Ground Motions and Structures - Earthquake Engineering for Structural Design - page 368

368 Earthquake Engineering for Structural Design

- Also soil and foundations should be included in this balance.
If these principles are respected, the structure will have a correct response during an
earthquake as close as possible to the given prerequisites.

8.7.2TypicalConfigurationDeficiencies

Unfortunately, in many cases, the structural engineer does not play a dominant role in
the determination of the building shape. The objective of the architectural aesthetics,
and/or of specialized functions, may be in direct conflict with the goals of structural
design to provide a proper structural configuration, it having priority in the selection of
structural planning. Therefore, it is possible that the architectural demands are not
convenient from the point of view of seismic behavior. As a result of this conflict, many
building structures have non-symmetric floor plans, multiple towers and setback floors
or irregularities in vertical configuration. Such buildings are prone to earthquake
damage due to coupled lateral and torsional movements producing non-uniform
displacement demands in building elements and concentrations of stresses and forces in
structural elements. In these cases, the task of the structural engineers is to limit the
adverse features in a reasonable range.
Typical building configuration deficiencies include irregular geometry, a weakness
in a given story, a concentration of masses or a discontinuity in the lateral force
resisting system. Vertical irregularities are defined in terms of strength, stiffness,
geometry and mass. Horizontal irregularities involve the horizontal distribution of
lateral forces to the resisting structural system.
Geometric irregularity. As far as the vertical structural configuration is concerned, it
is generally recognized that the compact shapes are the most favourable, so that the
ideal configuration is obtained when the building has the shape of a parallelepiped (Fig.
8.61a). Irregular configurations are characterized by a variation of the boundary lines
along the height, giving rise to set-backs (Fig. 8.61b) or off-sets (Fig. 8.61c). They are
considered irregular, because the corresponding distribution of mass and stiffness is not
uniform. Some analyses have demonstrated that not all vertical irregularities produce an
increase of structural vulnerability (Mazzolani and Piluso, 1996).