The vulnerability of industrial roofs under exceptional loads, in particular the fire load, is a relevant problem mainly evident for thin and discrete systems as the shed roofs, which, for this reason, require particular attention. Analyzing the different components of such systems, their historical development and technological details, some weaknesses as the strong asymmetry of some elements, the extreme thin sections of others and the ineffectiveness of joints emerge. Although the elements themselves are often supposed to be resistant to the action of generalized fires represented by the standard ISO curve, a brittle failure of the system can occur due to localized fire, although of modest entity, which can generate different temperature gradients in different elements. Since a post-flashover model and a two zones pre-flashover one are ineffective for this analysis, a computational fluid dynamic model has been applied to evaluate the different temperatures on the system. To apply this approach, it is strictly necessary to define the geometry of an industrial building, which should be representative of the existing industrial constructions, and a localized fire scenario configured to generate asymmetric and symmetric load conditions on the elements. Therefore, from the CFD simulation it was possible to determine different surface temperatures for each element for a consequential investigation of their effects using a transient thermal analysis. Considering the obtained results and the conformation of the elements, the analysis of the structural response in terms of deformational behavior can be more effective than a traditional approach based on the resistant capacity, as confirmed by a displacement transient analysis which reveals that critical kinematic mechanisms occur before the achievement of the ultimate limit state. The analyses prove that brittle failure mechanisms of the system can occur under localized fire, so the vulnerability of these roofs under local and punctual actions is confirmed, as well as the benefits of using CFD models and transient analysis for such studies.
La vulnerabilità delle coperture industriali in caso di azioni eccezionali quali l’incendio, fenomeno rilevante in tali edifici in relazione alle attività svolte all’interno, risulta particolarmente accentuata per sistemi discreti e sottili quali le coperture a shed, richiedendo un più approfondito studio a tale proposito. Da un’analisi dei componenti di tali coperture, della loro evoluzione nel tempo e dai particolari costruttivi di realizzazione emergono l’asimmetria di questi sistemi, la sottigliezza di alcuni elementi e l’utilizzo di connessioni inefficaci. Sebbene essi risultino talvolta adeguati a resistere ad azioni quali quelle rappresentate dalle curve nominali d’incendio, fenomeni di collasso fragile del sistema potrebbero insorgere a causa di sollecitazioni termiche differenziate tra gli elementi generate da incendi localizzati, seppur di minor entità. Dimostrata l’inadeguatezza dei modelli post-flashover e di quelli pre-flashover a due zone, si è utilizzato un modello di fluidodinamica computazionale per la valutazione delle differenti temperature sugli elementi. Per l’applicazione di questo approccio è stato necessario definire un edificio tipo che ben generalizzasse il panorama industriale esistente e uno scenario di incendio localizzato e sufficientemente puntuale per permettere una sollecitazione termica simmetrica e asimmetrica del sistema. A seguito della modellazione fluidodinamica si sono determinate le differenti temperature superficiali degli elementi per poi valutarne gli effetti mediante un’analisi del transitorio termico. Alla luce della conformazione degli elementi e degli effetti riscontrati, risulta più utile indagare la risposta in regime deformativo piuttosto che mediante l’approccio tradizionale di verifica della resistenza, come confermato da un’analisi transitoria degli spostamenti, la quale mostra l’insorgere di criticità anche prima del raggiungimento dello stato limite ultimo. Da questa emergono possibili meccanismi di collasso fragile del sistema, confermando dunque la vulnerabilità di tali coperture a sollecitazioni locali avvalorando quindi l’utilizzo della modellazione fluidodinamica e del transitorio termico per tali analisi.
Vulnerabilità al fuoco di coperture industriali a shed
CONTESSA, ALESSIA;FRANCA, ALICE
2016/2017
Abstract
The vulnerability of industrial roofs under exceptional loads, in particular the fire load, is a relevant problem mainly evident for thin and discrete systems as the shed roofs, which, for this reason, require particular attention. Analyzing the different components of such systems, their historical development and technological details, some weaknesses as the strong asymmetry of some elements, the extreme thin sections of others and the ineffectiveness of joints emerge. Although the elements themselves are often supposed to be resistant to the action of generalized fires represented by the standard ISO curve, a brittle failure of the system can occur due to localized fire, although of modest entity, which can generate different temperature gradients in different elements. Since a post-flashover model and a two zones pre-flashover one are ineffective for this analysis, a computational fluid dynamic model has been applied to evaluate the different temperatures on the system. To apply this approach, it is strictly necessary to define the geometry of an industrial building, which should be representative of the existing industrial constructions, and a localized fire scenario configured to generate asymmetric and symmetric load conditions on the elements. Therefore, from the CFD simulation it was possible to determine different surface temperatures for each element for a consequential investigation of their effects using a transient thermal analysis. Considering the obtained results and the conformation of the elements, the analysis of the structural response in terms of deformational behavior can be more effective than a traditional approach based on the resistant capacity, as confirmed by a displacement transient analysis which reveals that critical kinematic mechanisms occur before the achievement of the ultimate limit state. The analyses prove that brittle failure mechanisms of the system can occur under localized fire, so the vulnerability of these roofs under local and punctual actions is confirmed, as well as the benefits of using CFD models and transient analysis for such studies.File | Dimensione | Formato | |
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https://hdl.handle.net/10589/135585