Thermal-mechanical analysis for structural response under the fire action: operative methodologies

The traditional approach in order to estimate the structural response to fire is by mean of prescriptive methods, based on parametric time-temperature curves. These mathematical laws should approximately describe the global behavior of the structural members when fire is occurring. Despite this method is much faster and simpler than the engineering-based one, it does not take into account of many relevant aspect, such as the geometrical features, the chemical reaction of the combustion and the position of the fire, that cannot be considered in a rigorous way. Several curves have been implemented by the current standards, in order to state the minimum requirements to satisfy security checks when the structure is subjected to fire. For this reason, this research has the goal to find a way, using advanced mathematical methods, to check the local and global structural behavior performing coupled thermal-mechanical analysis. Furthermore, a subsequent phase of study may focus the attention on the displacement-based failure criteria, through the execution of a force-control structural analysis. Indeed, through a profitable collaboration with the Italian National Fire Brigade (CNVVF – Corpo Nazionale dei Vigili del Fuoco) - Piedmont Division -, this research has been carried on to find an automatized method to perform thermal-structural using a commercial FEM codes – ANSYS Mechanical APDL, which is one of the most used Finite Element Method code worldwide, which allows all purposes engineering analysis - and FDS - a Computational Fluid Dynamic simulator for fire scenarios -, through a force-control static structural analysis. Since the exothermic analysis is not performed under the same calculation engine of the FEM code, the available procedures are not always automatized and this is a relatively new field of research. Nevertheless, the needed computing time and the Information Technology knowledge and resources may be very onerous. In order to do this and to optimize the needed time for studying the structural behavior of a building, an experimental interface called FDS2ANSYS has been developed – under Microsoft Windows Visual Basic .NET Framework 4.7.2 Environment - as a bridge-software between FDS and ANSYS Mechanical APDL. \\Nevertheless, the FDS setting phase should be also considered: in order to simulate realistic scenarios, a preliminary study on the characterization of the HRR curves has been done and a software, called HRRSprinkler Gen, has been deployed. Through this simple software – developed under Microsoft Windows Visual Basic. NET Framework 4.7.2 Environment -, it is possible to generate parametric or experimental HRR curves from the insertion of typical parameters or through images and to correct them with the sprinkler installation. This phase of study allowed to deploy FDS2ANSYS more easily.\\Although steel structures are the ones which suffer more the fire load – due to the high thermal deformation rate, typical of metallic materials -, a statistical survey carried on by the Italian National Fire Brigade (CNVVF – Corpo Nazionale dei Vigili del Fuoco) showed that the activities, for which a fire prevention analysis (both thermal and structural) may be required, are located mostly in concrete structures, both with ordinary and prestressed rebar configuration. Thus, FDS2ANSYS has been developed to study the structural behavior only for concrete structures.