Northridge Earthquake Case Study

explanatory Essay
2026 words
2026 words

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…

In this essay, the author

  • Explains that if there are defects in the material, the crack initiation step could be eliminated, causing the total fatigue life to decrease.
  • Explains that nf represents failure, ni is the number of cycles for crack initiation, and n p shows crack.
  • Explains that the northridge earthquake and the widespread connection failure in steel moment frame buildings posed a major question regarding the behavior of pre-northridge connections.
  • Explains that the sac steel project was initiated by fema as a joint venture between structural engineers association of california, applied technology council, and consortium of universities for research in earthquake engineering.
  • Explains that low-cycle fatigue combined with higher modes of vibration can increase the cumulative fatigue at critical connections and cause connection failures.
  • Summarizes the research done in the area of fatigue failure of steel moment connections and the concept of low-cycle fatigue.
  • Describes the results of the study and compares the pattern of cumulative fatigue at critical connections to the observed damage.
  • Explains that structural steel is an excellent building material, but there are still concerns with its fatigue behavior and possible cyclic fatigue failures of steel components.
  • Explains that fatigue is a type of fracture failure caused by cyclic or repeated loading. fatigue failure can be represented in three simplified steps.
  • Explains that the concept of high-cycle fatigue is often used to 7 represent the situation, whereas when the number of cycles is not large, the condition will be referred to as low-cyclic fatigue.
  • Explains that some of the sac task groups addressed the low-cycle fatigue issue in their individual reports.
  • Explains that bertero and popov (1965) discusses low-cycle fatigue as a potential cause of failure in the steel members through tests on beam specimens with large deformations.
  • Explains that partridge et al. (2000) show that fatigue is the principal failure mechanism of the pre-northridge connection. the low-cycle fatigue issue has been addressed in the literature before and after the northridge earthquake.

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 [37] 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 [37], [38].
Barsom (2000) [6], 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) [26] 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

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