Keywords: three-dimensional building, seismic response, passive control, magnetorheological damper.

Significant effort has been made during recent years on dissipative protective systems. Presently, increasing attention is paid to combine the reliable and cost-saving passive technology with high performance active strategies, by means of hybrid and semiactive devices. Magnetorheological dampers are considered promising devices to reduce structural vibration because of mechanical simplicity, high dynamic range, low power requirements, large force capacity and robustness [1]. Experimental test results on large scale models show how this type of device is suited for application to civil structures [2]. To date, the experimental verification of magnetorheological dampers to reduce the torsional motion of asymmetric buildings is confined to light models [3].

The paper presents an analytical discrete model to describe the three-dimensional dynamics of a beam structure, to reproduce a two-storey frame building. The model provides a preliminary insight into the seismic behaviour of a steel-made laboratory-scaled (2:3) prototype structure, physically realized at the Structural Laboratory of the DiSGG - Università della Basilicata (Italy). In the framework of the Italian DPC-ReLUIS Research Project, the structure, which is placed on a shaking table, constitutes a benchmark for experimentally assessing the effectiveness of different passive and semi-active strategies to mitigate the seismic-induced vibrations.

The model is formulated according to the direct displacement method. Once some simplifying hypotheses are introduced, and the rotations of the beam-column nodes are statically condensed, the structural motion is wholly defined by six free dynamical variables, corresponding to the barycentric planar displacements of each storey. External excitation is provided by an imposed mono-directional ground accelerogram.

The passive controlling action of two magnetorheological dampers is considered. Their hysteretic behaviour is described by a modified Bouc-Wen rheological model. The dampers apply two eccentric and independent forces, acting on the first-storey displacements along the excitation direction. The chosen set-up allows the mitigation of the lateral and torsional motion when a monodirectional ground motion is imposed on the non-symmetric structure. First, the system modal properties are assessed, considering also the potential presence of eccentric masses, which breaks the structural symmetry and couples the lateral and torsional components of the modal response. Second, numerical investigations simulate the seismic response to seven natural accelerograms. Synthetic performance indexes permit a comparison between the uncontrolled and controlled responses. They indicate that the passive control strategy effectively mitigates the structure vibrations for both symmetric and non-symmetric mass distributions. However the control efficacy in reducing the peak response amplitude may degrade for particular accelerograms with highly non-stationary properties.

1

Yang G., Spencer B., Carlson J., Sain M., "Large-scale MR fluid dampers: modeling and dynamic performance considerations", Engineering Structures, 24, 309-323, 2002. doi:10.1016/S0141-0296(01)00097-9

2

Occhiuzzi A., Spizzuoco M., Serino G., "Experimental analysis of magnetorheological dampers for structural control", Smart Materials & Structures, 12, 703-711, 2003. doi:10.1088/0964-1726/12/5/306

3

Yoshida O., Dyke S.J., Giocosa L.M., Truman K.Z., "Experimental verification of torsional response control of asymmetric buildings using MR dampers", Earthquake Eng. & Struct. Dyn., 32, 2085-2105, 2003. doi:10.1002/eqe.316

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