Gust buffeting of long span bridges: Double Modal Transformation and effective turbulence

The gust buffeting of long span bridges is analysed by the Double Modal Transformation, a method consisting in the joint transformation into principal coordinates of the structural displacements (modal analysis) and of the loading exciting the bridge (Proper Orthogonal Decomposition, POD). Even if an accurate representation of the turbulent field producing the loading requires a high number of modes, thanks to the orthogonality of structural and turbulence modes, a satisfactory estimate of the structural response can be achieved representing the turbulence field through only a few modes. Based on this observation, an effective turbulence field is defined as that part of the actual turbulence which really influences the structural response, i.e. the turbulence reconstructed using only the necessary POD modes. The analysis of the spectral properties of the effective turbulence field shows that its harmonic content is strongly reduced and concentrated in a restricted frequency band with respect to the actual turbulence; moreover, it is highly correlated also for large distances. These observations allow a critical selection of the time and space discretizations to adopt for numerical analyses, strongly reducing their computational burden.