Engineering Know-How: Lessons from Earthquakes and Rules for Seismic Design - Dynamic Behavior of Concrete and Seismic Engineering - page 345

Foundation structural elements have to be stiff enough to transmit the seismic
actions of the structure towards the soil or deeper foundations. Typically there
should be only one foundation type below a structure. If this is not possible, the
structure must be divided into dynamically-independent blocks.
During the general design stage for a structure, we can distinguish the main
elements that constitute the bracing from the secondary elements, which merely play
a supporting role. The secondary elements should not be considered as strong
resisting elements when calculating structural response to seismic action, and their
stiffness can be neglected when evaluating the dynamic behavior.
Except when the structure has been designed as non-dissipative, ductility should
also be considered by designers. Brittle failures, or the premature formation of
unstable mechanisms like hinges, must be avoided to ensure the overall ductility of
the structure. This is achieved by using capacity design (see section 9.3.4). Local
ductility can also be ensured by using the construction layouts described in later
chapters on material structure.
Non-structural elements must be verified as being able to withstand the
acceleration transmitted by the structure; this acceleration is calculated using a floor
spectrum . The calculation can be simplified by using acceleration results from a
standard estimate that uses a behavior coefficient specific to the element being
considered.
9.4.2. Regularity conditions
The regularity of a building must be considered in two parts: plane regularity and
vertical regularity. They are both desirable though not crucial.
Horizontal regularity is ensured by designing so as to restrict vertical axis
torsion phenomena. The criteria are twofold: the first relate to the symmetry and
compactness of the plane shape, and are the subjects of simple geometrical controls.
The existence of two main orthogonal planes over the whole height of the structure
reflects most of these criteria. The second planes are mechanical: we must ensure
that the floor diaphragms are stiff enough with regard to bracing elements that
diaphragm displacements on each level are stiff behaviors (overall translation and
rotation). A second set of conditions deals with the radii of torsion, defined as the
square root of the ratio of the torsion inertia around the torsion center with the
bending inertia in the considered direction. For each principal direction, the radial
deviation of the structure (i.e. the distance between the bending center of the bracing
system and the center of gravity, projected in the direction under consideration)