**5. Conclusion**

**Figure 25.** The finite element models of the bridges with first mode shapes.

186 Structural Bridge Engineering

This chapter presents a comparative study about the nondestructive measurement of bridges for structural identification. Ten different bridges, which have different type and carrier systems, are selected as case studies. The dynamic characteristics such as natural frequencies, mode shapes, and damping ratios are extracted using ambient vibration tests with operational modal analysis procedure. The experimentally identified dynamic characteristics are validated by the finite element results, and the differences are evaluated.

It can be seen that the ambient vibration measurements are enough to identify the most significant modes of all bridge types.

The first natural frequencies are obtained as 4–14, 0.7–2.3, 2.4–4.6, 0.9–4.5, 1.9–6.7, 0.7–2.7, and 0.9–3.2 Hz for historical masonry arch bridges, Kömürhan and Birecik long span highway bridges, base isolated bridge, footbridges, steel bridges, and old riveted bridges, respectively.

The mode shapes occurred as vertical, lateral, longitudinal, and torsional forms. Especially, longitudinal modes should be considered for a base isolated bridge.

The finite element analyses are performed, and the results are compared with each other. It is seen that there is a good agreement between the natural frequencies and corresponding mode shapes. The maximum differences are nearly within 10–20%.

To eliminate these differences, the finite element models of the bridges should be updated by changing some uncertain parameters such as material properties, boundary conditions, section areas, etc. It can be evaluated that the maximum differences are reduced from 10–20 to 2–5% after model updating procedure. More information can be found in the cited articles.
