Effect of soil–structure interaction and spatial variability of ground motion on seismic risk assessment of bridges - Seismic Risk Analysis and Management of Civil Infrastructure Systems - page 611

Apparently, the extent of this benefi cial phenomenon is very much depen-
dent on the assumptions made regarding the defi nition of the 'synchronous'
excitation, which, in contrast to the actual, recorded asynchronous case, is
not obvious. There are specifi c cases (i.e., out-of-plane bending moments
and displacements at the top of the two bridge pylons) where the asynchro-
nous excitation has a critical effect.
In all cases, the observed deamplifi cation or amplifi cation of the bridge
displacements was verifi ed by the reduced or increased amplitude of the
Fourier spectra, at selected frequencies, which correspond to specifi c modes
that have a strong impact on the vibration of the structure along the direc-
tions examined. Given the complexity of the problem studied, the excitation
of higher modes due to asynchronous excitation may be the key tool for
understanding the role of spatial variability of earthquake ground motion
on the overall seismic response of bridges.
22.5 Conclusions
The present chapter is an attempt to critically review the currently available
structural analysis capabilities for the assessment of SSI and spatial vari-
ability effects in the framework of seismic design and assessment of bridges.
Having discussed the most widely applied approaches and highlighted a set
of reasonable approximations together with their limitations, it was aimed
to contribute towards fi lling the gap between state-of-the-art research and
state-of-practice.
As a closure it can be stated that despite the major advances in terms of
computational modeling and software development and powerful hard-
ware, there is lack of ready-to-use features that would allow for effi cient
computation of the overall dynamic soil-foundation-superstructure response
under realistic earthquake ground motion scenarios especially in the case
of multiple inelastic mechanisms developing simultaneously within the soil
and the structure. Tools for developing realistic ground motion scenarios
are also valuable due to the inherent frequency dependence of the soil-
structure system response.
As a result, it is necessary that the wide expert knowledge gained during
the last decades is refl ected in both ready-to-implement analysis tools and
in modern seismic codes. Moreover, it is critical to utilize the large bulk of
experimental soil data available worldwide together with the extensive
databases of recorded motions during strong earthquakes. Finally, improv-
ing the methods for quick in situ assessment of the soil material properties
is of utmost importance in order to minimize material-related uncertainty
and focus primarily on the optimal numerical approximation required for
ensuring the reliability of both analysis and design.