ynamic effects on highway bridges are usually deemed to be covered by the allowance for impact included in live loadings. However, although such considerations may be sufficient structurally, the possibility of highway users being adversely affected shall also be considered. Complaints about the liveliness of highway structures have been made in Hong Kong as a result of the occupants of traffic stalled in one lane of a structure being subjected to oscillations caused by traffic moving in a neighbouring lane. Similar situations could recur at any time under the conditions prevailing in Hong Kong.
Highway structures oscillate in sympathy with passing vehicles oscillating on their suspensions as a result of road surface irregularities. The worst oscillations occur when the natural frequency of a structure lies within the range of forcing frequencies imposed by passing traffic.
Such forcing frequencies generally range between 2 Hz and 5 Hz. Highway structures shall accordingly be designed so that as far as possible their natural frequencies lie outside this range.
