ABSTRACT: The International Guadiana Bridge (Fig. 1) is a concrete cable-stayed bridge, formed by a central span of 324 m, two lateral spans of 135 m and two transition spans of 36 m, spanning the Guadiana River close to the border with Spain, at the southern part of Portugal. The bridge was designed by Câncio Martins (1992) and opened to traffic in 1991. Given the relatively severe wind and high seismic risk characteristics of the site, extensive studies were developed prior, during and after construction (Branco, 1987; LNEC, 1992; Branco et al., 1991, 1993). Despite the generally good performance under normal traffic and ambient conditions, the stay cables soon proved to be vulnerable to wind excitations, and that results in the occurrence of frequent oscillations of high amplitude, accompanied by a significant “rattling noise”. Furthermore, a study developed at commissioning stage by Pinto da Costa et al. (1994) pointed to some vulnerability of certain cables to the so-called phenomenon of parametric excitation.