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3D Numerical Analysis of a Suspension Bridge Anchor Block to Oblique-Slip Fault
Movement
Analyse numérique 3D d'un bloc d'ancrage de pont suspendu soumis à un mouvement oblique de
glissement dû à une faille de rupture
Avar B.B.
Flint & Neill Ltd, a COWI Group Company, London, United Kingdom
Augustesen A.H., Kasper T., Steenfelt J.S.
COWI A/S, Kongens Lyngby, Denmark
ABSTRACT: The paper presents results from 3D finite element (FE) analysis of the response of a suspension bridge anchor block to
oblique-slip fault movement. The study attempts to understand the behaviour of the bridge anchor block in terms of displacements and
stresses acting on the block as a result of fault movement in order to assess the robustness of the structure. The fault displacements
result in minor changes in horizontal stresses acting on the anchor block vertical faces. Due to the rotational displacement effect of the
vertical fault displacement an uneven vertical stress distribution at the bottom of the anchor block can be observed. The rotation and
translation of the anchor block have been evaluated to be within acceptable limits.
RÉSUMÉ : Ce document présente les résultats de l'analyse 3D aux éléments finis de la réponse d’un bloc d’ancrage de pont suspendu
à un mouvement oblique de glissement dû à une faille. L’étude vise à appréhender le comportement du bloc d’ancrage du pont, du
point de vue des déplacements et des efforts appliqués, résultant du mouvement de faille, afin de déterminer la robustesse de la
structure. Les mouvements de faille n’ont que peu d’incidence sur les efforts horizontaux appliqués aux faces verticales du bloc
d’ancrage. Toutefois, en raison de l’effet rotationnel engendré par les déplacements verticaux de la faille, on observe une distribution
plus inégale des efforts verticaux à la base du bloc d’ancrage. La rotation et les déplacement de ce dernier ont été calculés afin d’être
dans des limites acceptables.
KEYWORDS: Anchor block, suspension bridge, fault rupture, soil/structure interaction, FE analysis.
1 INTRODUCTION
The type of foundations used for structures such as bridges
affect the performance of the structure during faulting (Gazetas
et al. 2008). In recent years comprehensive studies including
field observation and numerical analysis supported by
centrifuge model tests have been conducted to develop a
methodology to analyse and design foundation/structure
systems against fault rupture (e.g. Gazetas et al. 2007, Faccioli
et al. 2008, Anastopoulos et al. 2008, Loli et al. 2012).
The studies show that the presence of the structure,
depending on the rigidity of its foundation, can divert the
rupture path as opposed to free-field fault outcropping.
Furthermore, depending on the relative rigidity of the
foundation with respect to the soil, the foundation and the
structure experience differential displacements and rotations
different from those of the free-field ground surface.
Massive concrete anchor blocks are often used to anchor the
suspension bridge main cables which carry the bridge deck. In
this paper, the impact of oblique-slip fault movements passing
under a massive gravity anchor block in terms of displacements
and stresses have been investigated using 3D finite element
(FE) analysis. The behaviour of the anchor block-soil
interaction has been compared with free-field fault movements.
2 PROJECT DESCRIPTION
The anchor block analysed in this paper was the initial solution
for a suspension bridge crossing the Izmit Bay in Turkey, which
is to be located southeast of Istanbul. The free span of the
bridge is 1550 m in length with two 566 m long side spans. The
anchor block is located at the south end of the bridge. The site
investigation proved the presence of secondary fault systems
under the anchor block.
3 NUMERICAL MODELING
The anchor block is located in a multi-layered soil medium. The
fault plane is situated at the base of the soil medium
representing the interface between soil and rigid bedrock. It is
assumed that vertical and lateral movements may occur
simultaneously creating an oblique-slip faulting.
Numerical modelling using PLAXIS 3D 2011 FE software is
adopted. The size of the model is 500 x 1400 x 130 m (see Fig.
1).
Figure 1. Indicative cross section showing the anchor block and the
model dimensions.
To reduce the effect of the outer boundaries of the soil
volume as well as to ensure sufficient calculation accuracy and
to correctly apply the fault displacements, an artificial low-
stiffness "cushion material" is placed around the soil medium.
This zone allows the general standard fixities in PLAXIS to be
introduced at the outer boundaries of the cushion material. The
unit weight of the cushion material is the same as the soil
