Challenges for the Next Generation of Seismic Codes - Earthquake Engineering for Structural Design - page 471

earthquakes, by increasing the accelerations, but this aspect is not sufficient to
consider all the effects of this earthquake type. Eurocode 8 proposed two types of
design spectra, in function of the magnitude range. This specification can be
considered as corresponding to two earthquake types: crustal interplate and
intraplate earthquakes. Therefore, for the next generation codes, it is required to
setup some clear criteria for establishing the pattern of design spectra in function of
the earthquake type.
It is a matter of fact that the code provisions are very poor for the ductility
demands. Instead of having methodologies to evaluate the ductility of structural
elements, the code provisions just refer to some doubtful constructional
requirements, in order to obtain a satisfactory ductility. Based on the SAC
connection tests performed after the Northridge joint failure (SAC, 1996), it is
accepted that members and connections should be capable to develop a minimum
plastic rotation capacity of the order of 0.025 to 0.30 radian (FEMA 267, 1997).
These values were adopted by many other codes. But some important questions
arise concerning these tests, which were performed using quasi-static procedures
with increasing displacement history and with a considerable number of cycles
until the connection failure. Can these tests interpret the dynamic actions of
ground motions with the same characteristics as the ones developed in near-source
zones? It is very well known that the pulse velocities can be so high that they
cannot be modeled in laboratory. Another question refers to the generality of the
limit values of rotation capacities, considering the great differences in demand for
different earthquake types, which must be reduced in case of intraplate
earthquakes. Therefore, the next generation codes must introduce proper provisions
for checking the ductility and fracture methodologies like the ones used for
displacements and strength (Gioncu and Mazzolani, 2002).

10.4 CHARACTERISTICS OF EARTHQUAKES IN FUNCTION OF
SOURCE TYPES

10.4.1 Main Factors To Be Considered in Seismic Design

In the frame of Earthquake Engineering, today the unique big challenge of seismic
design is to provide reliable structures resisting the ground motions resulting from
earthquakes. Earthquakes produce large forces of short duration which must be
resisted by the structure without causing collapse and preferably without
significant damage to the structural elements. The main factors, which must be
considered in design process, in function of earthquake type (Table 10.1):
- Interplate (subduction or strike-slip).

- Intraplate (convergence or crust fracture)

- Intraslab