Our global objective is to understand designers' activities and reasoning during periods of synchronous collaborative work in order to provide supporting methods and tools. In this paper we present a first step in this direction. As collaborative design in such a context involves computer-mediated synchronous human interactions, we based our study on a theoretical approach for interaction analysis coming from the human sciences. The analysis of interactions issued from 1) a design experiment we carried out in an educational environment and 2) an observation of a design study in industry (in progress), led us to propose the transposition of an analytical framework of debate (Rainbow) to one of collaborative design (Rainbow-D). This framework will aid us in understanding how designers' co-construct their interactions, allowing us to form hypotheses about improving these processes through the modification of methods and/or tools designers use.