Progressive damage accumulation and fatigue considerations in structures subjected to an earthquake and aftershock sequence Conference Paper uri icon

abstract

  • The Canterbury area of New Zealand has recently been subjected to a series of large earthquakes. This earthquake and aftershock sequence has been unique in terms of the length and magnitude of the aftershocks. This extensive sequence has comprised over 20 earthquakes with magnitude 5.0 or higher, with many having epicentres very close (<5km) to the city of Christchurch. This unusual aftershock sequence has highlighted the cumulative damage and demand imposed upon structures within the region. Two key factors lead to the overall damage effects on structures. The first is the maximum response displacement or drift that arrives during shaking. This maximum response is often the primary focus of structural design through spectral analyses. The second and often neglected effect is the duration of the earthquake and the cumulative damage effect caused by the repeated cyclic loading, both within an event and through multiple events during an aftershock sequence. The effects of low-cycle fatigue on civil structures under earthquake loading are very important. Although structural engineers are not explicitly required by code to design for low cycle fatigue effects, designers are required to be aware of the ramifications of low cycle fatigue. This manuscript investigates the cumulative demand placed on structures subjected to the Canterbury earthquake sequence. This investigation considers all major earthquakes (M5.0 or higher) that occurred, from the September 4, 2010 main shock, up to an including the events of February 2011. The investigation presented proposes an analysis option to undertake rapid assessment of the cumulative demand in the days following a major earthquake. This approach applies simplified, rapid, cycle-counting techniques to time-history responses to approximate the overall number of equivalent design cycles that structures have undergone. These techniques are applied with different fatigue exponents to represent concrete-critical fatigue, reinforcing-steel critical fatigue and structural-steel critical fatigue consideration. These different fatigue exponents place different weighting on smaller amplitude cycles to provide an indication of overall demand. The results are presented as fatigue demand spectra. Cyclic demands are plotted for structures with natural periods from 0.1 to 5.0 seconds. For each structural period, time-history responses are normaliszed to an equivalent number of cycles at the amplitude specified for that period within the New Zealand Structural design code. The results can then be used to approximate the cumulative demand and provide an estimate of the remaining fatigue life of the structure. While the normalizations presented are specific to the New Zealand design code, the analysis methods are generalizable to equivalent design code requirements worldwide.

published proceedings

  • NCEE 2014 - 10th U.S. National Conference on Earthquake Engineering: Frontiers of Earthquake Engineering

author list (cited authors)

  • Rodgers, G. W., & Mander, J. B.

complete list of authors

  • Rodgers, GW||Mander, JB

publication date

  • January 2014