Several damage detection and localization techniques proposed in the literature are based changes in the flexibility matrix, which can be constructed using natural frequencies and mass-normalized mode shapes from both the un-perturbed and the perturbed system. However, in operational modal analysis (OMA), the forces are unknown, and the modal masses cannot be estimated, i.e., only the un-scaled mode shapes can be identified for each mode. In order to overcome this inconvenience, proportional flexibility matrices can also be used for damage detection without loss of accuracy. In this paper, a technique is proposed to estimate the linear relationship that exists between the modal masses of the different modes, allowing the construction of a proportional flexibility matrix.