength domain (design value of member resistance at time t ≥ design value of relevant effects of actions at t) or the temperature domain (design value of material temperature ≤ design value of critical material temperature). 

Nonlinear structural analysis under fire loading.

The Australian Building Codes Board (ABCB) provides a range of support material outlining the context and benefits of fire safety engineering, including a freely downloadable copy of the International Fire Engineering Guidelines (IFEG) 2005 that is commonly used for complex proposals in Australia.