Structural behaviour of cold-formed steel beams subjected to fire

Abstract

The cold-formed profiles have got recently a significant role in the civil engineering industry. Main reason for this is the fact that these profiles offer numerous advantages in comparison with hot-rolled sections. Cold-forming manufacturing process nowadays makes it possible to have practically any desired shape and dimension of product which considerably increases design flexibility and moreover allows very attractive, economical and sustainable solutions. Another huge advantage is the possibility of prefabrication and mass production, which combined with a very high load capacity-to-weight ratio these profiles have, may naturally lead to enormous construction speed, easy erection and great structural performance. For these reasons, cold-formed profiles found its practical use in numerous building applications. However, due to the thin shape and thus very slender sections there are several key drawbacks of the cold-formed profiles worth of mentioning; one of them is definitely their sensitivity and vulnerability to buckling phenomenon with often very low critical loading and short buckling lengths. Another problem with the cold-formed profiles is very poor fire resistance. The main reason for this is because steel has high thermal conductivity while the cross sections are not massive and therefore the section factors are several times higher than an average hot-rolled profile. The buckling phenomena of the thin-gauge profiles, however, have been widely examined and the design procedures in which these problems are taken into account are now completely known and can be found in almost every design code. On the other hand, studies on fire performance of cold-formed steel members are still fairly rare and the knowledge in this particular area is rather obscure. The aim of this study is thus to help and stimulate the development of a new knowledge in this particular area of fire safety design and to spur further investigation. This thesis mainly intends to examine the structural behavior of the cold-formed steel beams in the role of purlins with “Omega - Ω“ shape of the cross-section under the fire condition. For this purpose an industrial building in Portugal is taken as a study case, where 30 minutes resistance is requested by the Portuguese Legislation for purlins above the mezzanine floor where offices are placed. For the considered study case, the zone model curve obtained in Ozone program is used. The analytical approach is conducted according to Eurocode and these results are further compared with the numerical ones, carried out by the use of finite element analysis software – ABAQUS. More specifically, the objective is to determine an adequate fire protection which should aim to help these cold-formed purlins to meet the required fire resistance. Also for this purpose, both analytical and numerical results are obtained and compared. Furthermore, a parametric study was performed in order to find out the influence of several parameters on the structural performance of the CFS purlins, such as the static scheme of purlin (simply supported beam and continuous beam), initial geometric imperfections, the finite element mesh size and the thickness of both steel section and fire protection. It was established that numerical analysis gives slightly higher resistances of the purlins in comparison with the analytical results, and that the initial imperfections play an important role in overall behavior of the cold-formed profiles subjected to fire by reducing their resistance. The final goal of this study was to seek for the optimal economical solution taking into account all the mentioned parameters, which should make these purlins a competitive solution on the market and hopefully initiate their mass production. It is concluded that continuous beams are much better solution than simply supported beam, making the savings in expensive fire protection material up to 50%.