A critical review of “travelling fire” scenarios for performance-based structural engineering

Many studies of the thermal and structural behaviour for large compartments in fire carried out over the past two decades show that fires in such compartments have a great deal of non-uniformity (e.g. Stern-Gottfried et al. [1]), unlike the homogeneous compartment temperature assumption in the current fire safety engineering practice. Furthermore, some large compartment fires may burn locally and tend to move across entire floor plates over a period of time. This kind of fire scenario is beginning to be idealized as travelling fires in the context of performance-based structural and fire safety engineering.This paper presents a literature review of the travelling fire research topic and its state of the art, including both the experimental and theoretical work for the past twenty years. It is found that the main obstacle of developing the travelling fire knowledge is the lack of understanding of the physical mechanisms behind this kind of fire scenario, which requires more reasonable large scale travelling fire experiments to be set up and carried out. The demonstration of the development of a new travelling fire framework is also presented in this paper, to show how current available experimental data hinder the analytical model development, and the urgent need that the new travelling fire experiments should be conducted.     

Nonlinear behaviour of unprotected composite slim floor steel beams exposed to different fire conditions

An efficient nonlinear 3D finite element model has been developed to investigate the structural performance of composite slim floor steel beams with deep profiled steel decking under fire conditions. The composite steel beams were unprotected simply supported with different cross-sectional dimensions, structural steel sections, load ratios during fire and were subjected to different fire scenarios. The nonlinear material properties of steel, composite slim concrete floor and reinforcement bars were incorporated in the model at ambient and elevated temperatures. The interface between the structural steel section and composite slim concrete floor was also considered, allowing the bond behaviour to be modelled and the different components to retain its profile during the deformation of the composite beam. Furthermore the thermal properties of the interface were included in the finite element analysis. The finite element model has been validated against published fire tests on unprotected composite slim floor steel beams. The time–temperature relationships, deformed shapes at failure, time–vertical displacement relationships, failure modes and fire resistances of the composite steel beams were evaluated by the finite element model. Comparisons between predicted behaviour and that recorded in fire tests have shown that the finite element model can accurately predict the behaviour of the composite steel beams under fire conditions. Furthermore, the variables that influence the fire resistance and behaviour of the unprotected composite slim floor steel beams, comprising different load ratios during fire, cross-section geometries, beam length and fire scenarios, were investigated in parametric studies. It is shown that the failure of the composite beams under fire conditions occurred for the standard fire curve, but did not occur for the natural fires. The use of high strength structural steel considerably limited the vertical displacements after fire exposure. It is also shown that presence of additional top reinforcement mesh is necessary for composite beams exposed to short hot natural fires. The fire resistances of the composite beams obtained from the finite element analyses were compared with the design values obtained from the Eurocode 4 for composite beams at elevated temperatures. It is shown that the EC4 predictions are generally conservative for the design of composite slim floor steel beams heated using different fire scenarios.     

Structural behaviour during a vertically travelling fire

This study considers a multi-storey composite frame subject to a fire which travels vertically between three floors. Previous work has analysed the behaviour of this structure when subject to simultaneous fires on three floors. It highlighted the importance of the cooling regime adopted and the relative axial stiffness of the steel beams to the overall behaviour of the structure. This paper extends that work by investigating the more realistic case of a vertically travelling fire. Various inter-floor time delays are considered as well as two floor beam sizes. It is found that the inter-floor time delay affects the global behaviour substantially. The behaviour is also in part dependent on the stiffness of the floor beams. Axial forces caused by thermal expansion in individual floors may induce cyclic loading on the column which is not normally considered in structural fire design but may be important in determining structural behaviour. Identifying a worst-case rate of vertical fire spread is not possible due to the range of structural responses, so it is recommended that designers consider several rates of spread and ensure structural integrity for each.     

A new correlation for gas temperature inside a burning enclosure

A new correlation for predicting enclosure gas temperature is presented in this paper based on the energy balance for adiabatic conditions, an estimate of the heat flux imposed on the enclosure boundary and the transient thermal response of the boundary. This correlation has been verified being able to predict enclosure gas temperature in both well- and under-ventilated fires in comparison with the existing experimental results. It is also compared with the well-known and widely used McCaffrey, Quintiere & Harkleroad (MQH) correlation.     

Analytical assessment of the structural performance of composite floors subject to compartment fires

The behaviour of composite steel–concrete building floors exposed to fire conditions is examined in this paper. The work represents part of a large project dealing with the numerical modelling and assessment of structural behaviour based on the fire experiments undertaken on a full-scale multi-storey steel-framed building at Cardington, UK. After providing brief details on the analytical tools and modelling approaches adopted in this investigation, the structural models constructed to simulate the fire tests are described and salient findings are highlighted. Although the detailed models provide some insight into key behavioural aspects such as the paramount influence of restraint to thermal expansion, the complex structural interactions that occur under fire conditions may not be readily demonstrated. In order to illustrate a number of underlying response mechanisms, the analytical results obtained from an idealised structural system, in which a single compartment is subjected to fire, are summarised. Assessment of the structural response using such simplified models provides a clear interpretation of the behaviour and, with further refinements, may be employed for undertaking detailed studies aimed at developing improved design recommendations.     

The influence of travelling fires on a concrete frame

When building fires occur in large, open compartments they rarely burn uniformly across an entire floor plate of a structure. Instead, they tend to travel, igniting fuel in their path and burning it out as they move to the next fuel package. Current structural fire design methods do not account for these types of fires. This paper applies a novel methodology for defining a family of possible heating regimes to a framed concrete structure using the concept of travelling fires. A finite-element model of a generic concrete structure is used to study the impact of the family of fires; both relative to one another and in comparison to the conventional codified temperature-time curves. It is found that travelling fires have a significant impact on the performance of the structure and that the current design approaches cannot be assumed to be conservative. Further, it is found that a travelling fire of approximately 25% of the floor plate in size is the most severe in terms of structural response. It is concluded that the new approach is simple to implement, provides more realistic fire scenarios, and is more conservative than current design methods.     

Physical scaling of water mist fire extinguishment in industrial machinery enclosures

Froude-based scaling relationships had previously been theoretically extended to, and experimentally validated in the laboratory for, water mist suppression of fires in open environment and in enclosures, which were shown applicable to gas, liquid and solid combustible fires. Before applying these relationships to real-world settings, their applicability needs to be further evaluated for the intended protection. This paper presents such an evaluation on scaling water mist fire extinguishment in an industrial machinery enclosure. In this evaluation exercise, a full-scale water mist protection set-up tested for a 260-m³ machinery enclosure was selected as the benchmark. A ½-scale machinery enclosure test replica was then constructed, together with a ½-scale nozzle whose orifices were geometrically similar to those of the full-scale nozzle. Spray measurements indicated that the ½-scale spray closely met the scaling requirements, in terms of discharge K-factor, water mist flux, droplet velocity and droplet size distribution. Two spray fires and one pool fire, which were scaled with the respective full-scale fires, were used to challenge the water mist protection in the ½-scale enclosure. At least five replicated tests were conducted for each of the four tested fire scenarios. Overall, the instantaneous local gas temperature and oxygen concentration measured inside the ½-scale enclosure for each fire scenario agreed reasonably well with those measured at the corresponding locations inside the full-scale enclosure, meeting Froude modeling's requirement that scalar quantities be preserved in different scales. The fire extinguishment times obtained from the ½-scale tests for each fire scenario were also statistically consistent with that observed in the corresponding full-scale test. Based on the obtained results, it is concluded that, for machinery enclosures and other similar occupancies, the previously laboratory-validated scaling relationships for water mist fire suppression can be used to determine the fire extinguishing performance of a full-scale water mist protection in a ½-scale test facility.     

Flame radiation from polymer fires

Enclosure fires can be divided into two ventilation regimes: well-ventilated and under-ventilated combustion. The influence of fire ventilation on flame radiation is very important in enclosure fires especially for under-ventilated conditions. An approximate model for predicting flame radiation for both well- and under-ventilated fires is proposed on the basis of the Γ-correlation, in which the role of flame sootiness and heat release is considered. This paper is an extension of work performed earlier and reported in Tewarson et al. (Combustion and Flame, 95 (1993) 151-169)[1]. The results are calculated for several typical polymers, and the relationship between the flame radiant fraction and the fuel’s smoke-point has been examined. Additionally, flame radiation is predicted by the slightly modified Global Flame Radiation (GFR) Model of de Ris and Orloff. The comparisons between experimental and predicted data are satisfactory. This study attempts to provide: (1) a deeper scientific understanding of the effect of ventilation conditions on flame radiation, and (2) available correlation, for the analysis of enclosure fires.     

The behaviour of reinforced concrete slabs in fire

In this paper a robust model is presented based on the previous layer procedure developed by the author to also take into account the effects of concrete spalling on the behaviour of concrete slabs under fire conditions. In this study, a detailed analysis of a uniformly loaded reinforced concrete slab subject to different degrees of concrete spalling under a standard fire regime is first carried out. Further, a series of analysis of floor slabs with different degrees of concrete spalling is also performed on a generic reinforced concrete building. A total of 16 cases have been analysed using different degrees of spalling on the slabs, with different extents and positions of localised fire compartments. It is clear that adjacent cool structures provide considerable thermal restraint to the floor slabs within the fire compartment. And it is evident that the compressive membrane force within the slabs is a major player in reducing the impact of concrete spalling on the structural behaviour of floor slabs in fire.     

火灾下钢结构楼板的薄膜作用

通过对真实火灾中的足尺火灾试验和观察显示，合组合楼板和承载钢梁的建筑物的结构承载力比现行杭大设计方法的建议值高出许多。因此规范中规定所有承载钢梁都要添加被动防火保护是不必要的。现行设计方法和实际结构性能之间产生这种差异是由于设计方法中忽略了楼板的薄膜作用。本根据国外有关资料给出了几种简单计算方法，允许在钢结构杭大设计中考虑楼板的薄膜作用。从而可以更精确地评估火灾下建筑物的真实承载能力，在给定的耐火时间内能减少相当数量钢梁的防火保护。     

Structural performance of a post-tensioned concrete floor during horizontally travelling fires

This paper highlights the structural performance of a bonded post-tensioned concrete floor subject to fires that travel horizontally between zones within the floorplate. The floorplate was previously analysed by the authors based on experimental and numerical investigations on one-way spanning bonded post-tensioned concrete slab strips. In the previous studies, a nonlinear finite element model was developed for the floor that considered the mechanical and thermal material nonlinearities of the floor’s components, interfaces between the components, different natural fire severities, different applied static load during the fire and different restraint conditions. The previous studies highlighted the importance of investigating the whole-building behaviour and provided a useful insight into the temperature distribution throughout the floor slab, failure modes, comparisons with current design rules and time-displacement behaviour of the floor under fire conditions. This paper extends the previous studies and uses the validated finite element model to investigate different horizontal travelling fire scenarios between zones and different inter-zone time delays to represent fire travelling time. The time-temperature distribution throughout the floor slab was predicted at different locations in the floor subject to travelling fires. Furthermore, the time-deflection and time-axial displacement relationships were predicted at different locations in the floor. The current study has shown that horizontally travelling fire scenarios and the inter-zone time delay affect the time-deflection behaviour considerably. The change in heating/cooling scenarios between zones has resulted in a cyclic deflection pattern, which has previously not been considered when designing post-tensioned concrete floors against fire. Based on the analysis of the results presented, it is shown that the worst case in terms of maximum vertical defection or maximum residual deflection, at a given point in the floorplate, could occur either under the assumption of a uniform fire or a travelling fire. It is therefore recommended that designers should consider the integrity of floorplates using various travelling fires.     

Motivation,drivers and barriers for a knowledge-based test environment in structural fire safety engineering science

Structural fire testing has traditionally relied on the use of the standard fire resistance test (i.e. furnace test) for assuring regulatory compliance of structural elements and assemblies, and in many cases also for developing the scientific understanding of structural response to fire. Conceived in the early 1900s and fundamentally unchanged since then, the standard testing procedure is characterized by its high cost and low repeatability. A novel test method, the Heat-Transfer Rate Inducing System (H-TRIS), resulting from a mental shift associated with controlling the thermal exposure not by temperature (e.g. temperature measured by thermocouples) but rather by the time-history of incident heat flux, was conceived, developed, and validated within the scope of the work presented in this paper. H-TRIS allows for experimental studies to be carried out with high repeatability, imposing rationally quantifiable thermal exposure, all at low economic and temporal cost. This works aims at demonstrating that a rational, and practical, understanding of the fire performance of structural systems during real fires is unlikely to be achieved only by performing additional standard fire resistance tests. Hence, H-TRIS presents an opportunity to help promote an industry-wide move away from the contemporary pass/fail and costly furnace testing environment.     

Mechanical response of a partially restrained column exposed to localised fires

Recent trends in structural fire engineering research have focussed on the response of buildings with large open plan spaces to so-called travelling fires. These fires travel horizontally across the floor plate of a building and result in time and spatially varying thermal exposure and response of the structure to the fire. What has received little attention, however, is the effect that non-uniform thermal exposure has on columns. Recent tests conducted at SP demonstrated the effect of a small non-uniformity of thermal exposure, resulting in a thermal gradient of around 1 °C/mm, on a column exposed to a pool fire. The curvature resulting from a non-uniform thermal exposure where the column is pinned, or in cases where the column is partially restrained, will result in an eccentricity in the column’s loading and large second order effects.This paper describes the effect of thermal exposure varying in both the horizontal and vertical axes to columns by means of including this thermal boundary in a solution of classical Euler beam theory. The resulting solution allows for a variation in the stiffness of the rotational restraint at both ends of the column and a non-uniform temperature exposure through the column’s section and along its height. The resulting equations help to understand better the impact of the assumptions of ‘lumped capacitance’ on steel columns, suggesting a challenge to this assumption in some instances, as well as to enhance understanding of the impact of non-uniform fires on steel columns.     

Temperature of connections during fire on steel framed building

To study the global structural and thermal behaviour of buildings in fire, a research project was conducted including a fullscale test on a three storey steel frame building at Mittal Steel Ostrava before demolition. The main goal of the experiment was to verify the method for predicting joint temperatures and to improve it for the cooling phase. The fire compartment extending over a floor area of 24 m² was built on the second floor. The fire load was 140 kg/m² of wood and the ventilation was limited to an opening of 1,400 × 1,970 mm. This paper presents the time-temperature curves showing the development of the fire in the compartment and in the primary and secondary beams and its header plate connections. Comparisons are made between the test results and the temperatures predicted by the structural Eurocodes. The sensitivity of the connection behaviour to the estimated temperatures and associated degradation in material properties during the fire is demonstrated.     

Wall fire behavior in an under-ventilated room

An experimental setup has been designed and built to study, at a laboratory scale, the behavior of a wall fire in a semi-confined compartment both in naturally ventilated and vitiated (combustion products) atmospheres.A diffusion flame is stabilized along a vertical porous flat burner located at the rear of an enclosure. The combustion is supplied by injection of propane through the vertical burner surface. Air enters into the compartment by natural convection through a door, topped by a soffit, opposite the burner. After reaching a thermal steady state, the temperature field in the compartment is characterized. Then, the door is closed leaving only a horizontal free slot (0.06 m height) between the top of the door and the bottom of the soffit. The flame behavior completely changes, the intensity of the spontaneous flame emission decreases drastically and a weakly blue vertical flame leaves the burner surface and moves, at low velocity, through the chamber, up to the open slot. Visualizations of the flame and measurements of the temperature and main stable chemical species fields are performed in order to characterize the behavior of a flame referred as a ghosting flame.This flame displacement mode has been already observed in full-scale fires by Audouin (Fifth International Symposium on Fire Safety Science, 1997, Melbourne, p. 1261–1272). After the initial “flame propagation”, combustion can be stabilized at the room aperture that participates to the development of the fire outside the compartment. This work contributes to a better understanding of this phenomenon in order to prevent such a fire.     

Theoretical Analysis and Experimental Study on Whirling Flames in Enclosure Fires

Whirling flames are not commonly observed in enclosure fires, but some special hazards will be induced for their characteristics once they occur. Employing the theories of vorticity transport and fire dynamics, the formation mechanism of whirling flames in enclosure fires has been analyzed, and the factor expression governing the formation of whirling flames occurring in single rooms with ceiling and lower openings, which are familiar to us in industrial buildings, has been deduced. Using a small-scale firebox with a lower door and a ceiling opening, the experiments of whirling flames in enclosure fires were carried out. Based on the experimental results, the characteristics of whirling flames in enclosure fires were studied, and the basic parameters of fires with and without whirling flame were compared. Combining the results of the theoretical analysis and experimental study, the formation criterion of swirling flames in enclosure fires were derived. Furthermore the reasons that whirling flames cause increases in some parameters, such as hot gas layer temperatures and floor radiant heat flux, were analyzed.     

Whole-building behaviour of bonded post-tensioned concrete floor plates exposed to fire

This paper discusses the whole-building behaviour of post-tensioned concrete floor plates under fire conditions. Based on the results of eight fire tests on one-way spanning bonded post-tensioned concrete slab strips, recently conducted by the authors, a nonlinear finite element model was developed to model an entire typical concrete floor plate. The considered floor plate was post-tensioned using bonded tendons and was supported on traditional reinforced concrete columns. The overall objective of the study was to provide an understanding of the structural response and modes of failure of these floor plates under fire conditions. The mechanical and thermal material nonlinearities of the floor’s components, consisting of the concrete, grout, prestressing tendon, and the anchorages, as well as the reinforced concrete columns, have been considered in the model. The interface between the tendon and grout was also considered, allowing the bond behaviour to be modelled and the tendon to retain its profile shape during the deformation of the floor. The model has been validated against published finite element results on a floor plate under normal temperature conditions. The temperature distribution throughout the floor slab and supporting columns, together with the developed displacement and stress due to heating, and the overall fire resistance of the floor were predicted by the model. Furthermore, the variables that influence the structural behaviour comprising different natural fires, applied static load during fire, use of non-tensioned reinforcement, as well the difference between unbraced and braced frames were investigated in a parametric study. The study has shown that the failure mode of the floor under fire conditions is mainly due to tensile splitting along the tendons that extended to the top surface, while at ambient conditions the mode of failure is punching shear. The restraint provided by shear walls in the considered braced frame and the use of non-tensioned flexural reinforcement affected the vertical displacement behaviour under fire conditions, but did not affect the fire resistance due to the predicted tensile splitting failure mode. From the studies presented it is concluded that the design fire resistance of the floor specified in Eurocode BSEN1992-1-2 is acceptable, while that in the UK code BS8110 is unconservative and should be modified.     

Experimental behaviour of a steel structure under natural fire

Current design codes for fire resistance of structures are based on isolated member tests subjected to standard fire conditions. Such tests do not reflect the behaviour of a complete building under either normal temperature or fire conditions. Many aspects of behaviour occur due to the interaction between members and cannot be predicted or observed in tests of isolated elements. Performance of real structures subject to real fires is often much better than that predicted from standard tests due to structural continuity and the provision of alternative load paths.This paper reports on the results of a collaborative research project (Tensile membrane action and robustness of structural steel joints under natural fire, European Community FP5 project HPRI—CV 5535) involving the following institutions: Czech Technical University (Czech Republic), University of Coimbra (Portugal), Slovak Technical University (Slovak Republic) and Building Research Establishment (United Kingdom). It consists of an experimental programme to investigate the global structural behaviour of a compartment on the 8-storey steel–concrete composite frame building at the Cardington laboratory during a BRE large-scale fire test, aimed at the examination of the temperature development within the various structural elements, the corresponding (dynamic) distribution of internal forces and the behaviour of the composite slab, beams, columns and connections.