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4*2«12«3V*4»2*12*3(November, 1983)Pedestrians can be adversely affected by thedynamic behaviour of footbridges® To avoid unpleasantvibrations § the natural frequencies of footbridgesuperstructures aad columns should if possible exceed5 Hz*If the natural frequency of a footbridgesuperstructure or column in less than 5 Hz, the maximumacceleration should be limited to an acceptable value*Columns should only be used if their natural frequenciesexceed 2 Ez transversely and 1 Hz longitudinal ly*Appendix 4*2*2 gives guidance on the calculationof natural frequencies and accelerations, and quotesacceptable values for accelerations.The possibility of a group of small boys, orother pedestrians, deliberately causing a footbridge tooscillate resonantly should be borne in mind* Jbotbridgebearings should be designed to allow for this possibility,and prestressed concrete beams should be provided withsufficient untensioned reinforcement to resist a reversalof 10$ of the static live load bending moment. Guidesshould be provided where necessary to prevent any tendencyfor a superstructure to bounce off its bearings*4*2*13 Pj&gtfi ASM!In assessing dead load, the weight of concreteshould be taken as not less than 24*5 kN/m 3 » If thestructural concrete of the deck of a structure is to beused as the running surface, the assessment of dead loadshould include allowance for a minimum extra thickness of25 mm of concrete*If the running surface is to consist of asphalt fthe assessment of superimposed dead load should includeallowance for a minimum thickness of 100 mm of asphalt icsurfacing material*The values of dead load and superimposed deadload assumed for preliminary design purposes should becarefully checked against the final values, when known,and, if necessary, the calculations should be appropriatelyamended*