Due to the popularization of Internet, cooperative applications of project-enterprise type are expected to become commonplace on the WEB. By project-enterprise, we understand short-lived concurrent engineering enterprises which are created by aggregating several partners around a project and for the duration of this project. They require new technologies to organize their short duration networks. For this purpose, this paper introduces a flexible approach to build cooperation support software by assembling basic generic cooperation bricks. It shows how such a cooperation policy is implemented in a secure way thanks to a flexible transaction model. Our approach is illustrated through an example taken in the AEC domain.