The experimental modal analysis is often used to validate the accuracy of dynamic numerical models. It is also a good tool to obtain valuable information about current condition of the structures that could help to determine residual lifetime. The quality of modal testing results is highly dependent on the proper estimation of the natural frequencies from the frequency response function. This article presents the experimental modal test of the laboratory steel structure in which the natural frequencies and mode shapes are determined.

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Modern architecture leads to design subtle bridge structures that are more sensitive to increased dynamic loading than the massive ones. This phenomenon can be especially observed on lightweight steel structures such as suspended footbridges. As a result, it is necessary to know precisely its dynamic characteristics, such as natural frequencies, natural shapes and damping of construction. This information can be used for further analysis such as damage detection, system identification, health monitoring, etc. or also for the design of new types of construction. For this purpose, classical modal analysis using trigger load or harmonic vibration exciter in combination with acceleration sensors is used in practice. However, there are many situations where it is not possible to stop the traffic or operation of the bridge. The article presents an experimental measurement of the dynamic parameters of the structure at the operating load using the operational modal analysis.

Full–scale dynamic testing of bridge structures can provide valuable information on the service behavior and performance of structures. With the growing interest in the structural condition of highway bridges, dynamic testing can be used as a tool for assessing the integrity of bridges. From the measured dynamic response, induced by instructed passing trucks, modal parameters (natural frequencies, mode shapes and modal damping values) and system parameters (stiffness, mass and damping matrices) are obtained. These identified parameters can then be used to characterize and monitor the service of the bridge structure in the future. Analytical models of the structure can also be validated using these parameters [1,2]. The paper presents a procedure for estimating the traffic load bearing capacity of the steel arch highway viaduct Bridge Structure 205 (DC1–9, 755 m) constructed on Highway D1 in Nord Slovakia (Fig. 1) over the natural hollow basin via dynamic tests of the viaduct structures.

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