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This case study looks at the innovative designs and some of the major construction features of the Rion Antrion Bridge, which connects mainland Greece to the Peloponnesus.
The Rion-Antirion Bridge, located in the Gulf of Corinth, provides a fixed link between the Peloponnesus and Continental Greece over a stretch of water 2,500m wide. A multi-span, cable stay bridge with a continuous suspended deck of 2.252km, the bridge comprises:
Completed on prior to the 2004 Olympic Games opening ceremony, construction encountered a number of environmental challenges including:
This case study looks in more detail at the innovative designs of the bridge.
The structure spans a stetch of water of around 2500m. The seabed presents fairly steep slopes on each side and a long horizontal plateau at a depth of 60 to 70m. No bedrock was encountered during soil investigations down to a depth of 100m. Based on a geological study it is believed that the thickness of sediments is greater than 500m. General trends identified through soils surveys included:
The seismic conditions to be taken into account are presented in the form of a response spectrum at seabed level given in figure 1. The peak ground acceleration is equal to 0.48g and the maximum spectral acceleration is equal to 1.2g between periods of 0.2 and 1.0sec. This spectrum corresponds to a 2000-year return period, or 5% probability of exceedance for a life of 120 years. In Figure 1 the design spectrum of the Bridge is compared with the one required by Greek Seismic Code (EAK 2000) for the specific seismic zone of the bridge (III), with an importance factor of I=1.3.
The Peloponnese drifts away from mainland Greece by a few millimetres per year. Taking into account the 120 years design life of the bridge, contractual specifications required the bridge to accommodate possible fault movements up to 2m in any direction, horizontally and/or vertically between two adjacent piers, in addition with 1/500 vertical tilt of the pylons. The critical accidental seismic load combination consists of the specified earthquake combined with 50% of the tectonic movements.
As well as challenging sea and tectonic conditions, the Rion-Antirion strait is known for frequent and strong winds. The reference wind speed (hourly average, 10 meters high, 120 years return period) was estimated, from a large database of wind measurements, to be 32 m/sec. At deck level (57 meter above sea level) the reference wind speed is 50 m/sec. Flexible decks, however, such as cable-stay and suspension bridges are prone to aerodynamic instabilities. Existing standards require that the aerodynamic stability of flexible bridges with span larger than 200 meters be confirmed with aerodynamic tests. For the Rion-Antirion Bridge, the specifications required the critical wind speed for flutter instability to be higher than 74 m/sec, and that vortex shedding does not occur for speeds between 0 to 53 m/sec.
In addition, the bridge is capable of withstanding the impact from an 180,000 dwt tanker sailing at 16 knots. This corresponds to an equivalent horizontal static load of 28000 tons applied at a height of 67m from the foundation.
The challenging conditions required an original design based on large foundations able to sustain seismic forces and large spans in order to limit the number of these foundations. The bridge consists of:
Foundations consist of large diameter (90m) caissons, resting on the seabed (Figure 3). The top 20m of soils are rather heterogeneous and of low mechanical characteristics. To provide sufficient shear strength to these soil strata, which have to carry large seismic forces coming from structural inertia forces and hydrodynamic water pressures, the upper soil layer is reinforced by inclusions.
These inclusions are hollow steel pipes, 25 to 30m long, 2m in diameter, driven into the upper layer at a regular spacing of 7 to 8m (depending on the pier); about 150 to 200 pipes were driven in at each pier location. They are topped by a 3m thick, properly selected and levelled gravel layer, on which the foundations rest.
The cable-stayed deck is a composite steel-concrete structure with a width of 27.2m, carrying two traffic lanes in each direction. The steel frame was made of two longitudinal plate girders 2.2m high on each side of the deck with transverse plate girders spaced at 4m and a concrete slab, 0.25 to 0.35m thick (Figure 4). On top, an asphalt concrete pavement of 75mm thickness, including special waterproofing membrane, was placed. The deck superstructure is continuous and fully suspended by means of stay cables for its total length of 2252 meters.
In the longitudinal direction, the deck is free to accommodate all thermal and tectonic movements. At its extremities, expansion joints can accommodate the following movements:
Each pylon is composed of four legs 4 x 4m, made of high strength concrete, joined at the top to give the rigidity necessary to support unsymmetrical service loads and seismic forces (Figure 5). The pylons are rigidly embedded in a pier head to form a monolithic structure, up to 230m high, from sea bottom to pylon top.
The stay cables, forming a semi-fan shape, are in two inclined arrangements, with their lower anchorages on deck sides and their upper anchorages at the pylon top. They are made of 43 to 73 parallel-galvanized strands individually protected with an extruded layer of HDPE. Finally, all strands are placed inside a HDPE duct.
Following close monitoring of the behaviour of the cables for wind induced vibrations, in 2006 it was decided to improve the behaviour of cables, with length higher than 100m, by increasing the damping of the cable system. This was achieved with the installation of external dampers close to the bottom anchorage.
Finally, conservatory measures were taken for future installation of internal dampers on the short cables and secondary cross-ties, in case that the cables experience any additional type of vibration.
An innovative energy dissipation system connects the deck to the top of each pier and limits the lateral movement of the deck during the specified earthquake, while dissipating the seismic energy. The seismic protection system comprises fuse restraints and viscous dampers acting in parallel, connecting in the transverse direction the deck to the piers. The restrainers are designed as a rigid link intended to protect (to block) the hydraulic devices by withstanding the high wind loads up to a pre-determined force corresponding to a value slightly higher than the wind load ultimate limit state. Under an earthquake, the fuse restrainers are designed to yield, leaving the viscous dampers free to dissipate the energy induced by the earthquake into the structure. After the seismic event, new fuses can be re¬installed in few weeks.
From the beginning it was clear that the critical load for most of the structure is the design seismic loading.
The choice of design was made after examination of a wide range of possible solutions in term of span type (suspension spans vs. cable-stayed spans) and foundation concepts. Particularly with regard to the foundations, the bearing capacity was a major concern in these difficult environmental conditions characterised by poor soil conditions, significant seismic accelerations and large depth of water.
Alternative foundation concepts (such as pile foundations, deep embedded caissons and soil substitution) were investigated with their relative merits in terms of economy, feasibility and technical soundness1. This analysis showed that a shallow foundation was the most satisfactory solution as long as it was feasible to significantly improve the top 20m of soils.
Although these foundations resemble piled foundations, they do not behave as such: no connection exists between the inclusions and the caisson raft, which allows for the foundation to uplift partially or to slide with respect to the soil; the density of inclusions is far more important and the length smaller than would have been the case in piled foundations. The major concept for the selected shallow foundation is that the forces of the structure will initially be transferred through contact (mainly friction) to a gravel bed layer and then to the original soil that is reinforced with the metallic inclusions. The word "inclusion" was selected instead of "pile" in order to mark the difference in the foundation behaviour. This type of soil reinforcement is quite innovative and necessitated extensive numerical studies and centrifuge model tests for its validation in the Laboratoire Central des Ponts et Chaussées (France).
Another unique feature of the project lay in its continuous cable-stayed deck, which, in addition to being the longest in the world, is fully suspended. This created an effective isolation system significantly reducing seismic forces in the deck and allowing the bridge to accommodate fault movements between adjacent piers thanks to its flexibility.
In the transverse direction the deck will behave like a pendulum and its lateral movements must be buffered. This is achieved with the use of 4 hydraulic dampers at the pylon locations and two at the transition piers, having a capacity of 3500 kN each, operating both in tension and compression. They are able to limit the relative lateral displacements between the deck and the pylons and dissipate large amount of energy during a seismic event. In order to avoid deck transverse movements due to wind, a special metallic strut, called "fuse restrainer", connects the deck and each pier. This lateral restrainer incorporates a mechanical device that fuses at ±10500kN. The function of this device is to allow transversal deck motion only after a seismic event of low occurrence enabling the dampers to go into action. The dissipation system installed between deck and a pier is illustrated in the next figure.
The good performance of the dissipation system of the bridge was confirmed after a seismic event of a moment magnitude of Mw=6.5, that took place on June 08 2008 with an epicentre located approximately 36km SW of the bridge and a focal depth of around 30km. The recorded peak ground acceleration was 0.13g, while the maximum estimated under the piers was 0.18g. The fuses yielded and the dampers operated with a maximum estimated velocity of 0.28m/sec and a maximum stroke of 15cm.
An expansion joint with special fuse elements was designed by Maurer Sohne to allow the maximum design movements of the main bridge to the approach viaducts. While the expansion joint accommodates a service movement of 1.22m closing, 1.26m opening in longitudinal and +/- 0.10m in lateral direction, it is designed for a maximum opening of 2.20m closing, 2.81m opening in longitudinal and +/- 2.50m in lateral direction for the specified design earthquake. During this seismic event fuse elements within the expansion joint will open to minimize the forces induced to the bridge structure and keep the expansion joint with minor damages.
The construction of the pier footing followed the methods used for construction of offshore platforms. However some features of this project made the construction process of its foundations quite exceptional.
The dry dock was established near the site, 200m long, 100m wide, 14m deep, and could accommodate the simultaneous construction of two foundations. A dry dock utilised an unusual closure system: the first foundation was built behind the protection of a dyke, but once towed out, the second foundation, the construction of which had already started, was floated the front place and used as a dock gate.
Dredging the seabed, driving 500 inclusions, placing and leveling the gravel layer on the top, with a depth of water reaching 65m, was a major marine operation. This required special equipment and procedures, including a specially made tension-leg barge , based on the concept of tension-leg platforms but used for the first time for movable equipment.
The footings after being towed to the wet dock for the construction of the conical part were towed then sunk at their final position over the reinforced soil. Compartments created in the footings by the radial beams were used to control trim by differential ballasting. Then the pier bases were filled with water to accelerate settlements, which are significant (between 0.1 and 0.2m). This pre-loading was maintained during pier shaft and pier head construction, thus allowing a correction for potential differential settlements before erecting pylons and deck superstructure.
The deck of the main bridge was erected using the balanced free-cantilever technique, with prefabricated deck elements 12m long comprising also their concrete slab. The total weight of a prefabricated segment is 340 tons and was placed with a floating crane (TAKLIFT 7). The rate of erection of completed deck structure was up to 60m per week.
The Rion - Antirion Bridge had to overcome an exceptional combination of adverse environmental conditions: water depth up to 65m, deep soil strata of weak alluviums, strong seismic activity and tectonic movements. This resulted in a unique multi-span cable-stayed bridge with a continuous deck for the full length of the stretch (2252m), fully suspended from four pylons. Foundations consist in large diameter (90m) caissons resting on the seabed. The top 20m of soils are reinforced by means of metallic inclusions, based on an innovative concept.
The design and construction of this € 800 million project have been undertaken under a private BOT scheme, led by the French company VINCI. The whole project was completed on summer 2004, ahead of the contractual deadline by several months.
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