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There is a long tradition in the Czech and Slovak Republic to investigate the effect of moving load on the bridge structures [1], [2]. The theoretical approaches have been also supported with the experimental investigation [3], [4]. The dynamic experimental measurements of real bridge structures are an important source of data that can be used in different ways. Detection of modal characteristics is one of the most important [5]. Knowledge of them is an essential assumption for various applications like model updating, dynamic load identification and health monitoring, or a damage detection problem. There seems to be important to make the right decision on which suitable excitation technique should be used with respect to the conditions that are unique for each bridge structure. From a practical point of view, diagnostic methods, like operational modal analysis (OMA), are preferred because no excitation devices are used and the traffic on the bridge is not limited [6]. Even though that these techniques are useful, it is not clear if they are useful for all conditions. The main issue in this case is to apply various excitation sources and to analyze effect of them on the obtained modal frequencies of the bridge. The first source of excitation is the impact device, which loads the bridge with an impact impulse (impulse load) in the selected points. In this case, the intensity of impulse is also measured so the techniques of experimental modal analysis (EMA) are used to obtain the modal shapes and frequencies. The second source of excitation is a heavy vehicle passing over the bridge. In that case, the measured dynamic properties of the system vary over time as the vehicle changes its position on the bridge. The advantage is that the weight and position of the vehicle during the measurements are well-known. The ambient vibrations, caused by a train passing over the rail located under the bridge, is the third source of the excitation [7]. This kind of load can be defined as purely random ambient excitation. The speed and weight of the train are not clearly known, as they were not investigated. The same methodology of transformation of the obtained measured signals into the frequency-domain is chosen for all the three cases (sampling frequencies, filtering, time window, etc.). In the cases when the bridge is excited by an impact device and by the passing train, the frequency response functions (FRF) were also calculated by applying the techniques of OMA and EMA. The article also presents the application of measured data to verify the numerical model of the bridge. The presented study is focused on the road bridge situated between the two villages, Varin and Mojs, near the city Zilina in the Slovak Republic (Figure 1a). The full length of three span bridge is 87 m. Each span acts independently as a simple supported beam. The main girders 25 I-73 are prefabricated prestressed concrete structures with the length 29 m. The girders are placed in the transversal position with the mutual distance 1.441.45 m. The cross-section of the bridge contains eight girders. The shapes of the girders and the layers of pavement are showed in the Figure 1b.

Identification of Modal Frequencies From the Pre-Stressed Concrete Bridge Dynamic Response to Different Sources of Dynamic Excitation