Ground Motions and Structures - Earthquake Engineering for Structural Design - page 296

296 Earthquake Engineering for Structural Design

with resonance phenomena between the structure and the ground, and thus large inertial
forces arise. This frequently results in plastic deformation of the structure and
substantial damage with local failures, leading, in extreme cases, to the overall collapse
(Bachmann, 2003).

8.1.2EffectsofBuilding'sandCity'sWeightonGroundMotions

The free-field ground motions are usually considered as the source of the structure
motions. However, there is the evidence that the actual ground motions which shock
the structures depend on the building presence, being substantially modified due to the
building's weight.
Building's weight. For a proper structural design, it is very important to have
records in free-field conditions, measured at a convenient distance from the building
and at its base, in order to evaluate the effect of the structure weight. Generally, this
demand cannot be answered by the actual network of the instrumented sites, so only
the theoretical results can be considered. Due to the building weight, the soil is
overloaded, becoming stiffer and, therefore, producing a change in its behavior. In
some cases, especially for stiff soils, the weight effect is to reduce the acceleration peak
(Pitilakis, 1995 , Gioncu and Mazzolani, 2002) (Fig. 8.2). But for soft soils, it is
possible to observe a relative increasing in acceleration peaks due to the presence of a
building.
City-soil effect. The analysis of ground motion effects on structures generally
disregards the influence of the surface structures on the free-field motions in densely
urbanized areas. The effect of the building density increasing is studied by Semblat et
al (2000, 2002a,b, 2004), using the boundary element model (BEM) for alluvial basins
with soft soil (Fig. 8.3a). The idea is that a part of the vibrating energy of buildings is
released into soil through waves produced by buildings, significantly modifying the

Figure 8.2 Influence of building's weight
(Gioncu and Mazzolani, 2002, after Pitilakis, 1995)