Structural Analysis of Multi-Storey Steel Frames Exposed to Travelling Fire & Traditional Design Fires

Innovative architectural designs of new high rise structures pose previously unthought-of challenges to architects and engineers. For the design of buildings against fire the European building design code (Eurocode EN1991-1-2) considers a standard fire and parametric time-temperature curves. Standard fires are based on tests in small enclosures and thus have limitations on their applicability to large enclosures (>100 m2). In addition, the standard fire curves and parametric time-temperature curves are based on the unrealistic assumption of uniform fire conditions in a compartment. While this assumption may be suitable for small enclosures, fires in large open-plan compartments have been observed to travel resulting in highly non-uniform temperature distributions within the enclosure. Examples of such accidental events include the World Trade Centre Buildings 1, 2 & 7 (2001) and Windsor Tower fire in Madrid (2006) and One Meridian Plaza fire in Philadelphia (1991). In all of these accidents, the fires lasted for up to 7 or even 20 hours. Long fire durations like these are not considered in the current design codes. Non-uniform temperature distributions on structures have been shown to have a significant impact on the failure mechanism and time to failure. In studies on steel, reinforced concrete and post-tensioned concrete frames where a travelling fire was considered, large distortions and a cyclic behaviour of stresses and deflection patterns have been observed. However, to represent a travelling fire the studies, parametric curves were used and shifted from one bay to another after an arbitrary time. As noted by Stern-Gottfried and Rein such representation ignores the basics of fire spread, preheating of structural elements by hot smoke and spatially varying temperatures within the compartment. Recently, to account for travelling fires a design tool called Travelling Fires Methodology (TFM), has been developed. It considers a family of fires which depend on the fire spread rates and thus the percentage of the total floor area engulfed in flames. In TFM fire is divided into two regions near-field and far-field. Near-field represents the area where flames are directly impinging on the ceiling and far-field represents radially decreasing smoke temperatures further away from the fire. Therefore, TFM unlike parametric fires considers more realistic non-uniform temperature distributions in the compartment.

researchers about topic which will help us to decide about the subject, is as given below. A number of researchers have made important contributions to the development of our understanding of the experimental studies on travelling fires. Egle Rackauskaite, Panagiotis Kotsovinos, Ann Jeffers, Guillermo Rein (July2017)1 The aim of the study was to compare computationally the structural response of multi-storey steel frame subjected to both uniform design fires and travelling fires. A two-dimensional 10-storey 5-bay steel frame designed according to ASCE 7-02 was modelled in the general finite element program LS-DYNA. Different fire exposures are investigated. They included travelling fires, Eurocode parametric curves, ISO-834 standard fire and the constant compartment temperature curve from the SFPE standard. These fires were applied to different floors, one at a time, to explore the influence on the structural response, resulted in 80 different scenarios. The development of deflections, axial forces and bending moment was analysed. Uniform fires were found to result in approx.15-55 KN (3-13%) higher compressive axial forces in beams compared to small travelling fires. However, the results show irregular oscillations in member utilization levels in the range of 2-38% for the smallest travelling fire sizes, which were not observed for any of the uniform fires and depend mainly on the fire duration, but the locations in the frame and times when these peak displacements occur were different. The results indicated that travelling fires and uniform fires trigger substantially different structural responses which may be important in the structural design and selection of the critical members.

Chao Zhang, John L. Gross, Therese P. McAllister (July 2017)3 The paper investigates the effect of temperature gradients on the lateral torsional buckling behavior of steel wide flange(W) beams in fire conditions. The effects of localized fires and the temperature gradients they produce in steel beams were studied. Laterally unstrained beams of various uniform temperature condition in which the steel temperature was ramped linearly were used for comparison. The study shows that temperature gradients within a steel W-beam may have a detrimental effect on the lateral torsional buckling capacity of beams in fire. Angus Law, Jamie Stern-Gottfried, Martin Gillie, Guillermo Rein (March 2011)4 The paper applies a novel methodology for defining a family of possible heating regimes to a framed concrete structure using the travelling fires. A finite element model of generic concrete structure is used to study the impact of the family of fires; both relative to one another and in comparison to a 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 most severe in terms of structural response. It was concluded that the new approach is simple to implement, provides more realistic fire scenarios, and was more conservative than current design methods. Chao Zhang, Guo Qiang Li, Asif Usmani (March 2013)5 The paper numerically investigates the thermal and mechanical behaviours of restrained steel beams exposed to flame impingement from localized fires. Four steel beams with different dimensions and restraints were considered. Both developing and steady burning fires were investigated. The standard ISO 834 fire was also used for comparison. This study found that the temperature distributions within the steel beams subjected to flame impingement are highly non-uniform both across and along the beams. Along the beam length, the temperatures near the fire source may be hundreds of degrees higher than those far from the fire source. Due to different temperature distributions, the deformation mode for restrained steel beam subjected to a standard ISO 834 fire. The failure temperatures for restrained steel beams subjected to localized fires may be higher or lower than those for restrained beams subjected to the standard ISO 834 fire. Reliance on the standard fire may lead to an unconservative design if the potential real fires were localized fires.

Egle Rackauskaite, Catherine Hamel, Angus Law, Guillermo Rein (June 2015)8 In this study, TFM has been improved to account for better fire dynamics. Equations are introduced to reduce the range of possible fire sizes taking into account fire spread rates from real fires. It was found to be mainly dependent on the fire spread rate and the heat release rate. Location of the peak temperature in the compartment mostly occurs towards the end of the fire path.

This paper describes the methodology of an advanced analysis technique for studying the large-displacement inelastic behaviour of building frames subjected to localised fire. The main feature of the proposed analysis is its use of one element per member to model each structural component and thereby obtain a realistic representation of material and geometrical nonlinear behaviour of the overall framework. Both ISO fire and natural fires can be simulated by the analysis model. The transient heat transfer is calculated by using a refined finite element mesh, whereas the structural responses are calculated with a nonlinear finite element technique based on an elasto-plastic beam–column formulation. Numerical studies are carried out on portal frames and multi-storey frames exposed to natural fires. The computed results are compared and contrasted with those from the conventional approach based on ISO standard fire; so that the advantage of using advanced analysis for direct assessment of the performance of steel structures in fire can be highlighted. Effects of fire load and ventilation resulting in localised fires in building frames are studied. The design implication of fire and heating are taking place simultaneously is discussed.

This paper describes the results of finite element analyses on a small generic composite steel and lightweight concrete frame. It compares the structural behaviour of the frame during two different single floor compartment fires. The fire scenarios are modelled using Pettersson‘s (Fire Engineering design of Steel Structures, Swedish Institute of Steel Construction, Publication 50, Stockholm, 1976) post-flashover temperature-time curves assuming a fire load density typical of offices and opening factors of 0.02 and 0:08 m1=2: With an opening factor of 0:08 m1=2 the model fire is characterised by high temperatures but a relatively short post-flashover duration (‗‗short-hot fire‘‘). In contrast an opening factor of 0:02 m1=2 provides less ventilation leading to a post-flashover fire with lower maximum atmosphere temperatures but a longer post-flashover duration (‗‗long-cool‘‘ fire). The two fire scenarios create contrasting structural behaviour because the duration of the fire dictates the gradient through the depth of the composite slab. In the ‗‗short-hot‘‘ fire the steel beams achieve high temperatures but only the exposed face of the concrete begins to respond to heating. The rest of the slab depth stays cool. In the ‗‗long-cool‘‘ fire an extended post-flashover duration allows heat to penetrate further through the thickness of the slab. The slab has a higher mean temperature, thus the gradient through the composite is lower. During the analyses the columns and edge beams were protected to provide 60 min fire resistance, primary and secondary beams were unprotected.

This review outlines some of the major developments that have occurred in structural fire engineering design during recent years. The general principles used in fire engineering are discussed together with the various approaches that can be adopted by an architect or engineer to design a structure capable of withstanding the attack of fire without collapsing, using scientific concepts as a basis for determining the fire resistance and protection requirements. Details are provided of recent full-scale tests and research work either completed or in hand with the aim of producing design concepts and information which the fire engineer may use. Examples are discussed to show how structural fire engineering has been utilised in both large and small buildings to precisely define the fire resistance and protection requirements of steel frameworks. In some instances, estimates of the

This paper demonstrates the need for structural fire engineering in the design and specification of unusual structural arrangements. It was found that the required fire resistance of a structure cannot necessarily be achieved by considering only the fire resistance of individual elements of structure. A case study of columns that have a change of direction over their height (geometrically bi-linear columns) was used to illustrate how unusual arrangements may lead to early failure of key structural elements even where design guidance was followed. These arrangements were parameterised to allow the relative importance of a number of key variables to be studied. A methodology was developed to estimate the required limiting temperature for the governing elements of the assembly in order to deliver the required performance.

In this study, a performance based approach was developed for assessing the fire resistance of restrained beams. The proposed approach, based on equilibrium and compatibility principles, took into consideration the influence of many factors including fire scenario, end restraints, connection configuration (location of axial restraint force), thermal gradient, load level, beam geometry, and failure criteria in evaluating fire resistance. The validity of the approach was established by comparing the predictions from the proposed approach with results obtained from rigorous finite element analysis. The applicability and rationality of the proposed approach to practical design situations was illustrated through a numerical example. This paper presents theoretical descriptions of the key phenomena that govern the behaviour of composite framed structures in fire. These descriptions have been developed in parallel with large scale computational work undertaken as a part of a research project (The DETR-PIT Project, Behaviour of steel framed structures under fire conditions) to model the full-scale fire tests on a composite steel framed structure at Cardington (UK). Behaviour of composite structures in fire has long been understood to be dominated by the effects of strength loss caused by thermal degradation, and that large deflections and runaway resulting from the action of imposed loading on a ‗weakened‘ structure. Thus ‗strength‘ and ‗loads‘ are quite generally believed to be the key factors determining structural response (fundamentally no different from ambient behaviour). The new understanding produced from the aforementioned project is that, composite framed structures of the type tested at Cardington possess enormous reserves of strength through adopting large displacement configurations. Furthermore, it is the thermally induced forces and displacements, and not material degradation that govern the structural response in fire. Degradation (such as steel yielding and buckling) can even be helpful in developing the large displacement load carrying modes safely. This, of course, is only true until just before failure when material degradation and loads begin to dominate the behaviour once again. However, because no clear failures of composite structures such as the Cardington frame have been seen, it is not clear how far these structures are from failure in a given fire. This paper attempts to lay down some of the most important and fundamental principles that govern the behaviour of composite frame structures in fire in a simple and comprehensible manner. This is based upon the analysis of the response of single structural elements under a combination of thermal actions and end restraints representing the surrounding structure.

In this study when steel beams exposed to fire develop significant restraint forces and often behave as beam-columns. The response of such restrained steel beams under fire depends on many factors including fire scenario, load level, degree of restraint at the supports, and high-temperature properties of steel. A set of numerical studies, using finite element computer program ANSYS, is carried out to study the fire response of steel beam-columns under realistic fire, load and restraint scenarios. The finite element model was validated against experimental data. The results from parametric studies indicate that fire scenario, load level, degree of restraint at the supports, and high-temperature creep had

fire conditions.

This paper presents the results of a numerical study, using ABAQUS, of the large deflection behaviour of steel beams at elevated temperatures with different elastic axial and rotational restraints at the ends. A particular emphasis of this paper is the behaviour of axially restrained steel beams in catenary action. Although studies in the area of structural fire engineering has moved beyond the consideration of isolated elements to structural interactions in complete frames, catenary action in steel beams is an important aspect of structural interactions and is receiving considerable attention from researchers. After validating the capability of ABAQUS against available experimental results of fire tests on restrained steel beams, this paper describes the results of a numerical parametric study. The parameters investigated include beam span, uniform and non-uniform temperature distributions, different levels of applied load, and different levels of axial and rotational spring stiffness at the beam ends and the effect of lateral torsional buckling. It is concluded that the large deflection behaviour of steel beams can significantly affect their survival temperature in fire. Indeed, if a realistic amount of axial restraint stiffness is available at the beam ends and fire engineering design is not concerned with the amount of large deflection in a beam, it is possible that the beam can have virtually unlimited survival temperature. Clearly, fire engineering design will need to consider the effect of axial forces in the beam on the adjacent structure, including the connections.

In this study, structural response of a seismically designed steel moment-resisting frame subjected to travelling fire is investigated. This is to determine the structural strength of a generic frame designed for an extreme load when subjected to fire as another extreme load in addition to quantifying the effect of travelling fire size on its collapse behaviour. In this study, using the concept of travelling fire, and calculating the thermal field applied to structural elements, a generic frame was examined against a family of fires travelling across its first floor. In this regard, the resolved far-field gas temperatures dependent on the distance to the centre of fire were considered in order to calculate the temperature at the unprotected steel members. Analysis results revealed that fire size can deeply affect the total collapse time of a frame so that by reducing the fire size to a half or a respectively. It was also suggested that columns of such structures should be designed against travelling fire considering the effect of load redistribution by which axial forces of columns might be doubled compared to the nominal loads applied to them prior to fire.

In this paper, it is shown that some of the distortions caused by the fire are increased by progressive fire spread, as compared with simultaneous burning across the same range of compartments.

The Travelling Fires Methodology (TFM) framework (Stern-Gottfried & Rein 2012 Stern- Gottfried & Rein 2012) aims to incorporate the effect of non-uniform fires in large open-plan spaces. It does not supersede traditional design methods, but can be used in addition to them, and provides a range of possible fire dynamics. TFM provides an approach for generation of gas temperature-time curves at the ceiling at any location in the compartment. It considers design fires to be composed of two moving regions: the near field (flames) and the far-field (smoke). The near field represents the flames directly impinging on the ceiling and assumes the peak flame temperatures. The far-field model represents cooler smoke temperatures which decrease with radial distance away from the fire due to heat transfer and incremental mixing of hot gases with fresh air. Any structural element will experience cool far-field temperatures which correspond to pre-heating or cooling for much longer duration than the short hot near field.

TFM is flexible in a way that, unlike Eurocodes, it is not limited to one fire type. It covers a wide range of fire sizes depending on the coverage of the total floor area from 1% to 100%. The latter represents the whole compartment under uniform fire. The total fire duration depends on the fire size (Stern-Gottfried & Rein 2012). However, it is unlikely that a very thin line fire across the whole width of the compartment or a whole floor fire (e.g. 2500 m2 ) in large compartments will occur. The reason for that may be the physical fire limitations such as available fuel load, fire spread rate and burning rate. The aim in the present study is to provide a more realistic

passed to structural analysis, reduce computational time and neglect unrealistic results. In the methodology a fire is assumed to be fuel controlled. Therefore, fuel load density and heat release rate are used as main design variables. Based on these values a local burning time needed for an area involved in fire to burn out completely is calculated (e.g. 19min) in Eq1. Local burning time multiplied by fire spread rate gives the distance the trailing edge of the fire would travel until the burn out at the ignition point. If the limits of the realistic fire spread rates (i.e. trailing edge) are known these can be used to compute the limiting sizes of possible fires as in Eq2: tb = qf/Q‘‘ ...Eq1 Lf,min/max = Smin/max X tb ...Eq2 Where, tb = local burning time (s) qf = fuel load density (KJ /m2) Q‘‘ = heat release rate (KW /m2) Lf,min/ma =minimum and maximum possible fire size in terms of length along the fire path (m) Smin/max = minimum and maximum realistic fire spread rates in building fires (m/s)

1. New approach towards the design gives precise building behaviour exposed to travelling fires. 2. Fundamental to the new approach is the idea of the travelling fire that has consistently been observed in real accidental fires. Also key to this approach is the requirement for the fire engineer and the structural fire engineer to work together to create the most appropriate design fire. 3. In the context of finite-element modelling, this relies on a relatively small proportion of the available information about the structure. For example, temperatures at certain locations are used to diagnose the performance of the entire improvements can be made.

For concrete structures, the temperature of the tension reinforcement is often used as an indication of failure. In concrete structures or slabs, the rupture strain in steel reinforcement is often used as a basis for a failure criterion. Typically this measure is better suited to the numerical analysis of structures rather than fire tests because of the difficulties associated with instrumentation of rebar.

Travelling Fires Methodology has been developed to account for the travelling nature of fires. This indicate that in order to ensure a safe building design a range of different fires including both travelling fires and Eurocode fires need to be considered. While uniform fires might lead to higher axial forces travelling fires might result in larger displacements and either of them can lead to failure.

Angus Law, Jamie Stern-Gottfried, Martin Gillie, Guillermo Rein, (2011). ―The influence of travelling fires on concrete frame‖.

Applications in Structural Fire Engineering Design -- A Review‖, C.G. Bailey, I. W. Burgess & R. J. Plank, (1996) ―Analyses of the Effects of Cooling and Fire Spread on Steel-framed Buildings‖, Chao Zhang, Guo Qiang Li, Asif Usmani, (2013). ―Simulating the behaviour of restrained steel beams to flame impingement from localized fires‖. Chao Zhang, John L. Gross, Therese P. McAllister, (2013). ―Lateral torsional buckling of steel W beams subjected to localized fires‖. E. Ellobody, C.G.Bailey, (2011). ―Structural performance of a post tensioned concrete floor during horizontally travelling fires‖. Egle Rackauskaite, Catherine Hamel, Angus Law, Guillermo Rein, (2015) ―Improved formation of travelling fires and Application to concrete and steel structures‖,. Egle Rackauskaite, Panagiotis Kotsovinos, Ann Jeffers, Guillermo Rein, (2017). ―Structural analysis of multi-storey steel frame exposed to travelling fire and traditional design fires‖.

Farshad Hashemi Rezvani, Hamid R. Ronagh, (2016) ―Structural response of a MRF exposed to travelling fire‖,. J.Y. Richard Liewa, L.K. Tanga, Tore Holmaasb, Y.S. Choo, (1998). ―Advanced analysis for the assessment of steel frames in fire‖, Jamie Stern-Gottfried, Guillermo Rein, (2012). ―Travelling fires for structural design-Part 1 Literature review‖. Jamie Stern-Gottfried, Guillermo Rein, (2012). ―Travelling fires for structural design-Part 2 Design methodology‖.

of a small composite steel frame structure in a ‗‗long-cool‘‘ and a ‗‗short-hot‘‘ fire‖,

PG Research Scholar, Civil Engineering Department, JSPM‘s ICOER, Wagholi, Pune, Maharashtra, India