Engineering Know-How: Lessons from Earthquakes and Rules for Seismic Design - Dynamic Behavior of Concrete and Seismic Engineering - page 341

Under seismic actions that are reduced by the behavior coefficient, the code
demands that the engineers must demonstrate the overall stability of the structure,
the strength of the soil (without any intensive permanent strain) and the strength of
the foundations.
In addition, the non-structural elements should neither be dangerous for people
nor affect the dynamic behavior of the structural elements.
At damage limitation condition, the code recommends that strains be limited
(relative displacements between floors) to avoid partition walling and facade
damage according to their brittleness.
9.3.4. Capacity-design method
To design a structure to withstand seismic action economically, incursions into
the post-elastic field are tolerated. Because such incursions take place in cycles, it is
necessary to avoid reaching the limit strain of the constitutive material of the
structure, as beyond these points, integrity cannot be ensured, as the material
deteriorates, lowering strength. Good design not only seeks to ensure the strength of
constitutive materials, but to preserve their ductility and stable behavior during
cycles as well. The ductility objective is achieved by adopting some design and
verification rules that are more restrictive than those used in non-seismic situations,
by taking incursions into the post-elastic field into account, and setting up specific
construction layouts.
Ductile behavior is quite well illustrated in the case of frame structures, whatever
their constitutive material. Passing into the post-elastic field is evidenced by the
appearance of one or several plastic hinges, typically near the connections
(beam/column intersections). When the stress increases in a monotonic way, hinge
rotation increases and other plastic hinges may appear. This cyclic hinge formation
step does not correspond to any alternating characteristic of seismic action but
absorbs a lot of energy, thus turning the structure into a dissipative structure , which
can limit the damage accrued in an earthquake. For such behavior to be possible,
plastic hinges have to be capable of withstanding important rotations without
significant damage, so that the strength capacity of the structure and its capacity to
dissipate energy do not decrease. Transverse reinforcements and construction
layouts ensure such ductility. It also appears that hinge position inside the
mechanism is relevant for construction work safety, as localized effects in the
columns have to be avoided (Figure 9.15a). Finally, the degree of hyper-staticity of
the structure determines the number of plastic hinges that develop within the
framework to reach the mechanism condition, and is therefore an important
determinant of the total energy dissipation capacity of a structure.