Experimental Modal Analysis of Civil Engineering Structures

Sound and Vibration, Jun 2006 by Cunha, �lvaro, Caetano, Elsa

The gradual development of all these methods, which are extensively described by Maia, et al,1 tend to be completely automated systems of acquisition, analysis, processing, and identification, instead of interactive programs initially. Beyond that, the best-performing methods have been implemented in robust modal analysis software.2 A special class of modal identification methods, called tuned-sinusoidal methods (e.g. Asher, Mau) corresponds to the particular type of tests that are based on the application of a sinusoidal excitation at each natural frequency, which can be implemented using eccentric mass vibrators.

Examples of Forced Vibration Tests. The performance of classical input-output modal identification tests in civil engineering structures can be of interest both for physical models and for prototypes. Figures 4 and 5 show a physical model of Jindo Bridge (South Korea), which was extensively tested to analyze the importance of dynamic cable-structure interactions in terms of seismic response analysis.3 Several forced vibration tests were performed using electro-dynamic shakers (at the University of Bristol and ISMES) and considering two alternative configurations for the model. First, additional masses were distributed along the cables according to the similitude theory to idealize the cables' mass and consider lateral cable vibration. In a second phase, no distributed additional mass were introduced along the cables, but equivalent masses were concentrated at their extremities. This study identified the existence of different sets of multiple modes; some being pure cable modes and others coupled modes. Each of these sets presents a common shape for the deck and towers and different cable motions. The corresponding natural frequencies are very close, always in the vicinity of a global mode of the primary system (Figure 6).

Several large civil engineering structures, like buildings, bridges or dams, have also been subjected to forced vibration tests in the past using heavy excitation devices only available at well equipped laboratories. That was the case of EMPA, where Cantieni and other researchers have tested a significant number of bridges and dams.4-6 Figures 7 through 9 show some examples of that remarkable activity, presenting in particular some of the modes of vibration accurately identified at the Swedish Norsj� dam.

Output-Only Modal Identification

The main problem associated with forced vibration tests on bridges, buildings, or dams stems from the difficulty in exciting the most significant modes of vibration in a low range of frequencies with sufficient energy and in a controlled manner. In very large, flexible structures like cable-stayed or suspension bridges, the forced excitation requires extremely heavy and expensive equipment usually not available in most dynamic labs. Figure 10 shows the impressive shakers used to excite the Tatara and Yeongjong bridges.

Fortunately, recent technological developments in transducers and A/D converters have made it possible to accurately measure the very low levels of dynamic response induced by ambient excitations like wind or traffic. This has stimulated the development of output-only modal identification methods.