Progressive deterioration of concrete bridge structures has become a serious problem. The reasons of this phenomenon are especially a repeated great static loading a dynamic loading alkali silica reactions and frost-thaw influence. The importance of an observation on real structures in order to obtain the experimental results necessary for the development of theories for prediction of service life increases.

The investigated road bridge across the river Radbuza at Pilsen town was built in 1979. Today the majority of the heavy traffic passes over this bridge from south Germany to Prague in three traffic lanes and so this bridge is under great static and dynamic load. The bridge was in detail experimentally investigated. The passages of two trucks over the bridge were carried out in April 1997. The modal analysis of this bridge was executed repeatedly in April 1997 and in May 1998.

For both executed modal analyses the organisation of measurements was identical. To determine the modal parameters of the bridge (frequencies, modes of natural vibration and corresponding damping values), the construction was excited by the electrodynamic exciter TIRAVIB 5140 that produced the driving force of random character. A response of the bridge structure was measured in the selected network of points on the upper bridge surface. The total number of network points was 176 (22 lateral profiles, 8 points in each profile).

The frequency, corresponding damping values and modes of natural vibration of the observed bridge structure were evaluated using the Star software of the Spectral Dynamic company. In total, eight basic frequencies and modes of natural vibration were determined experimentally. The resulting modal parameters of both periods of the measurement were compared. The comparison of the appropriate natural modes is realised by means of a MAC and a COMAC. During the observed period (April 1997 - May 1998) the demonstrable decrease of the all compared natural frequencies of the bridge occurred. In the majority of cases the values of the change of frequencies were situated in the range from 3,5 % to 5 %. All frequencies of damping decreased too.