Keywords: model uncertainty, numerical simulation, finite element, punching, concrete slabs, nonlinear analysis.

Advanced numerical simulations are more often used because of the increasing possibilities of computer technology. These models are based on generally valid principles of continuum mechanics, in which nonlinear constitutive relations are employed for material descriptions. As a result of the global nature of nonlinear numerical analysis, the design condition is based on a global safety format. It is expected that in the near future the reliability of structures will be assessed by use of these sophisticated calculations. For this purpose, it is firstly necessary to evaluate the model uncertainty of the numerical model. Model uncertainty directly affects reliability of the final design. The correctness of its determination is therefore crucial for the safe design of structures. Recommended values of model uncertainties differ in individual scientific literary sources. Euro Code and Model Code set the partial safety factor for model uncertainty of nonlinear analysis at a relatively low value. Many of the numerical simulations carried out confirmed that this value is valid only for simple arrangement (beam bending, axial compression). Some comprehensive works dealing with model uncertainties mentioned the importance of evaluating model uncertainties with respect to failure mode. It is generally accepted that simulations of members exposed to shear load show larger model uncertainty. However recommended values of model uncertainties differ according to individual studies and/or individual programs. In this context it should be noted that model uncertainty has to be evaluated for each program/model separately. In each program a different formulation of constitutive equations is implemented (especially in case of quasi-brittle materials), although these physical laws directly affect the final accuracy of program. Also the presented method requires relatively few experiments when the partial safety factor is derived. This is very useful in practice, because many times a small number of executed experiments. The research presented deals with the description of the uncertainties of numerical models for the resistance of punching capacity of slabs. The model uncertainties were identified and subsequently a recommendation for the safety factor for model uncertainty is proposed.

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