Modal properties of beam-and-block pre-cast floors

This article describes a combined experimental and analytical approach to investigating modal properties of beam and block (B&B) building floors. A rarely performed dynamic testing of a representative sample of four nominally identical B&B open plan suspended ground floors in a real-life building has been carried out using state-of-the-art modal testing featuring a pair of simultaneously operating electrodynamic shakers and multi-channel data acquisition. The testing demonstrated that the floors have considerable transverse stiffness able to engage sizeable parts of the floor area in a manner similar to orthotropic plates. Although nominally identical, the four floors have somewhat different modal properties. In general, although open plan, the B&B floors tested developed considerable levels of non-proportional damping. This is possibly a consequence of friction between non-monolithic floor elements (in the beam-to-block and/or block-to-block contacts). By back analysis, using experimentally measured static stiffness and modal properties, a simple mathematical model of a rectangular simply supported orthotropic plate was verified as an applicable method for the calculation of modal properties of B&B floors tested. However, somewhat surprisingly, for this method to work, it was necessary to assume approximately 2.5 times higher bending stiffness than that of just the beams in the B&B floors, which are the only structural elements in this type of construction. Transverse stiffness was estimated to be 8% of this enhanced main stiffness. This is probably due to composite action of the in situ cast non-structural sand and cement screed and/or engaged friction stiffness between non-monolithically connected beams and blocks