Recent events such as natural catastrophes or terrorism attacks have highlighted the necessity to ensure the structural integrity of buildings under exceptional events. According to Eurocodes and some different other national design codes, the structural integrity of civil engineering structures should be ensured through appropriate measures. Design requirements are proposed in some codes but are nowadays seen generally as not satisfactory. In particular, it is not demonstrated that, even if these requirements are respected, the risk of a progressive collapse of the structure subjected to an exceptional event will really be mitigated. 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 allowing to ensure the structural integrity of steel and composite structures under exceptional events through an appropriate robustness. In particular, one substructure test simulating the loss of a column in a composite building was performed at Liège University. The present paper describes in details this substructure test. In particular, the development of membrane forces is illustrated and their effects on the behaviour of the beam-to-column joints are discussed.