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Modelling of soil-structure interaction for seismic analyses of the Izmit Bay Bridge
Modélisation de l’interaction sol-structure pour l’analyse sismique du pont de la baie d’Izmit
Lyngs J. H., Kasper T., Bertelsen K.S.
COWI A/S, Denmark
ABSTRACT: The Izmit Bay Bridge will carry the new Gebze-Orhangazi-Bursa-
İ
zmir motorway across the Sea of Marmara at the
Bay of Izmit, Turkey. The suspension bridge with 1550 m long main span will be subjected to strong seismic events. The tower
foundations are prefabricated reinforced concrete caisson structures that are installed on prepared gravel beds on Pleistocene deposits,
reinforced by driven steel pile inclusions. The design allows for limited permanent displacements in the subsoil and in the gravel-
caisson interface during high magnitude seismic events, in order to limit the forces imposed on the superstructure. The displacements
and forces in the bridge during seismic events are calculated in displacement-based time history analyses in a global finite element
model. This paper describes how the behaviour of the gravel bed and the reinforced subsoil is modelled by distributed sets of vertical
and horizontal translational springs, dashpots and gapping elements. The nonlinear horizontal springs are based on hyperbolic
relations, generalised in two dimensions, coupled with the local vertical stress and capable of producing hysteresis according to the
extended Masing rules. The paper demonstrates an advanced, yet feasible, modelling method that has been put into practice.
RÉSUMÉ : Le pont de la baie d'Izmit portera la nouvelle autoroute Gebze-Orhangazi-Bursa-Izmir dans la baie d'Izmit en Mer
Marmara, en Turquie. Le pont suspendu avec une travée principale 1550 m de long sera soumis à de fortes contraintes sismiques. Les
fondations des pylônes sont des caissons en béton armé préfabriqués installés sur des assises granulaires préparées sur les dépôts du
Pléistocène renforcés par des pieux en acier. Afin de réduire les forces exercées sur la superstructure lors d’événements sismiques de
magnitude élevée, le concept de fondation autorise des déformations permanentes à l’interface entre le caisson et l’assise granulaire
ainsi que dans les matériaux du sol naturel. Les déformations et efforts dans la superstructure lors d'événements sismiques sont
calculés sur la base d’accélérogrammes de déplacements dans un modèle global aux éléments finis. Cet article décrit les méthodes de
modélisation du comportement de l’assise granulaire et du sol renforcé par un ensemble de ressorts verticaux et horizontaux,
amortisseurs et ouvertures. La modélisation des ressorts horizontaux non-linéaires est basée sur des relations hyperboliques,
généralisées en deux dimensions, associées à la contrainte verticale locale et capable de produire une hystérésis selon les règles
généralisées de Masing. L'article montre une méthode de modélisation complexe, néanmoins réalisable et qui a été mise en pratique.
KEYWORDS: Foundation design, Suspension bridge, Seismic time history analysis, Soil-structure interaction, Finite element
MOTS-CLÉS : Études des fondations, Pont suspendu, Analyse sismique par accélerogramme, Interaction sol-structure, Éléments finis
1 INTRODUCTION
The Izmit Bay Bridge, shown in Figure 1, is a suspension bridge
with a steel superstructure
The bridge is currently under construction and has a 1550 m
main span and two 566 m long side spans. The bridge deck is a
steel box girder, which carries six road lanes and is located
between 60 and 80 m above the sea level. The top elevation of
the tower is 252 m above the sea level. Each tower has two steel
legs and two steel cross-beams, and their foundations consist of
a pre-fabricated concrete caisson placed on a gravel bed on
improved soil at 40 m water depth. The anchorages of the
bridge are concrete gravity structures.
Figure 1. Global geometry model of the bridge.
The project site in the north-western part of Turkey is
located in the area of the North Anatolian Fault with high
seismicity. The North Anatolian Fault stretches over
approximately 1600 km in east-western direction along the
Black Sea coast of Turkey. It is a major right lateral strike slip
fault that forms the tectonic boundary between the Eurasian
Plate and the Anatolian block of the African Plate. The North
Anatolian Fault zone forms a narrow band that splays into three
strands in the eastern Marmara Sea region. The northern strand
occupies the Izmit Bay and projects across the project
alignment, presenting the greatest seismogenic hazard source in
the site area. A number of earthquakes with moment magnitudes
M
w
between 7 and 8 have been recorded in the region. A
project-specific seismic hazard study has been carried out based
on a detailed review of the literature.
Three different seismic events with different return periods
and seismic performance criteria were defined as a basis for the
seismic design of the bridge: For the Functional Evaluation
Earthquake (FEE) with a return period of 150 years, immediate
access to normal traffic and minimal damage (essentially elastic
performance) are required. For the Safety Evaluation
Earthquake (SEE) with a return period of 1000 years, limited
access within days, full service within months and repairable
damage without closure to traffic are required. For the No
Collapse Earthquake (NCE) with a return period of 2475 years,
significant damage without collapse is allowed. For the FEE,
SEE and NCE, the rock outcrop peak ground acceleration is
0.25g, 0.65g and 0.87g, respectively.
Time history analyses with a global finite element model (cf.
Figure 3) formed the basis for the seismic design of the bridge.
For each of the three seismic events, seven sets of near surface
displacement time histories with three orthogonal components
were derived from site response analyses for each bridge
foundation.
This paper focuses on the soil-structure interaction
modelling of the tower foundations by means of distributed sets
of vertical and horizontal translational springs, dashpots and
gapping elements, which form the support of the tower
foundation caissons in the global model and to which the
