More and more frequently, the organisation of design fits into a project organisation where different designers have to cooperate with flexibility and reactivity. In order to help these cooperative design processes, we have to respond to new types of needs: a relatively unformalised coordination that requires permanent mutual adjustment, the fact that members of the team are geographically distant, the difficulty of building a shared reference via design documents and technical and organisational decisions that structure the project. In order to provide computer support for these needs, asynchronous Group Ware is often recommended. However, the efficiency of these tools depends on the models that structure the work processes and the knowledge exchanged. In this paper, we present ABRICo, a new Design Rationale formalism that permits the recording of cooperative decision-making processes in complex design situations. We describe the formalism and explain how we intend to use it to specify cooperative design tools usable via an Intranet for computing projects.