ارزیابی های بعد از آتش سوزی و بازگرداندن سازه های فولادی Post-fire assessment and reinstatement of steel structures

1. Introduction The last decades, the post-fire performance of structural steel elements has attracted many researchers. Although the fire safety of a structure is of paramount importance, the reinstatement of fire damaged structures is in the center of interest nowadays. Since 1960’s, the research is focused on the mechanical properties of the material as well as the holistic behavior of the structure, taking into consideration both low strength [1-7] and high strength steels [8-11], while cold-formed [12] and stainless steel [13] have gained considerable attention as well. In particular for cold-formed steel, distinguish from hot-rolled sections is deemed mandatory, taking into consideration the manufacturing process and the sensitivity to stability phenomena. Despite the recent interest in the post-fire performance of steel structures, an important study published by Kirby et al. [4] in 1986 contributed to the onset of several specifications. Apart from steel features, elevated temperature properties of wrought iron have been documented therein; furthermore, an extensive investigation of the residual properties of cast iron was conducted recently by Maraveas et al. [14]. The mechanical behavior of connections also plays a substantial role in the heating or cooling phase of an axially restrained member. For this purpose, many experimental studies deal with high strength bolt failure after exposure to high temperatures and subsequent cooling down [15-20], to identify both the range and the causes of deterioration. In general, although the mechanical behavior of both structural steel and bolts depends on various factors (such as the steel type, manufacturing process, heating temperature and duration, cooling rate, etc.), the majority of studies indicate that insignificant strength reduction was observed after heating to temperatures up to 500oC. Also, valuable results were observed via experimental fire development on full-scale steel structures. Thestorey steel framed building, while a cold-formed steel portal frame was tested by Johnston et al. [22]. Both of the studies above, which can be observed in Fig. 1, adequately represent the true behavior under fire conditions, aiming to the rational guidance establishment. Despite the importance of fire design of steel structures, specific guidelines have not been established by the current design codes for the determination of the remaining capacity of steel members after the fire event, with the exception of some recommendations proposed by the British Standards [23], the updating of which is essential since it regards limited steel types and specific fire temperatures. In addition, it is worh to consider that steel members perform differently when as part of a structure, owing to certain force and stress redistribution or materials over-strength, compared to the one of individual members; an important detail which is not taken into account by the present regulations. A characteristic example, which has been verified in the literature through both experimental and analytical studies, is the beneficial effect of secondary structural elements on the overall resistance of the structure, even for cold-formed structural systems.