58 K. Worden et al. 7.2 The Experimental Rig and Data Capture For the purposes of the present work, a small-scale simulated shear building model was constructed. This was chosen to correspond closely with a structure previously designed and built at Los Alamos National Laboratories (LANL). Within LANL, the experimental rig was referred to informally as the ‘bookshelf’ rig and this informal nomenclature is also adopted here. The bookshelf structure, illustrated in Fig. 7.1 has four levels, floors or shelves, with the lower level being considered the base. Each shelf/floor is composed of a substantial rectilinear aluminium block with a mass of 5.2 kg and dimensions 35 25:5x 0:5cm.L w h/. The shelves are joined by upright beams at each corner; each beam having a mass of 238 g and dimensions 55:5 2:5 0:6cm. The blocks used to connect the main plates and the upright beams have a mass of 18 g and dimensions 2:5 2:5 1:3cm. For each block, four bolts were used, each of Viraj A2-70 grade and with a mass of 10 g. The structure was mounted on a rail system which was securely clamped onto a substantial testing table. In order to introduce the excitation into the structure, an electrodynamic shaker with a force transducer was connected to the base. The output response of the structure was recorded using four accelerometers positioned, as shown in the pictures, at the middle of each main plate. So far, the system is linear, and the intention of the first identification tests was that it remain so in order to simplify the identification problem. However, as the longer term goal of this work was to consider nonlinear systems, provision was made for nonlinearity. In the original LANL structure, nonlinearity was introduced via a comparatively complex series of bumpers connected between shelves; in the current structure, nonlinearity is introduced in a much more basic fashion in order to impose simpler equations of motion. The nonlinearity here will be introduced via a vertical cantilever beam which contacts (via a bolt) the topmost shelf of the structure at negative displacements of that shelf. The experimental data were acquired using an LMS CADA system connected to a SCADAS-3 interface. A total of 93,184 points per channel were recorded at a sampling frequency of 1024 Hz. Lateral accelerations were recorded for each shelf from piezoelectric accelerometers fixed to the edges. ‘FRFs’ (transmissibilities) between the relative accelerations of the floor and the base acceleration are given in Fig. 7.2; the structures shown indicate that a three-DOF model of the rig is likely to capture the main dynamics. Fig. 7.1 The ‘bookshelf’ experimental rig showing shaker attachment and guide rail system
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