Structural Systems - Bridon

Technical InformationStrand Catenary CalculationsWhen strands are used in level or inclined spans, theshape taken by the strand is a catenary. If individual unitloads are attached between the supports as in the case ofa suspension bridge, then the strand will form a series ofcatenaries. For most practical cases where the sag tospan ratio is low, it may be assumed that the form takenby the strand is that of a parabolic arc. The followingformulae are based on this assumption.Level Span - Uniformly LoadedSTxy ytyS/2TIn the case of anchored spans it is also assumed thatthere is no displacement of supports under increasedload. If displacements occur the result is larger sag.The slight error in the load calculation will result in greaterdegree of safety for the cable assembly than thatcalculated.Where multiple loads are suspended in a span, areasonable accuracy can be obtained by assuming thesystem to be uniformly loaded, and adding the weight permetre of the applied loads to that of the strand.For wind loads, the pressure may be taken as0.01 kg/cm 2 at a wind speed of 150 km/h on the projectedarea of the cable. Under ice conditions, the radialthickness may be as high as 15 mm and the weight of ice0.9 g/cm 3 . Under combined wind and ice conditions, itshould be assumed that the wind is acting on thecombined ice and cable diameter, and the resulting forceshould then be found and used as the combined weightper metre.Calculation:horizontal tension t = WS2 or y c = WS28y c8tmaximum tension T = t where tan = 4y c0cos0Svertical deflection at any point in spany =length of rope L = S + 8y c 2W x (S - x)2t3S(approximately)Level Span - Single Load at CentreTSTytPHorizontal catenaryCalculation:horizontal tension t = S(2P+WS) or y c = S(2P+WS)8y c8tmaximum tension T = t when tan 0 = P+WScos02t36BRIDON Structural Systems