Structural fire analysis

Three successive steps are involved in determining the structural fire response of a building; they are fire analysis, thermal analysis, and structural analysis (Abu, 2014).

Fire analysis

The principal objective of the fire analysis is to assess the fire development and thermal exposure (heat flux, gas temperature) on building structures. Natural fire models (e.g. parametric fires) or the standard fire curve is commonly used depending on the design scenario. Other types of fire models exist; they are dependent on the significance of the fire development (i.e. either localised or fully developed fire) in the compartment and they can be used for fire modelling, e.g. zone and field models [e.g. computational fluid dynamics (CFD)] as shown in Table 2.1.

The simple models are based on experimental evidence; they can be used to predict gas temperatures given localised and fully developed fires. The zone and field models are used for the calculation of the effects of temperature, smoke and fire spread, etc., in a building compartment.

The 1-zone model allows for the assumption of homogenous conditions in modelling a compartment fire. It also uses simplified assumptions, and its application is restricted to fire

compartments of simple geometry (e.g. small compartments with small ventilation openings) Manuals, 2008). In using the 2-zone model, the compartment fire conditions follow the smoke separation theory whereby two different layers, upper and lower layers are modelled. The upper layer contains more heat and smoke; while the lower layer is smokeless. The layers are then calculated by simulation to predict gas temperature evolution over time

(Tavelli et al. 2014).

Table 2.1. Model levels for different fires (Manuals, 2008)

Model levels Localised fire Fully developed fire

Simple model Hasemi model; Heskestad model Parametric fires

Zone models 2 zone model 1-zone model

Field models CFD CFD

Field models (computational fluid dynamics (CFD) models) are used to model compartment fires based on 3-dimensional configurations, application of conservation laws (e.g. energy, mass etc.) and time-dependence. The fire compartment, in this case, is split into several thousand and often millions of cells depending on its geometry, the needed accuracy and computer speed. Typically, the outcomes of CFD simulations include heat and smoke spread, time of sprinkler and smoke detector activation, flashover time and temperatures in the cells (Manuals, 2008). Importantly, the sophistication and simulation time needed for CFDs are best utilised for complex compartment geometries and are rarely used to predict structural fire performance. However, CFD models can be used to predict fire resistance if the CFD model represents that of a furnace. As mentioned earlier in Section 2.1, a set of travelling fires can also be used to model realistic fire scenarios idealised as two regions of flames and smoke beyond flames (i.e. ‘far-fields’) for large compartments (Stern-Gottfried, 2011). Importantly, it was demonstrated that travelling fires have more severe effects on structural resistance than conventional fully-developed fires. Hence, travelling fires have been proposed as realistic design fires for structural fire analysis.

Thermal analysis

The temperature development or heat transfer to structural members is assessed after a suitable fire model has been selected. Established theory of heat transfer is employed in thermal analysis, given appropriate assumptions and design needs. In the structural fire

analysis of steel elements, prediction of uniform temperature across the steel cross-section is allowed (Purkiss and Li, 2013). For standard fire exposure, a hand calculation best-fit-method may be used, or an iterative (incremental) procedure which treats the entire steel cross-section as a lumped mass at a uniform temperature may also be used (Buchanan and Abu, 2017). The lumped mass method can be used with any design fire curve. Advanced computational models based on the finite element or finite difference techniques can also be used to estimate temperatures within cross-sections of 2D and 3D structural steel assemblies. In localised and post-flashover fire conditions, non-uniform temperature distributions may exist along steel structures needing accurate prediction which can be achieved with 2D or 3D thermal analysis. Notably, an understanding of the thermal properties of the steel structure and insulation materials is essential; these properties include density, specific heat, and thermal conductivity. The thermal properties of some insulation materials can be found in the

literature (Wang et al. 2013; Kodur, 2014; Buchanan and Abu, 2017).

Structural analysis

In fire conditions, the mechanical response of steel structures is determined by structural analysis; this is similar to structural analysis at ambient conditions except that thermal actions, fire limit state gravity loading and reduced material properties of steel at elevated temperatures are considered. Structural analysis may proceed by either the analysis of each structural member or structural component (i.e. part of the building structure) or whole (global) structure.

Eurocode 3, Part 1.2 (BSI, 2005a) provides three methods for assessing the mechanical

response of steel members in fire conditions; they are tabulated method; simple calculation

method and advanced calculation method. The tabulated method is based on sets of data,

which can be used only for specific members on exposure to the standard fire. The simple

calculation method applies to steel and composite member analysis and is sub-divided into two calculation methods namely, critical temperature (i.e. calculation in temperature domain) and resistance methods (calculation in strength domain). The advanced calculation method applies to the structural or mechanical assessment of parts or the global structure based on finite element methods, using sophisticated computer software. In the analysis of parts or global structure, several steel members are considered, which provides more clarity on steel structural behaviour, e.g. effects of load redistribution or structural deformations. The

designer’s decision on the appropriate fire protection material may follow the satisfactory determination of the mechanical response of steel structures in fire conditions.