Ground Motions and Structures - Earthquake Engineering for Structural Design - page 366

366 Earthquake Engineering for Structural Design

(a) (b)
Figure8.60 Bending moment diagrams: (a)Fixed-based schematization;
(b) Soil-foundation-structure interaction (courtesy Ghersi et al, 1999)

The soil-mass is extended in both vertical and horizontal direction, so the boundary
does not influence the frame behavior. Figure 8.59b shows the deformations in right
and left directions for the soil-foundation-structure system, which are different due to
the asymmetry of structure. Because of the strong asymmetry of the analyzed frame,
the direction of seismic action is obviously very important for the evaluation of
internal actions. The soil-foundation-structure interaction analysis, made by the
SOFIA computer program, shows significantly different bending moments with respect
of the classical fixed-base schematization. The main effect on the structural elements is
a not negligible change of bending moment values, especially in the columns of the
first level (Fig. 8.60). Contrary, no very significant bending moment changes are
observed in the beams. This aspect is particularly relevant, because an increase of
stress in the columns, not foreseen by the usual fixed-base column model, might lead
to a dangerous non-ductile story mechanism of collapse (Ghersi et al, 1999). Similar
conclusions were obtained by Saez et al (2006) and Lehmann and Borsutzky (2006).

8.7SEISMICVULNERABILITYFACTORS

8.7.1 Consequential Structural Configuration

The configuration of buildings is related to dimensions, form, geometric proportions
and location of structural components. The configuration of a building will influence its
seismic performance, particularly regarding the distribution of the seismic loads. A
well-designed structural configuration is expected to achieve some objectives, such as
predictable behavior, simplicity of design and construction,and minimized structural