This paper discusses the dynamic behavior of twin-box girder bridges under high-speed railway traffic. On the basis of several representative examples derived from recently built high-speed bridges, this contribution examines the effects of transverse bending in the upper slab of these structures and evaluates the bending moments in resonance conditions. The analysis was carried out according to one of the reference norms for the assessment of dynamic effects in high-speed bridges (Eurocode). The results demonstrate that the predicted dynamic response for shorter-span bridges can be unexpectedly higher than the static effects caused by the design loads as a result of transverse resonances induced by the absence of transverse diaphragms between the box girders and the movement of the sliding supports. Moreover, these strong impact coefficients may occur even when the maximum level of vertical vibrations in the deck is not severe.
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