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Traditionally, seismic resistant design of gravity retaining walls is based on a limit equilibrium-based
approach, in which the dynamic forces induced by an earthquake are calculated. A factor of safety was
then introduced to ensure that the retaining wall has sufficient resistance against the tendencies of sliding
and rotating. However, in recent years this design philosophy went through a lot of debating and
discussion because of its limitations. The main points of the discussion, as summarized by the
International Navigation Association (2001), are: 1) in a typical design, deformations in ground and
foundation soils and the corresponding structural deformation and stress states are key design parameters;
2) conventional limit equilibrium-based methods are not well suited to evaluate these parameters; and 3)
some residual deformation may be acceptable for the operation of most structures.
To overcome these limitations, the concept of performance-based design was introduced (lai and Ichii,
1998, Steedman, 1998). The main argument for this new design approach are: 1) if we demand that limit
equilibrium not be exceeded in conventional design for the relatively high intensity ground motions
associated with very rare seismic event, the construction/retrofitting cost will most likely be too high; and
2) if force-balance design is based on a more frequent seismic event, then it is difficult to estimate the
seismic performance of the structure when subjected to ground motions that are greater than those used in
the design.
Two of the most important steps in a performance-based design are to select the acceptable level of
damage for the structure designed and to calculate the deformation of the structure under the design
earthquake so as to check whether the deformation is acceptable. Some guidelines on how to choose the
acceptable level of damage were proposed by the International Navigation Association (2001). Table 1.1
is a summary of its recommendations.
TABLE 1.1
ACCEPTABLE LEVEL OF DAMAGE IN PERFORMANCE-BASED DESIGN
Structural
Minor or no damage
Controlled damage
Extensive damage in near
collapse
Complete loss of structure
Level of damage
Degree I: Serviceable
Degree II: Repairable
Degree El: Near collapse
Degree IV: Collapse
Operational
Little or no loss of serviceability
Short-term loss of serviceability
Long-term or complete loss of
serviceability
Complete loss of serviceability
Of course, for most structures, the purposes of performance-based design is to limit the level of damage to
degree I or maybe degree U in some exceptional cases. The acceptable level of damage depends on the
importance and the function of the structure. Once that is determined, the next step is to choose the design
earthquake. Then, the critical part of the design is to analyze the performance of the structure being
designed during the chosen design earthquake. For a gravity retaining wall, the performance is measured
by the displacement of the wall under the design seismic loading.