One of the major aftermaths of the Northridge Earthquake of January 17, 1994, was the widespread connection damage that posed a major question regarding the behavior of field-welded, field-bolted moment frame connections, also known as Pre-Northridge connections.
Before the Northridge Earthquake, Steel Moment Resisting Frames (SMRFs) were believed to have ductile behavior that would achieve high-cycle fatigue. As a result, fatigue was not considered to be a failure mode for these connections during a seismic event.
After the Northridge Earthquake and the widespread connection failure in steel moment frame buildings, it was concluded that many connections failed at what appears to be relatively few cycles. Observations after the Northridge Earthquake …show more content…
These buildings were two and ten stories, respectively, and used steel moment frames as the lateral load resisting system in both directions.
Chapter two of this document describes the definition of fatigue failure and summarizes the research done in the area of fatigue behavior of steel moment connections and the concept of low-cycle fatigue.
In chapter three of the current document, the analytical case studies are explained and the investigated buildings are described. Also, the observed damage in these buildings is studied.
Chapter four contains a series of linear and non-linear time-history analyses and includes a very thorough analytical study on the stress histories at the critical locations of the buildings. Furthermore, the contribution of each mode of vibration to total stress is investigated.
Chapter five establishes a comprehensive fatigue analysis procedure, which was developed using the Palmgren-Miner method. In addition, low-cycle fatigue behavior of Pre-Northridge connections are studied in this chapter, and 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. Fatigue analyses are performed …show more content…
The FEMA 350 commentary cites low-cycle fatigue as the main cause of failure in some laboratory connection tests but does not give any information or any possible recommendations on the issue.
Some of the SAC task groups addressed the low-cycle fatigue issue in their individual reports. For instance, the work done by Ricles et al. in 2000  has a chapter on low-cycle fatigue with a proposed method for predicting crack initiation and extension over the life cycle of a connection utilizing finite element analysis , .
Barsom (2000) , concludes that fatigue is the failure mechanism of the connection. This report was never distributed to the practicing engineers, as only selected SAC committee members received it.
The report by Krawinkler et al. (1983)  cites low-cycle fatigue as the failure mechanism of the Pre-Northridge connections. The concept of “cumulative damage” is discussed in this document. The author indicates that each connection remembers the past events, and these past seismic events consume part of the predictable and quantifiable life of a