Ground Motions and Structures - Earthquake Engineering for Structural Design - page 316

316 Earthquake Engineering for Structural Design

the structural response : stiffness, strength, ductility and robustness .

8.3.3 StiffnessandInterstoryDrifts

Stiffness is the property of a structure to resist displacement. Traditionally, strength
checking is considered by designers as the primary goal of the design process. In the
last time, due to the social and economical impact of the loss of functionality in
buildings, there is a particular focus on the control of damage through the stiffness
checking, in order to maintain the architectural integrity of non-structural elements.
The stiffness checking is performed for low seismic action, when the structure works
in linear elastic range . The top displacement of the structure can provide a good
indication of the structural damage, but it cannot adequately reflect the damage of non-
structural elements, which are more dependent on the relative displacement between
two storys, the so-called interstory drift (Fig. 8.18), the location of them depending on
the ground motion type. Deep earthquakes in far-field regions, the first vibration mode
being dominant, produce interstory drifts with maximum values in the lower part of
the structure (Fig. 8.19a). The structure deformations for ground motions in the near-
source regions are qualitatively different. In this case, the second and third modes are
dominant, so the maximum interstory drifts occur in the middle or at the top part of
structures (Fig. 8.19b). At the same time and consequently, in the first case the plastic
hinges take place in lower parts, while in the second case they are concentrated in the
middle or at the top of the structure.
The condition of non-damage to non-structural elements must be verified in elastic
range for given interstory drift limits, which take into account their integrity due to the
interaction with structural elements. Figure 8.20 presents the effects of this interaction
between a five levels framed structure with a core containing the staircase and
different panel types. One can see that an important reduction of drifts can be obtained
by considering this interaction, respect the behavior without panels.

8.3.4 Strengthand Redundancy

The strength is the property of a structure to resist forces. Redundancy is the property
to spread the damage among several elements, avoiding the concentration of failure at
an individual structural element. The elastic behavior ends when the first plastic hinge
occurs. It is followed by the elastic-plastic range, where a sequence of formation of
plastic hinges induces a very important non-linear mechanical behavior (Fig. 8.17).
The elastic-plastic range depends on the structural redundancy. Redundancy is a
positive and desirable feature of a structure. A non-redundant system is the one where
the failure of a component is equivalent to the failure of the entire system. Contrary, in
a redundant system, all the components are participating in carrying load and the