Issue 46

S. Motsa et alii, Frattura ed Integrità Strutturale, 46 (2018) 124-139; DOI: 10.3221/IGF-ESIS.46.13 125 experimental study presented in [4], it was observed that a greater network of cracks is formed on the side of the concrete exposed to fire and that crack propagation takes place along the thickness. Experimental and numerical studies in [5], revealed that the fire resistance of recycled aggregate concrete–filled square steel tubular columns increases with an increase in the thickness of fire protective coating and a decrease in axial compressive load ratio. In the experimental investigation conducted by [6] it was shown that the steel to concrete interface tends to fail, when exposed to elevated temperatures. Numerical models have also been proposed, for the investigation of the behaviour of composite and fire protected steel structures [7 - 14]. However, only a relatively small number of articles focusing on the gradual failure of the protection in fire conditions and its impact on the structural behaviour of steel, can be found. Delamination and/or damage of the fire protection material in fire conditions, strongly influence the performance of steel structures. The collapse of the WTC twin towers was partially attributed to the loss of insulation due to high impact and blast loads [15, 16]. In [17] the effect of fire insulation delamination on steel structures during fire following an earthquake or an explosion was studied. In the framework of a sequential thermal – structural analysis, a pre-defined delamination area (due to earthquake or explosion), restricted to a small length of the considered model, was adopted from a delamination study presented in [18]. The same authors presented in [19] a numerical model used to investigate the delamination effect of spray-applied fire resistive material (SFRM) under blast loading. In [20] the damage of a fire protection concrete board applied to a steel beam was considered, in the framework of coupled temperature – stress analysis. In [21] a study of the delamination effect during post-earthquake fire on composite steel frames, revealed that delamination of fire protection at the bottom of columns causes a large reduction in the fire resistance time, up to 70%. In the majority of the published research, the concurrent appearance of mechanical and thermal phenomena is not taken into account. Instead, first a thermal analysis is usually conducted, followed by a structural analysis. Within this framework, phenomena related to damage of the protection and their interaction with thermal/mechanical loads, as the fire event is evolved with time, are not taken into account. Therefore, the goal of this research is twofold: First, to recommend a simplified modelling technique for the consideration of the effect of the gradual failure of the protection, under fire conditions and mechanical loads. This is an effort towards imitating the behaviour of steel structures in fire conditions, where due to elevated temperatures, re-distribution of forces and failure of some steel members, both gradual damage of the passive protection and mechanical loads arise. Then, to present an insight on the structural performance of steel members under fire and mechanical loads, including among others yielding of steel, as well as reduction of the strength of the system, depicted by force – displacement diagrams. For the implementation of the mentioned ideas, a non-linear finite element analysis model is developed, where concrete fire protection boards are assigned to one, or more sides of a steel element. Both a standard temperature – time thermal load curve and a force – time mechanical load curve are simultaneously considered in the framework of coupled temperature – displacement finite element analysis. G ENERAL FRAMEWORK OF THE PROPOSED FIRE PROTECTION SCHEME steel, beam type structure with a length equal to 3m, has been chosen as the structural element of this research. In the perimeter along the length of it, one or more concrete boards are assigned, as fire protection. Between the boards and the steel, a unilateral contact - thermal condition is applied. Thus, opening between the boards and the steel can be freely developed, without any tensile resistance. This assumption relies on the fact that the structural influence of the fire protection board is not to be taken into account in this work. Instead, only its thermal contribution is examined. If at the interface between the protection and the steel some tensile and shear resistance were considered, then the protection would influence structurally, the performance of the steel. In [22] was experimentally shown that the interfacial normal and shear stresses between steel and fire protection coatings are very low (with values of 0.04MPa and 0.07MPa, respectively). In addition, adopting zero interfacial resistance is a safe assumption. To account for the influence of the gradual damage of the fire protection due to elevated temperatures on the structural performance of the system, the following, simplified concept is used. Fire is initially applied to the fire protection. Next, an effort of imitating a realistic sequence of phenomena during a fire event, is considered. Thus, after a chosen time period where fire is only applied to the board, the board is expected to fail due to elevated temperatures, and to no longer be able to protect the structure against fire. In that chosen time period, fire is applied to the steel surfaces too, which had initially been protected by the concrete board. It is worth mentioning that the experimental investigation conducted in [23], indicated that failure of fire protection coatings is related to delamination between the coating and the steel, as well as to the mechanical failure of the protection, A