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4.2.11Footbridge and subway coversV.4.2.11Amendment No. V-l/89 (Feb.)Covers should be designed to withstand all theappropriate permanent, temporary and transient loadsdescribed in BS 5400 : Part 2. In addition, covers shouldbe designed to resist a live load of 0.5 kN/m 2 , which shallbe considered as a secondary live load in conjunction withthe other loads and partial load factors appropriate tocombination 4 . No other primary or secondary live loads needbe considered.4.2.12 Dynamic effects4.2.12.1 Aerodynamic effectsThe possibility of wind-excited oscillationsoccurring should be considered, and due allowance made fortheir effects. If necessary, aerodynamic effects should beinvestigated by testing. Flexible structures such assuspension bridges, cable-stayed bridges and sign gantriesare particularly susceptible.4.2.12.2 Highway bridgesDynamic effects on highway bridges are usuallydeemed to be covered by the allowance for impact included inlive loadings. However, although such considerations may besufficient structurally, the possibility of highway usersbeing adversely affected should also be considered.Complaints about the liveliness of highway structures havebeen made in Hong Kong as a result of the occupants oftraffic stalled in one lane of a structure being subjectedto oscillations caused by traffic moving in a neighbouringlane. Similar situations could recur at any time under theconditions prevailing in Hong Kong.Highway structures oscillate in sympathy withpassing vehicles oscillating on their suspensions as a resultof road surface irregularities. The worst oscillations occurwhen the natural frequency of a structure lies within therange of forcing frequencies imposed by passing traffic.Such forcing frequencies generally range between2 Hz and 5 Hz. Highway structures should accordingly bedesigned so that as far as possible their natural frequencieslie outside this range.