Repetitive loading from locomotives and rolling stock can cause fatigue cracks to develop and grow in steel bridges. To guard against this possibility, a cyclic stress life is assigned to bridges, and allowable fatigue stresses are specified for various details as subsequently discussed.
Often, however, the damage is caused by secondary loadings that were not considered in design. For example, live loads may deflect one girder more that an adjacent girder in a multigirder bridge. This can cause the cross frames connecting the girders to induce large out-of-plane distortions and transverse bending stresses in the girder webs. Such conditions can usually be avoided by careful detailing.
The number of stress cycles (N) assigned to bridge members for design is based on the bridge span length or the number of loaded tracks, depending upon the component. It is  assumed that the structure has been designed for specified loadings in accordance with recognized acceptable practices. As indicated in Table 11.32, the number of stress cycles specified for the various components falls into one of two categories either 2,000,000 cycles or over 2,000,000 cycles. For bridge span lengths greater than 300 ft the number of relevant load cycles should be reviewed in accordance with the AREMA Manual, Commentary, Chapter 15.

The allowable fatigue stress range for various details has been determined by tests of large scale members. The details have been classified in categories designated A through F. In design, the member must be proportioned so that the stress range at each detail does not exceed the allowable range, which depends on N. Table 11.33 lists the allowable fatigue stress ranges for the various details in other than fracture critical members. The allowables are for bridges designed for E80 live loads. The details for the various categories are depictedin the AREMA Manual and should be carefully reviewed in usage that may differ from highway bridge design. Also, refer to the AREMA Manual for the fatigue design of members or components designated as fracture critical. (See Art. 11.10 and 6.22.)


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