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Multi-physics modelling for the safety assessment of complex structural systems under fire. The case of high-rise buildings
Full text linkAmong all structures, high-rise buildings pose specific design challenges with respect of fire safety for a number of reasons, in particular the evaluation of both the fire development (fire action) and response of the structural system to fire (structural behaviour).
In relation to the fire action, large compartments and open hallways often present in modern high-rise buildings don’t let themselves to be designed within compliance to current codes and standards. A comprehensive analysis of the fire environment is required to understand the fire dynamics in these cases. A Computational Fluid Dynamic (CFD) model allows a quite accurate representation of realistic fire scenarios, because it takes into account the distribution of fuel, the geometry, the occupancy of individual compartments and the temperature rise in structural elements that are located outside the tributary area of fire scenario.
In relation to the structural behaviour under fire, the passive fire resistance of structural elements and the intrinsic robustness of the system are the only measures to rely on in order to maintain the structural integrity of the building during and after the fire and avoid major economic losses due to structural failures and prolonged inoperability of the premises. Disproportionate damages induced by fire can be avoided with a proper design of the structure, aimed at reducing the vulnerability of the elements to fire (i.e. their sensitivity to fire) or at increasing the robustness of the structural system (i.e. its sensitivity to local damages).
The topic of this thesis is the evaluation of the structural safety in case of fire by means of advanced multi-physics analyses with direct reference to the modern Performance-Based Fire Design (PBFD) framework. A fundamental aspect is how some basic failure mechanisms can be triggered or modified by the presence of fire on a part of a structural system, such as three hinge mechanism, bowing effects, catenary action, thermal buckling and snap-through, sway and non-sway collapse. High rise buildings, which are expected to be susceptible to fire-induced progressive collapse, will be investigated. Critical elements will be identified in the system and countermeasure for enhancement of structural integrity will be suggested. The investigation of the response of such a complex structures subjected to fire scenarios requires the use of CFD and Finite Element (FE) models for a realistic evaluation of the fire action and of the structural response respectively
On the role of the numerical analyses in forensic investigations of fire-induced progressive collapses of tall buildings
No full textFire safety engineering is introduced in the forensic context, and
responsibilities of various actors operating for fire safety in buildings are
defined in relation to the different available fire safety strategies. The
responsibility of the designers concerning the structural behaviour is
highlighted with specific reference to the requirement of structural robustness
in high-rise buildings. In case of complex structures, the robustness assessment
can be made by the avail of advanced structural numerical analyses, which are
nowadays the most reliable tools for understanding, governing and addressing
this kind of problems. In addition, the same importance is given to the
numerical analyses in case where these are used in forensic investigation for
obtaining a back-assessment of the occurred fire-induced collapse in tall
buildings. The complexity of the problem, the correct approach to this kind of
analyses and the correct interpretation of the obtained results are discussed with
reference to a high-rise steel building
Influence of panic on human behaviour during emergency egress for tunnel fires
No full textTunnel fires represent a complex multidisciplinary problem of great importance for the Safety Engineering, where different aspects converge and add difficulties in studying and modeling the phenomenon. The confined geometry, traffic conditions and different, sometimes inap-propriate, safety measures are combined with the unavoidable presence of humans inside vehicles, making tunnel systems highly dangerous and risky: an extreme action like a fire or an explosion can cause severe effects depending on the fire development and the occupant characteristics, both physical and cognitive. In such environment, panic often drives people actions: the main negative consequence is the increase of the evacuation time, called RSET (Required Safety Evacuation Time). As the evacuation time grows, the exposure time at high temperatures and smoke increases and so the possibility for people of getting intoxicated.
This paper is thought as the prosecution of other two papers: “Risk analysis for severe traffic accidents in road tunnels” (Di Santo et al., Proceeding IF CRASC’ 15) and “Computational fluid dynamics simulations for the assessment of a road tunnel fire safety” (Baroncelli et al., Proceeding IF CRASC’ 15) in which the concepts of hazard and risk for tunnels are widely described.
The aim of this paper is to highlight the importance of considering human behaviour for the Fire Safety Engineering and the possibility of its modeling with numerical codes as FDS+Evac, giving as example the study of different egress scenarios for a 3 km – long – tunnel placed in the Catania – Syracuse Highway
Effects of combustible stacking in large compartments
No full textThis paper focuses on the modelling of fire in case of various distributions of combustible materials in a large compartment.Large compartments often represent a challenge for structural fire safety, because of lack of prescriptive rules to follow and difficulties of taking into account the effect of non uniform distribution of the combustible materials and fire propagation.These aspects are discussed in this paper with reference to an industrial steel building, taken as case study. Fires triggered by the burning of wooden pallets stored in the premises have been investigated with respect to different stacking configurations of the pallets with the avail of a CFD code. The results in term of temperatures of the hot gasses and of the steel elements composing the structural system are compared with simplified analytical model of localized and post-flashover fires, with the aim of highlighting limitation and potentiality of different modelling approaches
Structural response of steel high rise buildings to fire:System characteristics and failure mechanisms
Full text linkDue to the significant vertical elevation and complexity of the structural system, high rise buildings may suffer from the effects of fire more than other structures. For this reason, in addition to evacuation strategies and active fire protection, a careful consideration of structural response to fire is also very important. In this context, it is of interest to investigate the characteristics of the structural systemthat could possibly reduce local damages or mitigate the progression of failures in case of fire. In this paper, a steel high rise building is taken as case study and the response of the building is investigated up to the crisis of the structure with respect to a standard fire in a lower and in a higher storey: the comparison of the fire induced failures at the different height allows highlighting the role played in the resulting collapse mechanisms by the beam-column stiffness ratio and by the loading condition
Investigation of fire-induced collapse scenarios for a steel high-rise building
Full text linkThe paper deals with the problem of understanding and evaluating the structural response of steel buildings to fire and outlines a general framework for the structural fire safety design of high-rise building. Among all building typology, the fire design of high-rise buildings is particularly challenging with respect to both non-structural and structural design aspects: the enhanced design difficulties in providing i) a safe and prompt vertical evacuation of the building and ii) an effective vertical compartmentalization for avoiding vertical fire spread, refer both to non-structural aspects (architectural design choices and active measures) and won’t be investigated in detail in this paper; the paper focus instead on structural design aspects, with specific reference to the iii) enhanced susceptibility of high-rise buildings to disproportionate collapse, due to the significant vertical elevation of the structural system and to the complex and often untraditional design. With reference to fire-induced collapses, a particular dangerous situation is represented by the spread of failures to elements not directly involved in the fire, i.e. element that due to their location or because of greater insulation have still a relatively low temperature at the time of failure: in this respect, the example of a high rise building is presented in the paper, where, depending on the fire scenarios considered and as different beam-column stiffness ratio, fire damages can remain localized to the heated zone or involve other elements in the failure. Disproportionate damages induced by fire can therefore be avoided with a robust design of the structural system. This requires however a study of the response to fire of the structure as a whole, which is a quite difficult task, especially for complex structures such as high-rise buildings. Aim of the paper is thus to exemplify a method for performing this kind of investigations and to outline problematic aspects in the modeling and in the interpretation of the results
Modeling of the structural response to fire of a high-rise steel building
No full textObservations from the tests and the real fire investigations have consistently shown that the performance of a whole steel-framed building in fire is very different from the performance of its individual members (Usmani et al, 2000). In this context, it is of interest to investigate the failures mechanism of a structure under fire and to highlight a possible disproportionate influence of a local failure triggered by the fire on the response of the whole structure (Bontempi et al., 2010). The propagation of the collapse has to be traced up to the global collapse of the structure and numerical problems due to the triggering of local mechanism should be overcome to this purpose. In this paper, a steel structure has been considered as case study and the response of the structural system to fire and fire effects has been investigated with the avail of a finite element commercial code. These kinds of investigations require the consideration of a full nonlinear response of the structure, due to material degradation under fire, possibility of buckling, large displacements and deformations suffered by elements and exploitation of plastic reserve of the structure. The assessment of structural performances and the computation of the structural system behavior have been conducted by considering some remarkable fire scenarios. Basic preliminary investigations are carried on a substructure, which represents a single floor of the building (Fig.2 bottom) for understanding the effect of beam failures on the adjacent elements and for highlighting a possible propagation of the failures to zones of the structure not directly involved in the fire. In order to show the influence of ratio of stiffness between beams and columns, the behavior of two different floors has been studied
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