Ground Motions and Structures - Earthquake Engineering for Structural Design - page 320

320 Earthquake Engineering for Structural Design

First plastic hinges

Collapse mechanism

Figure 8.22 Effect of redundancy during earthquake (Gioncu and Mateescu, 1998)

8.3.5GlobalandLocalDuctility

Ductility is the ability of a structure to develop its bearing capacity in inelastic range
and to maintain it during an earthquake. Under normal conditions, a structure
experiences elastic deformations, deforming when the force is applied and returning to
its original shape when the force is removed. However, extreme earthquake forces may
generate inelastic deformations, from which the structure does not return to its original
shape after the force is removed. The ductile structure is built by ductile elements
(local ductility). Therefore, structures composed by such elements tend to withstand
earthquakes in very good conditions (global ductility).
In the phase of structure pre-collapse, when the number of plastic hinges is
sufficient to produce a plastic mechanism, the structural analysis can be performed by
means of the kinematic theorem of plastic collapse , which is based on the concept of
mechanism equilibrium curve (Mazzolani and Piluso, 1996). Basically, it has been
observed that the collapse mechanism of frame subjected to the lateral forces can be
classified in four types: global mechanism, mechanism affecting the lower or upper