2869
Integrating Nonlinear Pile Behavior with Standard Structural Engineering Software
Analyse non linéaire de fondations par pieux à l’aide d’un code industriel
Szép J., Ray R.P.
Széchenyi István University, Győr, Hungary
ABSTRACT: It is common in the practice of bridge design to analyze the superstructure, substructure, and foundation components
separately. Applying this kind of modeling, soil-structure interaction effects can only be approximated with moderate accuracy. The
foundation stiffness can greatly influence the internal forces, stresses, and displacements of superstructure. This is especially true for
portal frame and integral bridges. Better modeling of soil-structure interaction can use three-dimensional geotechnical FEM programs,
where the true soil-stucture environment can be analyzed. It is possible to use nonlinear constitutive models; capable of modeling soil
behavior accurately, however it is difficult, time consuming, and costly in day-to-day practice.
RÉSUMÉ : Dans la pratique, le dimensionnement des structures et des fondations se fait dans des notes de calcul séparées. En
conséquence les interactions sols-structures ne sont décrites qu’approximativement. Par exemple, la rigidité de la fondation peut
affectée la répartition des charges et les déplacements de la structure. Cela est d’autant plus vraie dans le cas de fondations par pieux
réparties sur une ou deux rangées. Le recourt à des codes de calcul tridimentionnel permet une approche plus réaliste des interactions
sols-structures et la prise en compte de lois de comportement non linéaire pour le sol. La presente étude vise à comparer les rigidités
des fondations par pieux réparties sur une ou deux rangées à l’aide d’un code de calcul industriel.
KEYWORDS: piles, nonlinear, structural, software
1 INTRODUCTION-DESIGN TRANSITIONS
Geotechnical and structural engineering software has evolved to
where most common design tasks are performed on the
computer. However, the levels of sophistication of these
software packages are somewhat divergent. Geotechnical
engineers prefer to consider soils as nonlinear materials with
properties that vary throughout a site. These properties may also
vary over time and loading conditions, yielding a highly
complex set of behaviors. While this is an encouraging trend,
structural engineers are often required to take a step back from
such a sophisticated and time-consuming approach. The need
for a timely and straightforward design that meets all aspects of
building code requirements as well as budget and time
constraints often forces structural designers to simplify
geotechnical solutions. This is often reflected in structural
design software that use beam elements and elasto-plastic spring
elements to represent soil support behavior. This paper
examines the methods to achieve these simplifications without
creating an overly conservative or incorrect design.
Representing the full spectrum of three-dimensional soils-
foundation-structure interaction is a laudable goal, but rarely
achievable within the present design environment, hardware,
and software. As a reasonable approximation, one may model
the foundation system via sophisticated geotechnical software
(Plaxis, Midas, Flac) and produce a family of foundation
response curves which can then be approximated in the
structural design model with simpler elements and material
behaviors (Strom and Ebeling 2001).
2 SPECIFIC FOUNDATIONS CONSIDERED
This paper examines several simple examples of single piles
and pile groups that support a bridge abutment. The structural
concept of the foundation is rather simple: vertical loading due
to structural loads, traffic, and other factors is generally small.
Lateral loading due to wind, braking forces, temperature, and
seismic contributions may control design. However, the
geotechnical loading requirements can be quite demanding:
approach embankments add lateral stresses to the piles; soft
layers may exist in widely different thicknesses; and effects of
consolidation may need to be considered (Szép et al 2009).
Additionally, the determining design quantity may be
limiting lateral displacements. At the foundation, this will mean
both lateral translation and rotation are important. Such
deformations play an important role in the determination of
global stiffness, hence deformations and moment/stress
distribution throughout the structure. This is especially true for
types of bridges (portal bridges, integrated bridges) where the
connection between the foundation and the superstructure is
moment-resistant (Hetényi 1964). In this paper we limit our
investigation to determining how the stiffness of single row and
double row pile foundations compare. In other words, what is
the meaning of the common design approach, expressed in
numbers, that the double row pile foundation is much more
rigid than the single row one.
To accomplish this task we used several methods; all
commonly used by structural and geotechnical designers.
However we try to specifically address the problem of moving
across
the
“structural-geotechnical
divide”
in
the
analysis/design process. We combine the results of structural
