MULTI-LEVEL OPTIMIZATION OF INPUT PARAMETERS FOR MODELING OF FIBRE REINFORCED CONCRETE Radomír Pukl, Tereza Sajdlová Červenka Consulting, Praha 5, Na Hřebenkách 55, 150 00, Czech Republic David Lehký, Drahomír Novák Institute of Structural Mechanics, Faculty of Civil Engineering, Brno University of Technology, Brno, Veveří 331/95, 602 00, Czech Republic Abstract Appropriate material input parameters for numerical models of steel fibre reinforced concrete were identified from measured response of four-point bending beams using inverse analysis at several levels of complexity including advanced stochastic analysis and neural network technology. Based on the obtained results the optimal material input data sets are suggested for practical utilization of various numerical material models of fibre reinforced concrete in the nonlinear computer simulation of response and damage of FRC structures and structural parts. Keywords: Computer simulation, Fibre reinforced concrete, Nonlinear material models, Identification of material parameters, Fracture analysis 1 Introduction The nonlinear finite element simulation is recently a well-established approach for analysis of reinforced concrete structures and it has a big potential also in the field of fibre reinforced concrete (FRC) structures. Special material models at macroscopic level are available for modelling of FRC- material in the numerical simulation of FRC-based structures, taking into account higher ductility of FRC. This can be represented by larger fracture energy in the material models. Appropriate input material parameters for these numerical models are basic precondition for successful analysis of the FRC structures. Requested values, in particular the tensile material properties, can be identified using inverse analysis method from results of available tests on simple structures such as bending beams. 2 Nonlinear material models for fibre reinforced concrete Special constitutive material models have been developed for description of FRC-material in the nonlinear finite element analysis (Červenka (2011), Pukl et al. (2005)). They account for the high toughness and ductility of FRC, as well as other properties differing FRC material from conventional plain concrete (e.g. shape of the descending branch of the crack opening law). Several levels of FRC modelling at the material levels are available for performing the nonlinear numerical analysis. The first choice could be utilization of the material models developed for the plain concrete with appropriately adjusted material parameters (tensile strength, fracture energy). The shape of the tensile descending branch is in this case an exponential function, which is not optimal for the description of FRC response, but its use is rather pragmatic – it is of advantage that the models for plain concrete are very well verified and exhibit rather stable behaviour. In order to improve modelling of the FRC tensile behaviour material laws with special forms of the tensile descending branch more suitable for FRC were formulated and implemented. Two models are designed especially for steel fibre reinforced concrete (SFRC). They are derived from plane stress material law for plain concrete. If the fracture energy is known, an objective material law based on the crack band approach can be used. After cracking, the tensile stress drops to certain fraction of the