Keywords: robustness, timber structures, ductility, brittleness, reliability.

Robustness of structural systems has attracted a renewed interest due to a much more frequent use of advanced types of structures with limited redundancy and serious consequences in the case of failure. This interest has also been stimulated due to the severe structural failures such as that at Ronan Point in 1968 and the World Trade Centre towers in 2001. In order to minimise the likelihood of such disproportionated structural failures many modern building codes consider the need for robustness in structures and provides strategies and methods to obtain robustness. One of the main issues related to robustness of structures is the definition of robustness. The most general definitions are very similar to each other, particularly those taken from codes, despite the use of different terms (robustness, structural integrity and also progressive collapse prevention). These definitions are focused on the prevention from an escalation of damage within the structure, given a certain initial (localised) failure or damage.

Due to many potential means by which a local collapse in a given structure can propagate from its initial extent to its final state, there is no universal approach for evaluating the potential for disproportionate collapse, or for robustnes. However, for reduction of the risk of collapse in the event of loss of structural element(s), a structural engineer may take the necessary steps to design a collapse-resistant structure that is insensitive to accidental circumstances. For example one can incorporate characteristics like redundancy, ties, ductility, key elements, alternate load path(s) etc. in the structural design. In general these characteristics can have a positive influence on robustness of a structure however, in Eurocodes ductility is only considered for concrete- and steel structures but not for timber structures. However, timber structures codes do not consider that ductility will result in semirigid behavior plus a higher level of safety due to a lower probability that premature brittle failures occur and possible redistribution of forces for statically undetermined structures either internally in the joints or to other structural elements. This paper investigates this ductility issue for a wide span glulam truss structure.

The robustness of the truss structure is determined using first-order reliability methods (FORM) where a reliability index is estimated based on limit state functions for each of the considered failure modes. The probabilistic analysis is performed with a stochastic model for the strength parameters for whole structural elements, and not to the strength for the single laminates and the glue. Second order effects are neglected for beams subjected to compression and combined compression and bending, respectively. Buckling problems and lateral buckling is taken into account as in Eurocode 5 with deterministic coefficients. For the structural analysis a linear finite element analysis has been performed where the glulam truss has been modelled by beam and truss elements. Furthermore, only permanent and snow loads are considered in the probabilistic analysis. A measure of ductile behaviour is introduced and for different values of this measure the robustness indices are estimated. The results indicate that the robustness of a timber truss system can be increased by taking the ductile behavior into account.

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