Selected methods of reducing the deck transverse vibrations in long-span steel footbridges – case studies

Horizontal vibrations of the deck in lightweight long-span footbridges, especially those made of steel, with a low damping level may significantly reduce the comfort of use of the whole structure. The natural frequencies of such footbridges are very often in the range of 0.5 ÷ 1.0 Hz. This can lead to the excitation of resonant vibrations of these structures by walking people generating horizontal transverse forces characterised by a large amplitude of harmonic components corresponding to two frequencies 0.5fs and 3/2fs (where fs – frequency of steps during walking. The article presents examples of structural solutions used in long-span suspension footbridges built in Poland in order to reduce the horizontal transverse vibrations of the footbridge decks. The results of dynamic field tests and numerical calculations of two footbridges were presented and analysed. The obtained results indicate a positive effect of reducing the horizontal vibrations of the deck in both footbridges and changes in the frequencies and the mode shapes of the analysed footbridges caused by applied solutions.