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 591

circumstances (Sextos et al. , 2003b; Zerva and Zervas, 2002). Similar to SSI,
the perception often prevails that the uncertainty in modeling the variation
of earthquake ground motion in space is higher than the uncertainty induced
by ignoring the problem itself. Hence in most cases, asynchronous excitation
of long structures is either neglected or taken into consideration in a very
approximate manner (Sextos, 2009). Considering the two problems from a
physical point of view, it can be made clear that SSI and SVGM are essen-
tially two strongly coupled phenomena.
The scope of this chapter is to critically discuss methods for considering
the above two phenomena in the design and assessment of RC bridges. An
effort is therefore made to demonstrate a simple yet accurate means to
account for SSI during the design process of bridges based on state-of-the-
art scientifi c publications, seismic code provisions, related background com-
mentary as well as expert engineering judgment. Additionally, a series of
modeling issues are discussed in terms of 'optimal' selection and application
of seismic input (with emphasis on site response, varying angle of wave
incidence, multiple support and rotational excitation). The seismic response
of the soil-structure system (particularly in terms of soil-pile-pier and
embankment-backfi ll-abutment-superstructure interaction and various
sources of material and geometrical nonlinearity) is also discussed. The
chapter concludes with a critical overview of the methods presented and a
discussion of the future trends in research and design.
22.2 Soil-foundation-pier-superstructureinteraction
The design of the foundation of piers and abutments primarily depends on
(a) the depth, stiffness and bearing capacity of the bearing stratum, (b) the
structural system and the anticipated vertical and lateral loads, and
(c) potential restrictions of differential settlement due to the type of bridge
deck (i.e., multi-span continuous decks can tolerate smaller differential
settlements than simply supported ones). These foundations generally fall
into two categories: strip footings and pile foundations. Combination of the
two (i.e. piers supported on piles and abutments on surface foundation) is
also feasible given the soil inhomogeneity. In the following subsections, only
pile foundations will be examined as it represents the most complex case
of the foundation systems, in terms of modeling, analysis, design and
construction.
22.2.1 Piers founded on pile shafts (single or groups)
Static loading
Numerous studies have been made on the effect of SSI under static loading.
The most commonly adopted engineering method for calculating the