FEB 2012
Introduction to structural fire engineering: Adding value in tough times
A common misconception, particularly related to steel and timber structures, is that elements require fire protection to satisfy the requirements of the Building Regulations. This is not the case. They need ‘fire resistance’. Fire resistance, based on years of experimental experience, can be achieved on a prescriptive basis using simplified design rules for a given structural material. Such rules, if followed, would allow for a structural element or a building product to survive the required amount of time, in a fire resistance test, under the very specific heating regime and boundary conditions inherent in the testing and assessment process.
Fire resistance and the use of the standard fire test are widely understood by designers and regulators. However, there is a continued misconception that a fire resistance rating, for example 60 minutes, refers to the ability of a building to survive for the same period in a real fire. This correlation cannot be made for a number of simple reasons:
• Firstly, the stochastic nature of fire, due to variability in fire loadings and ventilation from building to building, means that a standardised fire curve cannot represent the full, or even majority, of the types of fires that would occur in practice.
• Secondly, the standard fire test is a measure of the fire resistance of a single component acting in isolation. However, in a real structure, complex interactions occur which affect the behaviour of a component and may enhance (or degrade) its expected fire resistance. An example of this is the interaction between floors, walls, and the elements connecting them (bolts, rivets, nailing plates, etc.) which are not considered in a standard fire test.
• Thirdly, real fires do not continuously increase in temperature; they will eventually cool down. However, the thermal response used in fire resistance tests is that of an ever increasing temperature without consideration of cooling. This implies that the fire has an infinite fuel source, which is of course not the case.
It follows that the prescribed detailing rules derived on the basis of such tests do not accurately consider the shortcomings of the fire resistance test procedure and the potential implications they may have in real fire events. Although the prescriptive philosophy has proved adequate over the years, the prescriptive nature of such an approach also provides a barrier to innovation and increases costs. In addition, as far as safety is concerned, the robustness of a structure in a real fire remains uncertain, as a result of adopting prescriptive based detailing and design.
In recognition of the limitations of the prescriptive based approach to achieving fire resistance, the concept of performance based structural fire design has evolved.
The concept of performance based structural fire design is simple. A building is designed based upon fires that are unique to the characteristics (occupancy, location, height etc.) of the given project. It is seen above that the standard fire curve is ‘non-physical’ as it continually heats for all time. In reality, fires have a finite life governed by a combination of compartment thermal inertia, ventilation and fire loading conditions all of which are unique to a given building. Intuitively, all of the products associated with fire (smoke, radiation, and toxic gases) are then also unique to a given scenario. Considering this, a structure can be designed to survive a given realistic design fire without collapse or significant irreversible damage, during both the heating and cooling phases, thus fulfilling its functional requirement as required by the Building Regulations.
To this end, structural fire engineering is a powerful discipline which allows for robust buildings, with quantifiable margins of safety, which are protected with a degree of passive fire protection which is proportionate to the foreseen risks. As a result, structural fire engineering can add value to a number of different types of project:
a) New Build - Structural fire engineering can be adopted to rationalise the degree of passive fire protection required for a structure by considering the fire risks, the potential fire loadings/severity and the consequence of failure. As a result, robust buildings are often achievable with a lesser extent of passive fire protection than would be required adopting a prescriptive code compliant solution. This can result in significantly reduced capital expenditure. For steel structures, passive fire protection can equate to as much as 15% of the steelwork package expenditure.
b) Refurbishments - Changes in building use or building refurbishments often require the fire resistance performance of a structure to be reviewed in light of current guidance. Structural fire engineering techniques can be adopted to appraise the existing achievable fire resistance of a buildings structure and review the performance against the foreseen fire risks when the works have been completed. It is often the case that refurbished structures inherently satisfy the functional requirements of the building regulations, without necessitating costly remedial works.
c) Tender support - Consideration of structural fire engineering and passive fire protection requirements during the tender process can result in strategic advantages when bidding for a project. Often main contractors can improve the likelihood of tender success by considering optimised passive fire protection regimes and associated installation practicalities prior to appointment. Once appointed alternative strategies can be negotiated with the project team and approval authorities. Where structural fire engineering has already been adopted it is valuable to seek a peer review to ensure the robustness and longevity of a given design.
d) Construction - The ‘buildability’ aspect of designs during construction can often highlight a number of instances where fire protection guidance, as set out in the fire strategy for a building, cannot be followed/adopted for practical reasons. In such cases, structural fire engineering tools and techniques offer an opportunity to arrive at practical solutions which work on site but do not compromise the safety of the building. This may include the local omission of passive fire protection in problem areas, or the introduction of redundancy to offset passive fire protection requirements.
Trenton Fire Engineers are experience in the application of a number of tools and design standards to arrive at innovative structural fire engineering strategies for different types of buildings. This includes the assessment of steel, steel composite, timber and concrete buildings using the latest in analysis tools and software. To discuss how structural fire engineering can add value to your projects, please contact:
Dr. Danny Hopkin, EngD MEng CEng MIFireE
Structural Fire Engineer
