Experimental and numerical analysis on the structural behaviour of cold-formed steel beams subjected to fire

Tese de Doutoramento em Engenharia de Segurança ao Incêndio, apresentada à Faculdade de Ciências e Tecnologia da Universidade de CoimbraA construção metálica é uma tecnologia cada vez mais corrente na construção civil que se baseia essencialmente na utilização de perfis pré-fabricados de aço estrutural. Estes, por sua vez, encontram-se em três formas distintas, nomeadamente, em perfis laminados a quente, em perfis soldados ou em perfis enformados a frio. Estes últimos, relativamente aos outros, têm fundamentalmente a vantagem de facilidade de produção, montagem e transporte, devido ao baixo peso dos perfis, conferido pelas espessuras reduzidas dos mesmos. No entanto, o facto de estes apresentarem estas espessuras pode originar a ocorrência de fenómenos de instabilidade local e distorcional. Neste sentido, diversos autores estudam experimental e numericamente estes fenómenos à temperatura ambiente. Outro fenómeno também problemático é quando os elementos de aço enformados a frio estão sujeitos ao fogo, existindo neste caso muito poucos estudos. Deste modo, este trabalho de investigação teve como objectivo estudar o comportamento de vigas em aço galvanizado enformadas a frio em situação de incêndio, com base num vasto programa de ensaios experimentais. Ensaios de flexão de quatro pontos foram assim realizados com o intuito de avaliar a influência (i) do tipo de secção transversal das vigas (secções C, I-enrijecido, R e 2R), (ii) da restrição axial à elongação térmica (0 e 15 kN/mm) e (iii) da restrição rotacional nos apoios (0 e 150 kN.m/rad). Por outro lado, como referência, ainda foram efectuados ensaios à temperatura ambiente para a determinação da carga de instabilidade das respectivas vigas, assim como, dos seus modos de instabilidade que foram responsáveis pela perda de capacidade de carga da mesma, a fim de comparar com os ensaios a altas temperaturas. Estas vigas, normalmente utilizadas nos edifícios com perfis aço enformados a frio, tinham 3000 mm de vão e eram compostas por um ou mais perfis de aço enformados (perfis C e U) aparafusados entre si, os quais tinham 2.5 mm de espessura, 43 mm de banzo e 250 mm de altura para os perfis C e 255 mm para os perfis U. O estudo experimental foi também complementado por uma intensiva análise numérica através do programa de elementos finitos ABAQUS. Neste domínio foi realizado um estudo paramétrico de modo a avaliar ainda o efeito da altura (entre 200 e 300 mm) e da espessura (entre 1,5 e 2,5 mm) das secções transversais das vigas, do comprimento do vão das mesmas (entre 2000 e 5000 mm) e, ainda, de outros valores de rigidezes de restrição axial (entre 0 e infinita) e rotacional (entre 0 e 1200 kN.m/rad) no desempenho estrutural deste tipo de vigas quando submetidas ao fogo. Finalmente, com recurso a esta base de dados procurou-se comparar os resultados com as disposições estabelecidas no Eurocódigo 3, para o aço laminado a quente, e propor possíveis novas equações simplificadas de cálculo para o dimensionamento de vigas em aço enformadas a frio, em situação de incêndio. As principais conclusões deste trabalho de investigação foram essencialmente que as vigas de aço enformadas a frio correntemente utilizadas neste tipo de edifícios apresentam uma resistência ao fogo baixa (inferior que 30 minutos), mas temperaturas críticas elevadas (maiores que 350 ºC, isto é, temperaturas superiores ao limite recomendado pelo Eurocódigo 3, Parte 1.2, para elementos de classe 4), na maioria das situações. Por outro lado, verificou-se que as temperaturas críticas das vigas podem reduzir substancialmente (30% em alguns casos) quando são restringidas à elongação térmica. Contudo, esta diminuição pode ser atenuada quando a restrição à rotação dos apoios da viga é significativa, comparativamente com a restrição rotacional da respectiva viga.The steel construction is increasingly an important technology for civil building. Actually, the steel industry is constantly in search of more and better uses for steel. The uses of hot-rolled, welded and cold-formed steel elements in the construction of buildings are ones of the solutions that can easily replace the traditional technology of construction. Indeed, cold-formed steel is one of which are becoming a very popular material in construction because they provide a high strength to weight ratio, are easy to produce, transport and assembly when compared to thicker hot-rolled steel members. Another advantage is the great variety of profiles available on the market which allow the building of different member cross-sections. However, they may behave poorly under fire conditions, especially when they are unprotected in fire case, due to the combination of the high thermal conductivity of steel and section factor of these structural members (small wall thickness), both of which lead to a rapid rise of temperature in steel in fire situation. In addition, cold-formed steel members usually have complex buckling behaviour, involving local, distortional, global buckling and their interactions. Studies on fire performance of cold-formed steel members are still fairly rare, are mostly of a numerical nature and are based on the structural behaviour of single and short elements at elevated temperatures. Hence, this research work intended to study the behaviour of cold-formed galvanized steel beams under fire conditions, basing on the results of a large programme of experimental tests. Four-point bending tests on cold-formed steel C-, lipped I-, R- and 2R-section beams were performed, both under fire conditions, under flexural loading conditions and under simply supported boundary conditions (roller and pinned supports) with different restraining conditions, including no restraints, partial axial restraint to the thermal elongation of the beam (15 kN/mm) and partial rotational restraint at the beam supports (150 kN.m/rad). In other words, it was investigated the influence of the cross-sections, the axial restraining to the thermal elongation of the beam and the rotational stiffness of the beam supports. These beams, commonly used in CFS buildings, had a span of 3000 mm and were made of one or more cold-formed steel profiles (channel, U, and lipped channel, C, profiles), which were 2.5 mm thick, 43 mm wide and 250 mm tall for C sections and 255 mm for the U sections. On the other hand, as reference, four-point bending tests on the same type of beams at room temperature and under simply supported boundary conditions were also carried out to assess their ultimate bending strength and to compare with the failure modes. A numerical study was still performed by the finite element program ABAQUS. So a great number of numerical simulations aimed to carry out a parametric study outside the bounds of the original experimental tests, in other words, to find out the effect of thickness (from 1.5mm to 3.5mm) and height (from 200mm to 300mm) of the beam cross-sections, of the beam spans (from 2000mm to 5000mm) and of other values of axial (from zero to infinite) and rotational stiffness (from zero to 1200kN.m/rad) of the surrounding structure to the beam. Finally, the results of the experimental tests and the numerical simulations were the basis of an analytical study for the development of simplified calculation equations for fire design of cold-formed steel beams. The main conclusions of this research study were that the cold-formed steel beams commonly used in this kind of buildings may have a quite low fire resistance (less than 30 minutes), but high critical temperatures (higher than 350 ºC - limitation enforced by Eurocode 3, Part 1.2) in most of the cases. On the other hand, the critical temperature may drop significantly (30% in some cases) with the axial restraint to thermal elongation of the beam. This decreasing is however minimized when the rotational restraint at beam supports is relevant comparing with rotational stiffness of the beam

Baseline study on the behaviour of cold-formed steel columns subjected to fire

In this paper is presented a state-of-the-art and future research pathways on cold-formed steel (CFS) columns subjected to fire, the experimental set-upbuilt at the University of Coimbra for testing CFS columns and some preliminary results obtained in the tests, comparing them with the ones existing in the literature.The main objective of the experimental research already carried out is to assess the fire resistance, mechanical behaviour and characterize the failure modes of cold-formed steel columns with restraining thermal elongation in case of fire, develop simplified calculation methods and provide experimental data for future numerical studies

Rural Fires: Causes of Human Losses in the 2017 Fires in Portugal

Extreme wildfires are increasingly rising to intense and uncontrolled fires, with dimension and destructive potentials that are greater than what has been seen and dealt with. The hazards posed by these fires increase significantly when they approach the wild–urban interface, with relevant environmental and socio-economic consequences. The 2009 Victorian bushfires and the 2017 Portugal wildfires are powerful reminders, and they have demonstrated the need to better understand why mitigation plans have failed to protect the community in these events and to improve community resilience. The year 2017 is a milestone in the history of wildfires in Portugal, not only because of the vast burned area but also due to the high number of fatalities. The two occurrences were at different times (June and October) but were geographically close (region of centre of Portugal). A total of 117 deaths occurred in both events and 92% of the victims were in wild–urban interface areas. This paper analyses and discusses the characteristics and causes of death of the victims of these two events: age, place of death, distance from place of death to place of residence and last-minute choices to aim to understand the actions that people took in the face of the approaching fire, which led to their death. In both cases, most people died fleeing the fire without any information from the competent authorities. In the end, it is possible to identify risk factors that lead to the death of civilians due to wildfires, such as the increase in demand for rurality by young people from big cities with no previous contact with wildfires; on the other hand, there is the ageing of the population residing in forest areas, who were previously physically and structurally prepared to deal with fires and are currently no longer able to.info:eu-repo/semantics/publishedVersio

Special Issue on New Challenges in the Civil Structures for Fire Responses

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Thermal conductivity of insulating refractory materials: Comparison of steady-state and transient measurement methods

International audienc

Development of Passive Fire Protection Mortars

During a fire event, the stability of steel structures may be compromised, and structural collapse may occur due to the loss of their mechanical resistance as the temperature increases. One of the solutions to reduce this problem is the protection with a coating using enhanced fire-resistant mortars. This paper reports a detailed experimental work aiming to develop gypsum and cementbased mortars for passive fire protection and evaluate their composition’s effect in the final thermal performance. Two types of specimens were tested: (i) small specimens composed of a mortar coating (10 mm thick) and one steel plate and (ii) square section short tubular steel columns with 20 mm of coating. The evaluation of the thermal protection was carried out by (a) measuring the thermal gradient between the exposed surface of the protected steel plate under high temperatures and the mortar-steel interface and (b) assessing the fire resistance of the short steel columns. It was concluded that the compositions with gypsum binder present better thermal insulation than the cementitious compositions. Additionally, the introduction of nano-and microparticles of silica still slightly improved the thermal insulation of the tested compositions