44 X. Wei et al. 10 20 30 40 50 1 x z y 10 20 30 40 50 x y 10 20 30 40 50 1 x z (a) (b) (c) Fig. 5.12 The third vertical bending dominated mode (f D2.21Hz, D0.96%) 10 20 30 40 50 0 1 x z y 10 20 30 40 50 x y 10 20 30 40 50 0 1 x z (a) (b) (c) Fig. 5.13 The fourth vertical bending dominated mode (f D2.71Hz, D1.89%) These nine modes include three lateral bending, five vertical bending and one torsional. The damping ratios of most modes were found to be >1%, and this provided evidence to justify a recommendation in vibration serviceability design to have the damping ratio at 1% or higher. Despite some vibration modes having natural frequencies excitable in resonance by human walking actions the relatively high damping in the Wilmott footbridge ensures satisfactory serviceability performance.
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