The Effects of Low-Cycle Fatigue in Steel Structures

Structural steel is known to have a finite fatigue lifespan. This paper briefly discusses the concept of fatigue in steel structures and investigates the contribution of this phenomenon to the connection damage observed in steel moment frames after the Northridge Earthquake. A comprehensive fatigue analysis procedure is developed based on the Palmgren-Miner method and the S-N curves established for the high-cycle fatigue range are extended to the low-cycle region using the limited test results that are available. An analytical case study on a ten-story steel moment frame building is carried out and fatigue analysis is performed at critical locations of a representative moment frame using the established procedure. Finally, the pattern of cumulative fatigue at critical connections and the observed damage are compared. Results of this analytical study indicate that the effect of low-cycle fatigue may be significant in evaluating and predicting various types of connection damage similar to those observed in steel moment frames following the Northridge Earthquake.