In large engineering projects, the complexity of organizational and decision-making structures is a challenge for efficient management. Middle management plays a crucial and often underestimated role in the daily life of these complex projects. Our goal is to provide theoretical tools to seize the complexity of large engineering projects from the point of view of the human actors. An analysis of the organization of the ATLAS detector within the LHC at CERN was conducted over a period of several years. This contribution presents work on engineering interfaces in collaborative activities by showing how middle managers (mostly engineers) act as interfaces with other stakeholders and deal with complex socio-economic and technical issues. The interface model described is human-centric and aims at reflecting the complexity of engineering management situations. Different types of interactions stem from this model, as well as the exchange spaces established through the interface actor. The potential of application of the model is illustrated through two project case studies, one in the field of big science and the second one in high-tech medical equipment.