Monitoring strain data provides possibilities for the fatigue assessment of the critical components in real structures under the operational conditions. This paper presents a case study on the investigation of fatigue performance of welded details. Long-term monitoring strain data of 4years between 2006 and 2009 collected by the structural health monitoring system mounted on the Runyang Suspension Bridge and Runyang Cable-stayed Bridge are utilized. The study focuses on two aspects: (i) the effects of different strain components in the raw strain data on the fatigue damage assessment and (ii) the necessity of long-term strain measurement for fatigue evaluation. The results indicate that temperature effect on the stress range spectrum is negligible. The enormous low-level stress cycles caused by random interference would result in erroneous equivalent stress ranges and number of cycles, and thus should be eliminated from the stress range spectrum. Through in-depth analysis of monitoring strain data in three special months and in 3years, respectively, it is revealed that short-term data monitored in a few days and medium-term data monitored in a few months are not adequate enough to reveal the actual fatigue behaviors of steel bridges and would give inaccurate fatigue life prediction. Finally, the fatigue life predictions of two types of welded details by considering traffic flow growth are carried out. Lessons learned from the long-term monitoring are expected to enhance our understanding in fatigue performances of steel bridges. Copyright (c) 2015 John Wiley & Sons, Ltd.