Truck Moment Range from Single Concentrated Load

Recently recommended guidelines for fatigue design and evaluation of steel bridges employ a truck with an increased spacing between back axles, compared to the standard truck used for strength design. Separate structural analyses must be performed for the fatigue truck because the change in axle spacing may not be accounted for by applying a linear correction factor to the results obtained for the strength‐design truck. The moment range caused by the fatigue truck may be determined very conveniently, however, from basic bridge design aids, and the use of customized bridge‐analysis software is not required. A series of coefficients is presented for symmetrical, constant‐EI bridges that relate the moment range caused by the fatigue truck to the maximum moment produced by the truck resultant. These coefficients enable one to perform a simple analysis involving the single placement of an isolated concentrated load to obtain the moment range, rather than more extensive analyses that require several trial placements of a three‐axled vehicle. Extremely quick checks may be conducted for preliminary design purposes, or relatively accurate results may be obtained with simple linear interpolation. Accuracy from the coefficients ranges from less than 1% below the correct value to a maximum of approximately 17% above the true range. Several examples are presented to illustrate the application of the procedure.