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

Robustness of structutral systems has received a renewed interest resulting from the more frequent use of advanced types of structures with limited redundancy and serious consequences in the case of failure. The interst has also been facilitated due to recently 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, but 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 the 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 the robustness of a structure however, in Eurocodes ductility is only applied to concrete- and steel structures but not to timber structures. However, timber structures codes do not assume that ductility will result in a 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 joint or to other structural elements. The present paper will investigate this issue using parallel and series structural system configurations and a strucutral reliability framework. The robustness of the systems is investigated with respect to the assumed ductililty levels, correlation between the strength of structural elements and load models for permanent and live loads.

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