Herrn Prof. Dr.‐Ing. Peter Schaumann zum 65. Geburtstag gewidmet
In DIN EN 1991‐1‐2:2010 1 ist aktuell ein vereinfachtes Verfahren enthalten, das die Berechnung von thermischen Einwirkungen von ...Bauteilen bei lokalen Bränden ermöglicht. Auf der Grundlage der Arbeiten von Heskestad und Hasemi kann für lokale Brände die Berechnung der Temperaturen im Feuer‐Plume und des Netto‐Wärmestroms im Bereich der brandbeanspruchten Bauteile im Deckenbereich (Ceiling Jet) vorgenommen werden. Das vereinfachte Verfahren wurde durch experimentelle Untersuchungen validiert und hat sich in der Anwendung bewährt. Im Zuge der Novellierung von EN 1991‐1‐2 Anhang C soll das vorgenannte Verfahren um neue analytische Bemessungsansätze erweitert werden. Der Vorschlag sieht vor, Netto‐Wärmeströme von brandbeanspruchten (vertikalen) Bauteilen, die in Bezug auf die Flammenachse des lokalen Brands versetzt angeordnet sind, zu berechnen. Im Rahmen eines DIBt‐Forschungsvorhabens 2 wurde die Plausibilität und Anwendbarkeit des erweiterten Bemessungsverfahrens geprüft. In diesem Beitrag werden die wesentlichen Vereinfachungen und die Anwendung des erweiterten Bemessungsverfahrens für Stahlstützen unter lokaler Brandbeanspruchung gezeigt. Ferner wird eine Validierung anhand von Vergleichsrechnungen zwischen den Ergebnissen des Bemessungsverfahrens, Simulationsergebnissen mit dem CFD‐Modell Fire Dynamics Simulator sowie anhand von Messdaten realer Pool‐ und Holzkrippenbrände vorgenommen.
Simplified design methods for steel columns under localised fire
DIN EN 1991‐1‐2:2010 1 currently contains a simplified design method that allows the calculation of thermal effects under localised fires. Based on the work of Heskestad and Hasemi, the temperatures in the fire plume and the net heat flux in the ceiling area (Ceiling Jet) of structural elements under localised fires can be calculated. The design method have been validated by experimental investigations and have proven themselves in application. In the Draft of EN 1991‐1‐2 Annex C, the mentioned design method are to be extended by new analytical design approach. The new design approach calculates the net heat flux of structural elements (vertical) which are offset to the flame axis of the localised fire. Within the scope of a DIBt research project 2 the plausibility and applicability of the new design approach was tested. In this article the simplifications and the application of the new design approach for steel columns under localised fire are shown. Furthermore, a validation is carried out on the basis of comparative calculations between the results of the new design approach, simulation results with the Fire Dynamics Simulator as well as measurement data of real pool and wood crib fires.
Existing design codes and guidelines for structures subject to normal loading are relatively mature and comprehensive, while the codified treatment of structural robustness for extreme loading has ...tended to be less rational and based on prescriptive rules. Although some codes already incorporate guidance for the assessment and design of structural robustness, this is not immediately applicable to the fire condition, and a considerable gap therefore exists between fire resistance and structural robustness research. This paper discusses key issues that should be addressed in the robustness assessment of steel-composite structures subject to localised fire, and proposes robustness assessment approaches that offer a practical framework for the consideration of such issues. Two alternative approaches are proposed within a design-oriented framework, namely, a temperature-dependent approach (TDA) and a temperature-independent approach (TIA). The TDA requires the definition of elevated temperature scenarios and can thus predict the fire response of structures more accurately. On the other hand, the TIA corresponds more closely to typical robustness provisions, which are intended to limit the progression of local damage under unforeseen events, and can be more easily applied in design practice. While the TIA is more practical than the TDA, it is shown that its application can be overly conservative under specific conditions. Illustrative examples are presented, which demonstrate the application of the proposed robustness assessment framework for steel-composite buildings subject to localised fire.
► Development of robustness assessment framework for buildings subject to localised fire. ► Proposal of temperature dependent (TDA) and temperature independent (TIA) approaches. ► Demonstration that conventional failure criterion for columns under fire is conservative. ► Investigation of cases where upper ambient floors may provide an alternative load path. ► Illustration of the benefits of the simplified TIA assessment approach.
Composite joints under M-N at elevated temperatures Haremza, Cécile; Santiago, Aldina; Demonceau, Jean-François ...
Journal of constructional steel research,
09/2016, Letnik:
124
Journal Article, Web Resource
Recenzirano
The Eurocodes recognise robustness as a way to ensure the structural integrity of a building frame subjected to an unforeseen event and therefore to avoid a so-called “progressive failure” mode in ...extreme loading situations. However, few practical guidelines exist nowadays which would allow a designer to design a structure accordingly. Within the European RFCS ROBUSTFIRE project, the behaviour of steel and composite car parks subjected to localised fire leading to a column loss was investigated. Under such a scenario, the beam-to-column joints play a key role in the global structural response. Indeed, these joints, initially loaded in bending, may be subjected to elevated temperatures and to combined axial load “N”, bending moment “M” and shear forces “V”. In this paper, a methodology to predict the mechanical response of bolted composite beam-to-column joints at elevated temperatures under M-N is presented and validated through comparison with detailed numerical results and experimental tests. This methodology is based on an analytical method able to predict M-N resistance interaction curves for steel and composite joints and which is in full agreement with the component model recommended by the Eurocode.
•Behaviour of steel and composite car parks subjected to localised fire leading to a column loss was investigated.•Methodology to predict the response of bolted composite beam-to-column joints at elevated temperatures under M-N.•Validation through comparison with detailed numerical results and experimental tests.•Methodology in full agreement with the component model recommended by the Eurocode.
Localised fires are an important accidental scenario for the offshore industry due to the extent of their potential damage. Its consequences may be localised damage, weakening of structural elements, ...blast when a storage tank reaches its critical temperature, or the structure's progressive collapse. The correct sizing of the fire protection (passive and/or active) system allows reducing or delaying the potential structure's damage. This article evaluates the use of the LF-ESF methodology at the design stage to estimate the efficiency of the passive fire protection layer (PFP) in upper offshore structures exposed to localised fires. Fire conditions are assumed to result from a typical process involving the combustion of hydrocarbons. Three fire scenarios are evaluated using the LF-ESF model proposed by the authors and based on updates of simple methodologies widely used in the literature. In order to verify its accuracy, its results are compared with a model developed in a CFD-based package for a fire scenario. Thermo-mechanical analysis is performed employing a finite element model that considers the temperature-dependent physical and geometric non-linearities. The estimated thermal load is entered into the thermo-mechanical model in combination with the pre-existing operational loads to evaluate the structure's behaviour. Despite the slight overestimation of the thermal field from the LF-ESF model, compared to the CFD-FEM methodology, the results obtained allow the selection of the PFP without being too conservative and at low computational cost.
Abstract
Herrn Prof. Dr.‐Ing. Peter Schaumann zum 65. Geburtstag gewidmet
In DIN EN 1991‐1‐2:2010 1 ist aktuell ein vereinfachtes Verfahren enthalten, das die Berechnung von thermischen Einwirkungen ...von Bauteilen bei lokalen Bränden ermöglicht. Auf der Grundlage der Arbeiten von
Heskestad
und
Hasemi
kann für lokale Brände die Berechnung der Temperaturen im Feuer‐Plume und des Netto‐Wärmestroms im Bereich der brandbeanspruchten Bauteile im Deckenbereich (Ceiling Jet) vorgenommen werden. Das vereinfachte Verfahren wurde durch experimentelle Untersuchungen validiert und hat sich in der Anwendung bewährt. Im Zuge der Novellierung von EN 1991‐1‐2 Anhang C soll das vorgenannte Verfahren um neue analytische Bemessungsansätze erweitert werden. Der Vorschlag sieht vor, Netto‐Wärmeströme von brandbeanspruchten (vertikalen) Bauteilen, die in Bezug auf die Flammenachse des lokalen Brands versetzt angeordnet sind, zu berechnen. Im Rahmen eines DIBt‐Forschungsvorhabens 2 wurde die Plausibilität und Anwendbarkeit des erweiterten Bemessungsverfahrens geprüft. In diesem Beitrag werden die wesentlichen Vereinfachungen und die Anwendung des erweiterten Bemessungsverfahrens für Stahlstützen unter lokaler Brandbeanspruchung gezeigt. Ferner wird eine Validierung anhand von Vergleichsrechnungen zwischen den Ergebnissen des Bemessungsverfahrens, Simulationsergebnissen mit dem CFD‐Modell Fire Dynamics Simulator sowie anhand von Messdaten realer Pool‐ und Holzkrippenbrände vorgenommen.
Abstract
Simplified design methods for steel columns under localised fire
DIN EN 1991‐1‐2:2010 1 currently contains a simplified design method that allows the calculation of thermal effects under localised fires. Based on the work of
Heskestad
and
Hasemi
, the temperatures in the fire plume and the net heat flux in the ceiling area (Ceiling Jet) of structural elements under localised fires can be calculated. The design method have been validated by experimental investigations and have proven themselves in application. In the Draft of EN 1991‐1‐2 Annex C, the mentioned design method are to be extended by new analytical design approach. The new design approach calculates the net heat flux of structural elements (vertical) which are offset to the flame axis of the localised fire. Within the scope of a DIBt research project 2 the plausibility and applicability of the new design approach was tested. In this article the simplifications and the application of the new design approach for steel columns under localised fire are shown. Furthermore, a validation is carried out on the basis of comparative calculations between the results of the new design approach, simulation results with the Fire Dynamics Simulator as well as measurement data of real pool and wood crib fires.
► Seven experimental fire tests were performed on a beam-to-column sub-frame. ► The heated joints were subject to combined bending moment and axial loads. ► The sub-frame showed sufficient robustness ...provision thanks to the ductility of the joint. ► A well designed joint is able to behave in a ductile manner under fire conditions.
This paper presents the results of seven experimental tests performed at the University of Coimbra on a two dimensional composite steel–concrete beam-to-column sub-frame. These experimental tests were part of a global study of the robustness of composite steel–concrete open car park buildings under localised fire. The tested sub-frame was extracted from a composite open car park building, using the real cross-section dimensions of the beams (IPE 550) and the columns (HEB 300), and using bolts M30 grade 10.9 in the composite connection, and was subject to mechanical (bending and axial forces) and thermal loadings (constant temperature equal to 20°C, 500°C or 700°C; or linear increase up to 800°C). The interaction between the bending moment and axial load in the heated internal joint, after the loss of the column, is the objective of the tests. In order to reach this goal, the effect of the axial restraint to the beam was simulated: three tests were performed without any restraint to the beam, two tests with total axial restraint to the beam and two tests with a realistic axial restraint to the beam (spring restraint). The results of the seven tests are analysed and the behaviour of the joint is investigated and discussed in detail.
In fires in large compartments like enclosed car parks, airport terminals and industrial halls, the uniform distribution of gas temperature of post-flashover stages are unlikely to occur; in these ...cases, the thermal actions of a localised fire must be taken into account. In order to design steel structures for a localised fire, very detailed data concerning the development of temperatures in steel is required. EN 1991-1-2 presents a simplified model for calculating the temperatures in ceiling slabs and in the beams that may support such slabs; however, no simplified calculation model for the heat transfer in vertical elements, such as columns, is yet available. There is a need for more experimental data on real scale structures exposed to localised fires. A research project on the evaluation of temperatures in steel columns exposed to localised fires was carried out at the University of Coimbra. Full-scale natural fire tests were used to test columns, instead of conducting the usual furnace tests. This paper presents and discusses the results of the experimental tests on unprotected hollow steel columns exposed to localised fires, each of them simulating a distinct fire scenario according to different fire loads, positions and ventilation conditions. During the fire tests, real measurements showed flame heights and burning times different to those preliminarily estimated: flame heights had been conservatively predicted; while, the duration of the burning had been significantly underestimated.
In terms of developing knowledge to enable more effective use of performance based engineering (PBE), one of the key limitations is the lack of an easy to use integrated computational tool that is ...also robust and comprehensive enough to enable automated modelling of more realistic fire scenarios, i.e., the structural response to localised or travelling fires. The main objective of this thesis is to establish such an integrated computational tool, which shall be based on the OpenSees software framework and facilitated by specially developed approaches to achieve higher efficiency of the integrated analysis. This includes the analysis of heat transfer from the fire to structural members, as well as the analysis of structural response to elevated temperatures during the fire. In this thesis, the research begins with the investigation of the feasibility of dimensional reduction for heat transfer analyses of structural members subjected to localised fire action (SFPE and Eurocode 1 fire models), which can be numerically represented by a linear or exponential correlation between incident heat flux and radial distance. Accurate estimates of the error induced by dimensional reduction are presented under strongly varying localised heat fluxes that represent the most non-uniform fire conditions in a building compartment. It is shown that beams and slabs can be adequately modelled with a lower dimensional heat transfer analysis for ordinary building fires. Using this approach, the complexity of heat transfer modelling and the required computing resource and user effort can both be significantly reduced, especially in cases where structural members are subjected to localised fire action. Thermo-mechanical simulations are presented to address the behaviour of structural members subjected to localised fire action, for which a ThermalAction- Wrapper is developed to approximate the temperature distribution from a mixed-order interpolation between sections (beam) or locations (slab). For concrete slabs subjected to localised fire, MITC4 based shell elements are used to account for material and geometric nonlinearities. An integrated simulation environment is developed, which is designed to be a computational tool that requires limited input but provides a comprehensive solution to the problem of simulating large structural frame and sub-frame response under realistic fire scenarios. A considerable amount of code has been written to create and operate the building model, and to process the heat fluxes from the design fires to the structure and the consequential structural response to the evolution of temperatures within it. Parametric studies have been performed to investigate the computational performance of the newly developed elements in modelling beams and slabs subjected to different cases of localised fire action. The results suggest that 3 to 6 force-based beam elements can adequately describe the localised response however more elements are required for quadratic distribution of incident heat flux and higher temperatures, which is due to the degradation of material strength that governs the accuracy especially when the members are heavily loaded. For slabs exposed to localised fires, centre fires are found to produce greater deflections than corner fires, while lateral restraints applied to the slabs may also lead to higher deflections. A small-scale three dimensional structural frame is modelled as a demonstration of the tool, tested against a number of localised fire scenarios. The global behaviour of the structure with the local effects induced by the fire action and partially damaged fire protection are investigated. Severe damage can be found in the members exposed to a single whole compartment fire, in contrast with the relatively small deflections that are observed when a fully protected column is engulfed by a localised fire. However if the passive fire protection is partially damaged, collapse may occur in the column as a result of load magnification because of the redistribution. To the author's knowledge this is the first piece of research that has been able to develop a practically feasible approach to enable efficient coupled computation of the response of structural frames to realistic fire scenarios on a freely available open source software platform. Currently this kind of analysis can only be carried out by just two or three large consulting firms because of the prohibitive commitment of analyst time and effort and to a lesser extent the need for significant computing resources. The work of this thesis will contribute enormously towards making high-end performance based engineering of structural fire resistance a much more practical proposition for small and medium size structural consultancies. Furthermore, the choice of OpenSees, which is a very well respected software framework for simulating structural response to earthquakes naturally enables this work to be extended to the simulating the multi-hazard structural resistance, such as in the event of a fire following an earthquake which may have locally damaged passive fire protection.
The methodology implemented in the tool OZone V2 to design steel elements submitted to compartment fires is presented. Input needed to define a fire compartment are first quoted. The procedure to ...define the design fire is then explained. This procedure enables to take into account the benefits of active measures on the fire safety. The combined use of a two- and a one-zone model is then presented. A particular attention is given to the criteria of choice of the model and to the different scenarios that can occur. The calculation of steel element temperature is then explained. The proposed procedure takes into account the localised effect of a fire with the help of Hasemi's model. The fire resistance is then calculated with the EUROCODE 3 methods. An application is finally presented.
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