Cable-deck dynamic interactions at the International Guadiana Bridge

The International Guadiana Bridge 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. After that a series of test campaigns under normal traffic use and with different wind conditions has been conducted (Caetano & Cunha 2003, Caetano et al. 2005). Recently, different mechanisms which cause the raising of high-amplitude cable vibrations in cable-stayed systems has been investigated by means of simple cable-supported beams (Fujino et al. 1993, Gattulli & Lepidi 2003, Gattulli et al. 2005). Their occurrence can be actually explained either due to direct cable excitation, in primary 1:1 internal resonance, or to different mechanisms of nonlinear cable-deck coupling, in 1:2 and 2:1 frequency ratio between global (deck) and local (cable) modes (Gattulli & Lepidi 2003). The measured data collected during the field observation of cables vibrations in the International Guadiana Bridge reveal a high dense frequency spectrum, with regular recurrence of internal resonance conditions (Caetano & Cunha 2003, Caetano et al. 2005). This data are used to update different finite element models of the bridge, which include a proper description of cable transverse motion (Caetano et al. 2000, Caetano & Cunha 2003, Caetano et al. 2005). Thus, from the results of a modal analysis, couples of interacting global and local modes are accurately selected to verify, and compare each other, the relevance of the different excitation sources. (Figure Presented). To this aim, the mechanical properties of a parametric cable-stayed beam model (Gattulli et al. 1999) are selected in order to ensure an effective synthetic description of the complex bridge dynamics. Extended analysis on the forced response of the model allow to recognize the importance of the different structural parameters which are involved in the cable excitation mechanisms. In conclusion, the study aims to verify the importance of previously identified nonlinear phenomena in cable-deck interaction in the measured response of the Guadiana Bridge. The investigation has been conducted starting from the amplitude levels of oscillations, at specified modal frequencies, measured on the bridge in the absence of direct measures of the complex aerodynamic and traffic excitation. On the light of the equivalent model solution and of the rough simplification of the external action, the measured low deck oscillation amplitudes seem to be still inconsistent with an effective justification of the observed high amplitude cable midspan displacement, neither assuming a simple linear behaviour, nor exploiting the effects of nonlinearities as sources of different known interaction mechanisms related to the occurrence of internal resonance conditions. Since the excitation of an extremely localized mode, with very low deck participation, seems to be the most promising hypothesis to match the experimental data, more studies should be made either in better modeling the complex external excitations or in verifying if any still unexplored mechanism (aerodynamic instability, other ratio beam to cable resonances,...) could really justify the observed dynamic behavior in the Guadiana cable-stayed bridge.