Experimental Modal Analysis of Civil Engineering Structures

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

Finite-Element Updating. The accurate identification of the most significant modal parameters based on output-only tests can support the updating of finite-element models, which may overcome several uncertainties associated with numerical modeling. Such updating can be developed on the basis of a sensitivity analysis using several types of models and changing the values of some structural properties to achieve a good match between identified and calculated modal parameters. This procedure has been followed recently to study the dynamic behavior of a stress-ribbon footbridge at the FEUP Campus (Figure 21). For that purpose, initial finite-element models were developed for the bridge deck as a set of beam elements with the geometry considered at the design stage or measured through a topographic survey (Models 1 and 2). Afterward, due to the clear nonlinear geometrical behavior of the bridge, a third model (Model 3) was developed. The deck was modeled in truss finite elements with the cable axial stiffness (neglecting bending stiffness) and adjusting the initial cable tension to obtain the measured longitudinal profile after progressive application of the loads. To also take into account the bending stiffness of the concrete slab, this model was subsequently adapted (Model 4) by discretizing the deck in truss finite elements with progressive loading and activation of beam elements connecting the nodes of the truss elements. Finally, this model was slightly modified to consider partial rotations between beam elements and to simulate the lack of sealing of the joints. The area and inertia of the beam elements was also reduced to simulate the effects of cracking and lack of adherence between precast and in situ concrete. After all these iterations, very good correlation between identified and calculated natural frequencies and mode shapes was achieved.40

Beyond this type of sensitivity analysis, more automatic finite-element updating techniques can also be used.41 The drawback of output-only modal identification is that it seems to be impossible to obtain mass normalized mode shapes. However, this inconvenience can be overcome42 by introducing appropriate mass changes.

Conclusions

Civil engineering structures have peculiar characteristics (large size and relatively low natural frequencies) that make the current application of classical input-output modal identification techniques difficult. Therefore, there is presently a clear tendency worldwide to explore and improve the potential of output-only modal identification techniques, whose efficiency and accuracy were clearly illustrated with the applications shown. The techniques that may be used under normal operation conditions can provide a solid basis for:

* Developing finite-element correlation analyses.

* Finite-element updating and validation.

* Defining a baseline set of dynamic properties of the initially undamaged structure that can subsequently be used for the application of vibration-based damage detection techniques.