DYNAMIC TESTS ON A LARGE CABLE-STAYED BRIDGE AN EFFICIENT APPROACH By A. Cunha 1 , E. Caetano 2 and R. Delgado 3 1 Assistant Professor, 2 Assistant and 3 Associate Professor at Faculty of Engineering of University of Porto Rua dos Bragas, 4099 Porto Codex, Portugal e-mail: acunha@fe.up.pt ABSTRACT: This paper describes the dynamic tests performed on a large cable-stayed bridge, Vasco da Gama Bridge, on the basis of a non-conventional testing system, comprehending several independent accelerographs conveniently synchronised by a laptop, as well as a laser interferometry system for non- contact dynamic measurements in stay cables. This system showed to be rather portable, efficient and accurate, leading to the creation of a very large high quality data base concerning the dynamic behaviour of the bridge. Subsequent processing of the data permitted to identify accurately all the significant modal parameters of interest from the aerodynamic and seismic point of view, which present a very good correlation with the corresponding values provided by the 3-D numerical finite element model previously developed at the design stage. INTRODUCTION The development of reliable analytical dynamic models is a crucial aspect of major importance in terms of the study of the dynamic response and of the health condition of both new and existing large span bridges under traffic, wind or seismic loads. Although sophisticated finite element codes are currently available for that purpose, the success of their application is strongly dependent on the possibility of experimental verification of the results. An appropriate experimental calibration and validation of such analytical models, so that they can reflect correctly the structural properties and the boundary conditions, involves the experimental identification of the most significant modal parameters of the structure (natural frequencies, mode shapes and modal damping factors), and their correlation with the corresponding calculated values. Dynamic tests for modal parameter identification on bridges can be generally classified according to the three following types: (i) forced vibration tests; (ii) ambient vibration tests and (iii) free vibration tests. Forced vibration tests are directly related with the application of standard techniques of Experimental Modal Analysis (Ewins 1984), previously developed and applied in the context of