Earthquake Return Period and Its Incorporation into Seismic Actions

Abstract

Seismic-resistant design of structures aims primarily at preventing loss of life and global collapse. The most common approach used worldwide is the design of structures with a large ductility capacity, thus, to be able to withstand inelastic deformation during strong ground shaking with a small collapse probability. However, the anticipated damage to structural and nonstructural components and contents that is caused by large deformations can have serious impact on postearthquake functionality and often require, if not demolition, certainly costly and disruptive repairs. On the other hand, the design of stiffer and stronger, non-ductile structures could minimize deformations and therefore damages. However, issues such as increased material quantities, structural accelerations, and damage to acceleration-sensitive nonstructural components and building contents have to be carefully assessed in such design approaches. An alternative approach toward the limitation of structural damages under strong ground shaking is the modification of structural characteristics, in a way that the capacity is increased and/or demand is reduced. Modern technologies that can be implemented for these purposes are damping devices, base isolation schemes, or a combination of both. Earthquake frequency occurrence or return period is directly incorporated into seismic actions and contemporary seismic norms, since in general, seismic-resistant structures must have (i) the capacity to resist frequent (with return period 50–100 years), minor earthquakes without damage, (ii) the capacity to resist with limited structural and nonstructural damage infrequent (with return period approximately 500 years) moderate earthquakes, and (iii) the capacity to resist without collapse and life safety endangerment very strong and rare earthquakes (with return period approximately 2,500 years). For more critical infrastructures, even higher seismic demand levels are imposed which is interpreted in terms of a very large return period, e.g., 10,000 years for maximum design earthquake (MDE) motions for major dams in New Zealand. The majority of seismic norms belong to the category of prescriptive design codes (or limit state design procedures), as their aim is to ensure that the structural design satisfies a number of checks in order to be considered safe and reliable against collapse. A typical limit state-based design can be viewed as a one-limit state, ultimate strength, or a two-limit state approach, serviceability and ultimate strength. Most of the prescriptive design codes are based on the assumption that structures are capable of dissipating energy through inelastic deformations. It is recommended to design