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

2. DUCTILITY AND SEISMIC RESPONSE OF STRUCTURES
3. Survivability-ultimate limit state (ULS) for earthquakes which may rarely
occur represents the strongest possible ground shaking. For these
earthquakes, both structural and non-structural damage are expected, but
the safety of inhabitants has to be guaranteed. In many cases damage is
so substantial that structures are not repaired and demolition is the
recommended solution.
Although it is recognized that the ideal methodology would use four or three levels
for design, today current European Code methodologies and seismic design
philosophy can be based on just two levels (Eurocode 8, 2002):
1. Serviceability limit state, for which structures are designed to remain
elastic, or with minor plastic deformations and the non-structural elements
remain undamaged or have minor damage;
2. Ultimate limit state, for which structures exploit their capability to deform
beyond the elastic range, the non-structural elements being partially or
totally damaged.
In the Eurocode 8, the accelerations corresponding to the serviceability limit state
are given as a fraction of the corresponding ones for the ultimate limit state.
Generally, this methodology cannot assure controlled damage, also because the
determination of this relationship is not clearly assigned in the code.
When the number of performance levels is discussed, one must recognize that the
use of four levels is the most rational proposal and that two levels represent the
minimum acceptable option. Since it is questionable to ask design engineers to
perform too much verification, it seems more rational to introduce no more than
three levels of verifications: serviceability, damageability and ultimate (survivability)
levels. To achieve these levels of verifications, the seismic design problem is laid
out through required-available formulation. Currently, the required-available pairs
of three mechanical characteristics considered in seismic design are:
Required rigidity < Available rigidity ( 2.3 )
Required strength < Available strength ( 2.4 )
Required ductility < Available ductility ( 2.5 )
The rigidity verification is a problem of the serviceability level, the strength
verification is related to the damageability level and ductility verification is a
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