Analysis and modelling of the seismic behaviour of high ... - Ingegneria

2. DUCTILITY AND SEISMIC RESPONSE OF STRUCTURES
against injury or death. 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). So, the first step in the performance basic design
philosophy is to define an acceptable level of damage due to an earthquake, and
this is the purpose of the design code.
Performance-based seismic engineering, elaborated by the Vision 2000 Committee
of SEAOC (1995) and ATC (1995), consists of a selection of appropriate systems,
layout and detailing of a structure, and of non-structural components and contents
so that, at specified levels of ground motion and defined levels of reliability, the
structure will not be damaged, beyond certain limit states. The performance levels
have been defined for four levels as a combination of damage to structure and to
non-structural elements, building facilities and required repairs.
1. Near collapse level: collapse prevention is directly related to the prevention
of casualties and of damage to the contents of buildings. The structure can
undergo serious damage during the major earthquakes, but it must be
standing after the ground motion.
2. Life safe level: the casualties in a building are usually caused by the
collapse of the building components during an earthquake. The evaluation
of the number of casualties as an economic damage in an optimisation
process, as suggested in some studies, poses very difficult ethical
problems.
3. Operational level: a distinction is made between structural damage which
cannot be repaired and damage which can be repaired. Irreparable
damage is a specific subject for individual engineering judgement of
experts. The damage refers both to structure and to non-structural
elements.
4. Fully operational level: 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 owner of a building wishes to avoid the cost
of interruptions, it is necessary to fulfil more than the minimum requirement
of design codes. By using stronger and stiffer designs, it is possible to
reduce or even eliminate, the interruption of the building function after a
strong earthquake, but this results in a more expensive structure.
These limit states are presented in Figure 2.4, as a function both of the structure
and of non-structural elements. In the seismic load-top sway displacement curve,
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