Compagnone, Alberto (2020) Probabilistc approach for simplified verification methods of resistance of steel frames in fire. [Tesi di dottorato]

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As alternative to formulation provided in Eurocode 3 for analyses on single members, this thesis shows the development of a probabilistic model to estimate critical temperature to take in account the parameters that influence the behaviour of framed steel structure in fire. A new simplified formulation has been developed to estimate the critical temperature of a steel member in the context of FSE. In other words the simplified model allows to use a general fire curve instead of the ISO standard fire also for single members, because takes into account the nonlinearities of the structure and the indirect actions generally considered with structural analysis through nonlinear FEM models. To develop the probabilistic regression model, results of FEM simulations were used and thirteen parameters were identified to describe the behaviour of the structure in fire, taking in account fluid dynamic aspects. Simple Monte Carlo simulation is not suitable to develop the regression model due to the large variability of the parameters affecting the problem. Therefore, the probabilistic study was conducted using two techniques in order to reduce the size of the sample and to preserve the quality of the results. With the Latin Hypercube Sampling (LHS) we generated a small number of samples with high meaningfulness and with the Simulated Annealing optimization method we achieved diagonal correlation matrix. The analyses were handled through a in house developed software that, starting from sampling data (i) automatically generates the input files for fluid dynamic, thermal and thermomechanical analysis, (ii) performs all analyses, (iii) stores the results in a database and (iv) generates the report file to analyse the results. The probabilistic model, created using the Bayesian updating, has been used to estimate collapse temperature of beams and columns, separately. The significant parameters have been defined through a step deletion process in order to simplify the formulation. Therefore the proposed formulations for the critical temperature take in account the effective stiffness of the structure, the non-linear structural behaviour and indirect actions due to thermal expansions and allow to exploit the resistance reserves due to the internal forces redistribution in steel framed structures. Moreover define the critical temperature as a random variable, to be easily used in reliability analysis and to develop fragility curves.

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