The dynamic characteristics of a multi-span pre-stressed concrete urban bridge before, during and after widening operations have been studied. The widening operations have been carried out by adding two new bridges on the either sides of the old bridge. The decks of the new bridges have been structurally separated from the old one by two longitudinal joints on the either side of the old bridge. The dynamic characteristics have been extracted from operational modal tests in three phases during different stages of the widening operations. The dynamic interaction between the new parts and the old part of the bridge has also been investigated. The results show that after the completion of the widening works, and when the whole bridge was placed in service condition, this interaction affects the natural frequencies of almost all the vibration modes of the bridge. It was also found that the longitudinal joints were unable to effectively isolate the new parts from the old part. Finally, in order to achieve a consistent behaviour in the first bending mode of the new and old bridges, a geometrical index has been introduced. This index includes both the effects of stiffness and mass properties of the bridge deck. In widening projects, where the span lengths, material properties and support conditions of the two bridges are taken the same, adopting equal geometrical indices leads to almost equal values of the natural frequencies and similar mode shapes in the first bending mode. The proposed index can be used in the initial design stage for such projects.

The fundamental frequency of vibration of a bridge deck is a key parameter that represents the dynamic characteristics and the condition of the superstructure. This paper reviews the different empirical and/or theoretical formulae that have been proposed in the past. The classical vibration theory of beams and plates was simplified in order to clarify and review the general philosophy behind the previously proposed formulae. On the basis of the results obtained from limited dynamic tests performed by the authors on a number of reinforced concrete bridges and the results of limited tests conducted by other researchers, a formula for the first bending natural frequency of a bridge deck was derived. Moreover, as new applications, new practical formulae are introduced that can be used for estimating the initial dimensions of a deck cross-section for simple span bridges, which are particularly suited to bridge-widening projects. The first bending natural frequency/period of the bridge deck and its corresponding cross-sectional properties are considered here as functions of the bridge main span length.

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