Composite joints in robust building frames

Recent events such as natural catastrophes or terrorism attacks have highlighted the necessity to ensure the structural integrity of buildings under exceptional actions. Accordingly, a European RFCS project entitled “Robust structures by joint ductility” has been set up in 2004, for three years, with the aim to provide requirements and practical guidelines for the design of steel and composite structures under exceptional actions. In this project, the importance of the structural joints has been shown; as a particular case, these ones experience additional high tying forces after the loss of a column, as a result of the development of membrane forces in the beams located just above the damaged column. Moreover a reversal of moments occurs in some joints. In this paper design models for the evaluation of the mechanical properties of joints in such extreme situations are presented. References are made to recent tests on joints in isolation and joints in frames recently achieved at Stuttgart and at Liège University in the framework of the above-mentioned RFCS project.