Importance of modal cross-correlations on wind loaded structures

In civil engineering applications the dynamic analysis of large structure is often
performed in a modal space. This method is known to offer an interesting decrease of the number
of degrees of freedom as well as a decomposition of the structure’s response in “uncoupled”
components. Even if the response in each mode can be computed independently from the responses
in the other ones, in the context of stochastic loading, the coherence of these modal responses
must be accounted for in the determination of the structural response. These quantities
are known as the modal cross-correlations. In this paper we will show that these crosscorrelations
can be important (contrarily to what is sometimes thought) even in case of wellseparated
natural frequencies. This will be illustrated on the analysis of the famous Viaduct of
Millau during an erection stage.