Identification of dominant modes of single-layer reticulated shells under seismic excitations

Single-layer reticulated shells are a type of spatial lattice structures with closely spaced natural frequencies and significant higher mode effects. An accurate calculation of the seismic responses of these structures necessitates the modal combination of dozens or even hundreds of modes when using mode superposition method. To precisely identify the modes that are significantly contributory to the seismic responses, the time-averaged modal strain energy ratio, denoted as TAMSER for brevity, is derived and then selected as the only evaluation metric, taking into account both the modal characteristics of a given structure and the spectral characteristics of an input ground motion. In terms of this metric, a methodology for identifying dominant modes is developed for structure systems under unidirectional or multi-directional seismic excitation. Based on this methodology, the dominant modes of two typical single-layer reticulated shells are identified and investigated, and then the responses resulting from the modal combination of these identified dominant modes are compared to that of the first 30 and 250 modes, as well as the time history analysis results. It is finally concluded that this proposed methodology is capable of accurately identifying the dominant modes of single-layer reticulated shells subjected to seismic excitations, and the incorporation of the identified dominant modes only into the modal combination is appropriate and sufficient to consider the higher modes effects and to get a satisfactory engineering accuracy in the mode superposition analysis.