In LRFD, fatigue is only checked if the fatigue load groups is selected and is carried out as specified in LRFD Art. 5.5.3. For AASHTO LFD, it is only checked if at least one combination for service load is selected and performed in accordance with Art. 8.16.8.3, with one modification to fmin.

For LRFD, Substructure checks the stress range in the reinforcement for fatigue load combinations. If all load combinations produce tension in rebar, the stress range is equal to the largest stress value. However, if some combinations produce compression and others produce tension in the reinforcement; the stress range is obtained by adding the two absolute stresses together. The minimum stress level, fmin, is computed due to larger of the permanent loads and the permanent loads plus creep and shrinkage accompanied with minimum live load.

The calculation of stress range and fmin are the same for AASHTO LFD.