The goal of this research project is to analyze the behavior of columns made with plain concrete and synthetic polypropylene/polyethylene fiber addition. In order to supplement the current data base, an experimental study was carried out on six columns of real size and ordinary concrete resistance confined by circular stirrups and subjected to constant axial loads and cyclic flexure. Three columns were built with concrete of normal resistance with 0% of synthetic fibers and three columns were built of concrete with 1% by volume of synthetic fibers.The used concrete strength varied from 25 to 42 MPa, the resistance of longitudinal steel was 510 MPa, that of the stirrups of 468 and 510 MPa and the axial factor load around 25% Ag f'c . Today, it is possible to explore the behavior in the plastic range of the structural elements under seismic loading. Predictions of behaviour of moment vs curvature as well as predictions of force vs displacement relationship were accomplished. Indeed, the comparison between the experimental results and the prediction made for the specimens with and without fibers shows a very good agreement. In the case of the prediction of the moment vs curvature relationship, the MNPHi software developed by [PAULTRE, 1996] was used and in the case of the prediction of force vs displacement relationship, software DISP96 developed by [LÉGERON, 1998] was used. Finally, the influence of tie spacement as well as addition of fibers were clearly observed.The results show that the presence of polypropylene/polyethylene fibers in the concrete matrix improves ductility, tenacity, and the capacity to dissipate energy. Also, they help to retain the cover concrete.