The role of textile reinforcements is particularly important in the development of composite materials with optimal performances in terms of mechanical behavior and weight, whose use is increasing in various fields such as tire, aeronautic and automotive industries. In our study we are interested in unidirectional textile reinforcements, made of bundles of fibers which are twisted together to form yarn. The global mechanical behavior of these yarns is very complex, and largely controlled by contact-friction interactions taking place between elementary fibers. This kind of structure is characterized by continuous features along its longitudinal direction, although it should be considered as a discrete assembly in its transversal directions. Since the trajectory of each filament within the yarn depends on the assembly process, the initial configuration of such structures can not be known a priori, and the assembly process needs therefore to be simulated. Besides mechanical properties at macroscopic scale, the determination of local stresses at the fibers scale is very important to predict the occurrence of damage. A finite element code, based on an implicit solver, has been developed in order to simulate the mechanical behavior of such fibrous materials [1, 2]. However, due to computational costs, the use of this approach is limited to structures made of few hundred fibers, whereas cords in the scope of our study are formed of few thousand fibers. The purpose of this work is to develop a multi-scale approach to characterize mechanical properties of such material with a large numbers of fibers, at different scales. This approach is based on the introduction of so-called macro-fibers, aimed at representing the behavior of bundles of filaments.