Modal testing and finite-element model updating of a lively open-plan composite building floor

This paper presents results of a combined experimental and analytical approach to investigate modal properties of a lively open-plan office floor. It is based on state-of-the-art finite-element (FE) modeling, FRF-based shaker modal testing, FE model correlation, manual model tuning, and sensitivity-based automatic model updating of a detailed FE model of this composite floor structure. The floor studied accommodates a fully furnished office. Such environments can be problematic regarding their vibration serviceability. However, there is a lack of reliable information about their as-built modal properties and the ability of designers to predict them. Therefore this paper has two aims: (1) to assess the ability to both predict and measure as accurately as possible the fundamental and higher modes of floor vibration, and (2) to correlate and update the initially developed FE model of the floor, so that its modes match as accurately as possible their measured counterparts. It was found that even a very detailed FE model, the development of which was based on best engineering judgment, missed the natural frequencies by 10-15% in some of the first four modes of vibration which possibly could be excited by walking. The key reasons for this were both over- and underestimation of the stiffness of the main composite beams, depending on the beam location. This was probably caused by uncertainties due to visible cracking of the lightweight concrete in the zone above the beams, effects of nonstructural elements, such as false flooring, and the inevitable uneven distribution of mass and stiffness in the real-life floor in operation. All of these factors are difficult to model explicitly in the floor, so their aggregate effect was taken into account via changes in the beam stiffness. This was found by performing a sensitivity-based FE model updating in which the first four vertical bending modes of the floor were successfully updated. Such updating was possible only after all perimeter walls were explicitly modeled. The obtained updated properties are by no means a unique solution which minimizes the difference between natural frequencies and maximizes the MAC values between the experimental and analytical mode shapes. Rather, it is a reasonable set of modeling parameters which quantifies the possible uncertainty when specifying FE modeling parameters for an open-plan floor structure like the one described in this paper.