Modal density and modal distribution in a ribbed plate

Plates reinforced by ribs or joists are common elements in lightweight building structures, as well as in other engineering structures. However, these structures are not well suited for the common simplified prediction models such as Statistical Energy Analysis (SEA) or the European calculation standard EN 12354. There are several reasons for this: One is that the overall vibration field is not diffuse due to the orthogonal behaviour introduced by the ribs. Another reason is that the modal density is not well behaving due to the periodicity introduced by the ribs. In the present paper these two aspects of the ribbed plate are investigated, focusing on the latter, using a modal model of a ribbed plate. Some suggestions are given regarding how to proceed towards a simplified prediction model for ribbed plates.