Determination of the steel fire protection material thickness by an analytical process—a simple derivation

Structural fire safety is assured if the design value of the effect of the actions (thermal and mechanical) is lower than the design value of each structural element fire resistance or, in other words, structural safety is assured when the steel temperature in a fire situation only reaches values less than the structure critical temperature. The critical temperature is the temperature that causes structural collapse in a fire situation. The temperature in the structural element can be determined either experimentally or analytically. In the case of a structure covered with thermally protective material, such methods serve, in practice, to determine the thickness of the protective material. In this work, a previously unpublished expression for the calculation of the temperature in thermally protected steel structural elements in fire is derived. Comparisons with international recommendations and with experimental and numerical analysis results are made. In view of its simple form and derivation, the use of such expression is recommended for the revision of the Brazilian Standard “Steel structures fire design”.     

A group-AHP decision analysis for the selection of applied fire protection to steel structures

Decisions related to selecting the most suitable fire protection for steel structures subject to fully developed fires are critical to addressing design and construction uncertainties. Fire design stakeholders are faced with the challenge of managing their divergent opinions to make design decisions given the many options or engineered solutions available to meet performance objectives. This paper demonstrates the viability of a group-Analytic Hierarchy Process (AHP) as a multi-criteria decision analysis (MCDA) technique for managing fire design stakeholder opinions on economic, safety, environmental and societal considerations toward selecting suitable fire protection for steel structures. Based on 22 structural fire design decision criteria gathered from literature and expert opinion, 36 New Zealand stakeholders from 12 fire design stakeholder categories have been interviewed to elicit ratings from their relative comparisons of the structural fire design decision criteria. The Geometric Mean Method-Analytic Hierarchy Process (GMM-AHP) is used to assess four applied fire protection options or alternatively to use unprotected steel. The results show the seamless aggregation of multiple stakeholder desires, the importance levels of different stakeholder opinions and the systematic approach in ranking the proposed fire protection options. The ranking shows that given a general selection of passive fire protection by stakeholders having equal weights, there is a stronger preference for the concrete encasement of steel.     

Composite steel-framed structures in fire with protected and unprotected edge beams

The structural behaviour of a steel-concrete composite frame subject to a natural fire is analysed using a numerical model. The behaviour is compared when fire protection is applied to only the external beams and when no beams are fire protected. The behaviour of the structure in the two cases is significantly different. When the edge beams are protected the floor slab tends to span in 2 directions because the edge beams provide sufficient support around the perimeter of the floor for tensile membrane action to develop. When the edge beams are unprotected the slab tends to span in only one direction in a manner similar to a beam in catenary action. Catenary action is a weaker load carrying mechanism than tensile membrane action. As a consequence of the weaker mechanism, when the edge beams are unprotected, the columns displace inwards towards the end of the fire indicating the possibility of imminent runaway collapse.The pattern of mechanical strains in the floor slab reinforcement depends on the load carrying mechanism and therefore on whether edge beam protection is included. Although the average mechanical tensile strains are higher when the edge beams are protected the highest strains occur when the beams are unprotected. Conversely, an instability in the primary beam occurs at much lower temperatures when the edge beams are protected.It is concluded that fire protecting the edge beams of the structural layout studied has a number of effects on the fire resistance of the structure, some beneficial, some detrimental, however, in general, fire protecting the edge beams provides an increased level of fire resistance.     

火灾中有保护和无保护边梁的组合钢结构框架

采用数字模型分析了钢混组合框架在火灾中的结构性能。两种情况下结构的性能是显著不同的。对只带防火保护的外梁与没有任何梁受保护两种情况的结构进行了比较,当边梁被保护时,底板趋向两个方向延伸,因为边梁对板周界应力的发展提供了足够的支撑;当边梁没有被保护时,板只朝一个方向延伸,其受力方式类似于悬链线。与拉伸膜相比悬链线梁为较弱的荷载载运机制。这种弱机制的结果是当边梁无保护时,柱位移向内预示出临近坍塌的可能性。混凝土楼板的应变模式取决于其承载机理,因此与边梁是否保护有关。当边梁有保护时尽管平均拉应变较高,但是最高拉应变都发生在边梁无保护时。反之,当边梁有保护时,在很低的温度,原梁就失稳。研究结论是边梁的结构布局在结构防火等方面具有一定的效果,有些有利,有些有害,然而,一般情况下,有保护边梁可以提供较高的防火等级。     

Temperatures during fire tests on structure and its prediction according to Eurocodes

The paper reports on an experimental programme to investigate the global structural behaviour of a compartment in the three-storey steel frame building in a plant of the Mittal Steel Ostrava exposed to fire before demolition. The research project of the Czech Technical University in Prague was focussed on the examination of the temperature development within the various unprotected structural elements and its connections, the corresponding distribution of horizontal forces and the behaviour of the laterally unrestrained beams during the natural fire. The experiment also allowed studying of the heating of external elements, the influence of connection in a wall of sandwich panels, the temperature development in light timber-based panel and the degradation of the timber concrete composite element. Before the compartment fire, a local fire was prepared to verify the models of the temperature development in an unprotected column. The comparisons to the simplified calculations by European standards are included in the text to show their strong and weak points in prediction of temperatures of gas and structural elements during fire.     

Sonderlösungen bei der brandschutztechnischen Bewertung von Konstruktionsdetails

Special‐purpose solutions in the fire‐protective evaluation of construction details Very often components or construction details in existing buildings but also in new buildings cannot be evaluated with regard to its fire resistance ability considering established technical building regulations. There are no regulations for the evaluation of such components or construction details as these specific construction details have not been evaluated by a certified test authority. However, these components and construction details very often can be classified in fire resistance classes e. g. by evaluating fire tests, with regard to DIN 4102‐4, on the basis of similar proof of usability or only by pragmatic considerations. Since introducing the technical approval of the fire protection Eurocodes more often engineering methods of fire protection are used based on these Eurocodes by using temperature assessment in order to secure the evaluation results. Within this essay we show methods and ways of fire‐protective evaluation for components and construction details. The approach of proof including dimensions of possible upgrade measurements if necessary are being illustrated by examples of use out of practical experience. At the same time different materials such as steel, concrete and wood will be treated with different fire‐proof products. The essay is to explain ways of evaluation of components and construction details involving fire‐proof requirements including upgrade measurements also especially for existing buildings.     

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.     

Wirklichkeitsnahe Brandeinwirkungen für die Planung und Nachrechnung von Tunnelschalen

Realistic fire loads for design and re‐analysis of tunnel linings Meanwhile, the development of numerical systems for the simulation of flow phenomena has reached a level, which even allows the analysis of complex scenarios such as tunnel fires, within acceptable time‐periods. On the one hand, this provides the possibility to critically question fire‐curves from standards (derived and calibrated for the general case) against the background of tunnel‐specific temperature fields. On the other hand, both temperature and smoke development as well as expected structural impairment may be assessed on a relatively realistic basis without large‐scale conflagration tests being necessary. In addition to general options of non‐linear structural analyses (i.e. utilisation of load re‐distribution and activation of additional bedding reactions) [1] the simulation‐based evaluation of fire loads particularly provides a valuable potential for the re‐analysis of existing tunnel linings. This especially applies in such cases where the calculation with conventional fire‐curves requires extensive strengthening and retrofit, whereas more favourable loading conditions may be expected by taking into account the tunnel‐specific boundary conditions. In the present paper, firstly tunnel fire accidents and the derivation of fire‐curves are generally discussed. Then, the possibilities, the theoretical background and limits of CFD modelling of fire scenarios are presented and finally, major influence factors on the thermal loading (in particular the cross‐sectional size of the tunnel) are investigated by sensitivity studies.