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(November, 1983)Covers should "be designed to withstand allthe appropriate pemanent^ i^iporary and transientloads described in BS 5400 t Part 2® In addition^covers should be designed to resist a live load of0*5 kl/in 2 | which shall be considered as a secondarylive load in conjimction with the other loads andpartial load factors appropriate to combination 4®No other primary or secondary live loads need beconsidered®4*2»124*2*12*1 Aerodynamic effectsThe possibility of wind-excited oscillation®occurring should be considered f and due allowancemade for their effects* If necessary , aerodynamiceffects should be investigated by testing* Flexiblestructures such as suspension bridges, cable-stayedbridges and sign gantries are particularly susceptible*4*2*12*2 Highway bridge®Dynamic effects on highway bridges are usuallydeemed to be covered by the allowance for impactincluded in live loadings* However t although suchconsiderations may be sufficient structurally! thepossibility of highway users being adversely affectedshould also be considered* Complaints about theliveliness of highway structures have been made inHong Kong as a result of the occupants of trafficstalled in one lane of a structure being subjectedto oscillations caused by traffic moving in aneighbouring lane* Similar situations could recurat any time under the conditions prevailing in BongKong*Highway structures oscillate in sympathy withpassing vehicles oscillating on their suspensions asa result of road surface irregularities* The worstoscillations occur when the natural frequency of astructure lies within the range of forcing frequenciesimposed by passing traffic*Such forcing frequencies generally raiagebetween 2 Ha and 5 HJS* Highway structures shouldaccordingly be designed so that as far as possibletheir natural frequencies lie outside this range,