Ground Motions and Structures - Earthquake Engineering for Structural Design - page 314

314 Earthquake Engineering for Structural Design

Stories

Time (sec)

Figure 8.16 Wave-propagation on a framed structure ( 75 m in 0.4 seconds)
(after Kohler et al, 2007)
Stiffness. Usually, the structure behaves in elastic range until the first yielding (or
plastic hinge) forms and both structure and non-structural elements remain without (or
with minor) damage. The structural behavior is linear and the traditional analysis
methods can be used to determine the structural response. In this range, the knowledge
of the actual stiffness is the main target.
Strength. When the structure is subjected to loads which overcome the elastic limit,
a number of plastic hinges forms, taking advantage from the structure redundancy.
The non-structural elements will be partially or totally damaged. The structural
behavior is strongly nonlinear and an elastic-plastic analysis method has to be used,
considering the progressive history of the plastic hinge formation.
Ductility. After the formation of a sufficient number of plastic hinges, the structure
is transformed in a plastic mechanism. In this field of the structure behavior, the
kinematic analysis method, which considers the element of ductility, must be used.
Multiple configurations of plastic mechanisms are possible for most structures. The
one caused by the smallest lateral forces is likely to appear before the others do. This
mechanism is considered to be the dominant mechanism.