Fatigue Provisions of AASHTO Specifications

The statistical properties of fatigue strength of various types of weldments are analyzed from which the allowable fatigue stresses are computed. Results compare favorably with those of the AASHTO Fatigue Specifications (1973). The North Carolina 1972 vehicular traffic data are used to derive the statistical properties of vehicle type, weight, and traffic intensity. The information of fatigue strength and vehicular traffic are used to compute the probabilities of fatigue failure of weldments of two bridges, one narrow and the other of the wider variety, both designed according to the AASHTO Specifications (1973). Results show that the probability of failure values of the weldments of these two bridges differ drastically, the difference being attributable to the lateral load distribution factor recommended by the specifications. The manner in which the lateral load distribution factor may be revised so as to yield structures with desired but uniform reliability against fatigue failure is given.