Synopsis
A model and simple explicit expressions are developed for the calculation of the natural frequency of multi span (reinforced concrete, steel, composite, timber and timber concrete) floors, which take into account the deflections of the supporting beams. The floor is modelled as an orthotropic plate, while the effect of the supporting beams is taken into account either with Föppl's expression or by the Rayleigh-Ritz method. The results are verified numerically and experimentally. The derived expressions can serve either as an independent check of more sophisticated FE calculations or they can be used in the preliminary design stage.