Advances in Seismic Design Methodologies - Earthquake Engineering for Structural Design - page 388

388 Earthquake Engineering for Structural Design

- Definition of multi-level appropriate design criteria and specification of ground
motion levels, corresponding to the different design criteria.
- Consideration of a conceptual overall seismic design in function of these levels
and option for a suitable structural analysis method for each level.
The first step in the performance-based design philosophy is to define an acceptable
damage level due to a given earthquake. There is no general agreement on this damage
level, but there are some generally accepted criteria for determining these
performances:
- Collapse prevention directly related to the life safety in order to prevent loss of
life, injuries and damage of the contents of buildings. The structure can undergo
serious damage, but it must stand after the earthquake.
- Reparable damage. A distinction is made between the structural damage which
cannot be repaired and that which can be repaired. Irreparable damage is a specific
subject for individual engineering judgment of experts. The damage refers both to
structural and non-structural elements.
- Acceptable business interruption, which can be borne by the owner. In some cases,
the value of the business is more important than the value of the buildings
themselves and the interruption of this activity is intolerable. If the building owner
wishes to avoid losses due to these interruptions, a stronger and stiffer design than
the minimum required by design codes is necessary. But the owner must be fully
aware about the supplementary cost for this requirement.
The second step in a comprehensive seismic design is the definition of multi-levels
appropriate design criteria to satisfy the performance objectives. A building can be
subjected to low, moderate or severe earthquakes. It may survive these events
undamaged; it can undergo slight, moderate or heavy damage; it may be partially
destroyed or it can collapse. These levels of damage depend on the earthquake
intensity. The low intensity earthquakes occur frequently; the moderate earthquakes are
rarer, while the strong earthquakes occur once or maximum two times during the
structure life period. It is also possible that a devastating earthquake will not affect the
structure during its life.
Under these conditions, the relation between multi-level design criteria and the
performance objectives can be summarized in the following:
- Frequent low earthquakes must be associated with the acceptable business
interruption.
- Rare moderate earthquakes must be considered for reparable damage.
- Very rare strong earthquakes are related to collapse prevention.
The next step is to adopt an appropriate conception for each level and a suitable
structural analysis method. The strategies for each performance level are:
- Elastic analysis of structure for frequent low earthquakes, so the non-structural
elements suffer only minor damage. For assuring their integrity, interaction
between structure and non-structural elements must be considered.