ABSTRACT

The structure design procedure on the basis of multi-level criteria is not a new concept. Under gravity, live, snow, wind loads, the limit state design considers the service and ultimate levels. In the case of seismic loading, the declared intent of building codes is to produce buildings capable of achieving the following performance objectives (Fajfar, 1998):

-to resist minor earthquakes without significant damage; -to resist moderate earthquakes with repairable damage; -to resist major earthquakes without collapse. However, as a rule, the majority of codes consider explicitly only one performance

objective, defined as protection, in cases of rare major earthquakes, of occupants against injury or loss of life. Criteria for structure checking to minor or moderate earthquakes that may occur relatively frequently in the life of the building are not specified explicitly. A review of 41 codes elaborated all over the world shows that 38 are based on just one level, the principal design being concentrated on strength requirements (Bertero, 1997). During recent earthquakes, including those of California and Japan, structures in conformance with the modern seismic codes performed as expected: the loss of lives was minimal. However, the economic loss due to sustained damage was substantial. Earthquakes in urban areas have demonstrated that the economic impact of physical damage, loss of function and business interruption was huge and the damage control must become a more explicit design consideration. So, in the recent years, a new philosophy for building design has been discussed within the engineering community, adopting a performance based seismic design philosophy. In the United States, the Vision 2000 Document provides the foundation concepts for this approach (Bertero, 1996a,b). The goal of this design philosophy is to produce structures that have predictable seismic performances under multiple levels of earthquake intensity (Leelataviwat et al, 1999).