The impact of sprinklers on structural fire resistance Structural fire resistance is always included in code requirements, its purposes being to ensure that the load-bearing capacity of the structural members and compartmentation of the building are maintained for a minimum time during a fire. Its evaluation with an analytical approach allows structural fire engineers to verify regulatory compliance in lieu of using methods in the Eurocodes, which often end in rejection or the necessity to apply expensive passive protection. Daniele Andriotto, Giovanni Cosma and Luciano Nigro of Jensen Hughes examine the evaluation procedure.
The evaluation of the resistance of a building according to the so-called natural fire curve allows fire specialists to determine the actual behaviour of structures during a realistic fire scenario, rather than by referring to the conventional temperature / time fire development curves. A full analytical procedure for structural fire design would address the behaviour of the structural system at elevated temperatures, the potential heat exposure and the beneficial effects of active and passive fire protection systems, together with the uncertainties associated with these three features and the importance of the structure. This fire safety engineering (FSE) approach applied to the fire resistance of structures can nowadays be considered a specialized discipline and branch of engineering. The general starting conditions common for any structural calculation are defined within the European standard series EN 1990 – also called Eurocodes – where to obtain an accurate calculation of fire resistance an in-depth study of the mechanical behaviour of the structural elements must be conducted, to analyse the possible interactions between elements when stressed in different ways and to evaluate the thermal conductivity of the structure. Within the prescriptive approach, the thermal effect of the fire on structures is determined using a nominal time-temperature curve defined within EN 1991-1-2, also referred as the ISO834 curve.
64
outlook
After having determined the thermal effects of the fire using this temperature-time curve, the fire engineer must evaluate the response of the structure, choosing an appropriate method to solve the heat transfer equations and determine the structural temperature-time curve. The second step according to this prescriptive approach is the assessment of the structural behaviour and the evaluation of any possible mitigation measure. Even the selection of passive structural fire protection can be made using a prescriptive approach, where the time is defined by national regulations. On the other hand, when following a performance-based approach, it is the fire engineer’s task to quantify the risk associated with the building and to demonstrate that acceptable thresholds are met. This study highlights the structural performance of a building subjected to a fire with a focus on how active fire safety systems can influence the result.
Performance based design
When structural fire resistance is calculated using the natural fire curve, the fire resistance performance must be determined through thermal and structural modelling of the entire structure or significant subgroups of structures to address all the indirect, imposed or hindered stresses that are generated by deformations or expansions. When the safety factors included within the nominal fire curves procedure are no longer considered, it is essential to analyse the indirect stresses, caused by the
