Ground Motions and Structures - Earthquake Engineering for Structural Design - page 327

Ground Motions and Structures 327

these exceptional situations, considering the possibility of formation of plastic hinges
in elasto-plastic structures. But this property is not always sufficient to protect the
structure in case of very strong earthquakes. The building collapses during the 1985
Mexico City, 1994 Northridge, 1995 Kobe, 1999 Kocaeli and 1999 Chi-Chi
earthquakes clearly exemplify the cases when the predictable ground motion
characteristics have been exceeded.
An important question is to know what happens to the structural system as soon as
its maximum loading response is exhausted. The answer to this question belongs to a
new field of research, which has not been sufficiently explored until now. According
to Earthquake Engineering, the robustness can be defined as the residual resistance
(Fig. 8.28), beyond the formation of the global plastic mechanism, when an
unexpected event occurs. In the range of residual resistance, the structural systems
respond in a ductile or brittle fashion, depending on their typological, geometrical and
organizational configuration, as well as on the materials they are made of (Vogel,
2005).
In a robust structure, the resistance to seismic loads does not disappear suddenly
after the maximum action is reached and the structure will stand up, permitting
evacuation and repair with an acceptable expense. In this condition, the structure
failure can be defined as ductile collapse. Contrary, if this condition is not satisfied by
a non-robust structure, after reaching the maximum loading value, a brittle
catastrophic collapse occurs.
The structures are designed according to the code loads, which are determined with
a high level of uncertainty for the earthquake events. In many cases, these values are

Structural Residual

Load resistance resistance

Displacement

Figure 8.28 Structural and residual resistance