Safety Format for Non-linear Analysis of RC Structures Subjected to Multiple Failure Modes

Sammanfattning: This thesis treats the safety format for non-linear analysis of reinforced concrete (RC) structures subjected to multiple failure modes. The purpose is to identify questions which are poorly understood and ambiguous about the behaviour of structures that may fail due to a number of possible failure modes (e.g., bending, shear, buckling, crack propagation, fatigue) which might be used as a focus for the development of a more comprehensive approach to the evaluation of the structural safety level.Nowadays non-linear analysis in concrete structures cannot be considered only as a research tool to improve the understanding of structural behaviour, but it is also a useful mean to design more and more enhanced structures and to estimate the actual safety level in existing structures. As a consequence, semi-probabilistic safety formats for the non-linear analysis of RC structures are of great practical interest for structural engineers.Safety formats for non-linear analysis have mainly been tested on beams and columns subjected to normal forces and bending moments. Only recently there has been a noticeable effort in understanding whether available safety formats lead to the intended reliability when they are applied to structures that also may fail due to shear forces. However, the road ahead is still long and challenging.The definition of a suitable safety format involves the clarification of (i) which values of geometric and material properties should be used in the non-linear analysis, considering that they all influence both the resistance and the ultimate behaviour of the whole system, (ii) when the incremental process of non-linear analysis should stop, and (iii) how to derive from the failure load the ultimate load that can be carried by the structure with the safety margins that are required by the semi-probabilistic approach. This thesis considers in some sense all three of these aspects.The following major conclusions are based on the studies described in the appended papers: (1) the scatter of the shear capacity of RC slender members seems to be mainly due to the randomness of both tensile strength of concrete and shrinkage; (2) the structural behaviour at ultimate load of RC structures designed according to Eurocodes 2 is not unambiguous and may significantly vary depending on the structural system, load configuration, and capacity design; (3) the resistance of RC structures subjected to flexural and shear failure modes seems to be mainly influenced by the combination of mechanical properties of both longitudinal reinforcement and stirrups, and tensile strength of concrete; and (4) the resistance of RC structures subjected to multiple failure modes may have a general multimodal probability density function, in which each mode represents a specific failure mechanism.