|In Istanbul, the Haliç Bridge, the fourth across the Golden Horn, has just been completed. The structure, which is made up of two access viaducts, a cable-stayed bridge and a swing bridge, is built on a historical site and will make it possible to extend a metro line in a city plagued by traffic jams.|
13.6 million inhabitants and covering an area of 5,170 km2 (50
times the size of Paris), Istanbul is the largest city in Turkey. It has
been a UNESCO world heritage site since 1985 and is the country's main
economic hub, experiencing unprecedented growth. The historical western
part of the city is located on either side of the Golden Horn estuary.
At 7.5 kilometres long and 750 metres wide, the estuary joins the Bosphorus,
which separates Asia from Europe and links the Black Sea to the Sea of
Marmara. Both a transit and trading route, the Bosphorus is the fourth
busiest waterway in the world, frequented day and night by numerous cargo
and container ships. |
1. The Haliç Bridge, background and origins of the project
A - The fourth bridge over the Golden Horn estuary
There were previously three bridges across the Golden Horn estuary, the Golden Horn Bridge, the Atatürk Bridge and the Galata Bridge. Since mid-2013, there has been a fourth crossing: the Haliç Bridge. Located between the Galata and Atatürk Bridges, near the Galata Tower and the Süleymaniye Mosque, the new link combines a cable-stayed bridge and a metro station.
At 919 metres long, the bridge connects two metro tunnels located on either side of the estuary. It will enable the extension of the M2 metro line beyond the Yenikapi terminus to Taksim Square, thus increasing the transport capacity from 200,000 to around 750,000 passengers per day.
The project is based on an original concept developed for Istanbul Metropolitan Municipality by Michel Virlogeux. The final version and the corresponding architectural and structural drawings were produced respectively by Hakan Kiran and Wiecon, and the contract was awarded to a consortium formed by Astaldi and Gülermak in December 2009.
B - A 919-metre long structure
The bridge is made up of three main sections, the longest being the 387-metre cable-stayed bridge across the Golden Horn, formed by a 180-metre central span and two 90-metre side spans. The structure is unusual in that there is a metro station on the main span. A 120-metre long swing bridge also allows maritime traffic to move up and down the Bosphorus. The two bridges are flanked by access viaducts at either end, connecting them to the tunnels. The cable-stayed structure comprises two steel towers 65 metres in height, with the platform and roof of the metro station sitting between them on the main span of the bridge.
C - A project modified to blend in with the landscape
Because the Haliç Bridge is built on a protected historical site, particular care was taken during the work to preserve the archaeological remains in the area. The 15 foundations for the viaduct piers were excavated by hand, and the spoil was washed and screened in order to retrieve any items of historical value. Excavations performed during construction revealed numerous Byzantine remains. An arch, the wall of a basilica and a cemetery were discovered near the pier foundations on the Unkapani district shoreline. These developments led to modifications to the design of the building housing the machinery for the swing bridge.
Finally, the height of the towers had to be changed twice during the project due to the bridge's location on a protected world heritage site. UNESCO's main concern was to reduce the bridge's visual impact and preserve the view of the Süleymaniye Mosque. In November 2009, Istanbul Metropolitan Municipality requested that the height of the towers be reduced from 82 metres to 65 metres, which resulted in the top of the stay cables being lowered from 63 metres to 55 metres. The height was revised again in July 2011, lowering the top of the stay cables to 47 metres, and this final height was only approved in February 2012.
The design of the pile foundations for the cable-stayed bridge and the swing bridge also had to be altered to take into account the geology of the soil; the new design was approved in July 2009 and a test pile installed in October of that year.
2. Description of the structure
The new structure carrying the metro across the Golden Horn is 919 metres long. It is made up of two access viaducts, a cable-stayed bridge and a swing bridge.
The 387-metre cable-stayed bridge comprises two side spans and a 180-metre central span. It is supported by a central plane of stay cables anchored to two steel towers. The towers are in turn supported by two sets of nine steel piles with a diameter of 2.5 metres and a maximum height of 75 metres, which are partially filled with concrete to strengthen them. The lower end of the tubes is sunk into the bedrock by means of a mixed steel and concrete extension 2.2 metres in diameter. A steel pile cap meets the tower slightly below water level, 0.5 metres down.
The deck is an orthotropic steel box girder 14.5 metres wide and 3.7 metres high. Two 4.4-metre wide pedestrian walkways are suspended underneath the box girder. At 28.65 metres wide, the deck assembly gives the structure distinctive, elegant lines.
There is 13.8 metres of clearance under the cable-stayed bridge, which is sufficient for most of the ships sailing on the Golden Horn. However, particularly as there is a shipyard upstream, larger ships sometimes need to pass under the structure. A swing bridge has therefore been built at its south-western end. Rotating 90° around a vertical axis, it provides a clear width of around 50 metres. The bridge is made up of a central pier that acts as the rotation axis, and two cantilevered spans 50 and 70 metres long respectively. The deck is also an orthotropic steel box girder, externally reinforced with high-strength steel stiffeners. As the two cantilevers are free during rotation, a counterweight at the end of the 50 metre span balances the deck. When closed, allowing the metro to run, the two ends are supported on piers and the pivot jack is retracted, thus transferring the load to two pot bearings on top of the pivot pier P2-2.
Two access viaducts complete the structure. The south-eastern access viaduct is made up of five spans with a total length of 169 metres. At the other end, the seven spans of the north-eastern viaduct straddle protected historical remains over a distance of 241 metres. The two elegant, slender viaducts have prestressed concrete decks. One hundred and seventy-two T15.72 19-strand cables have been installed.
3. The stay cables
The plane of stay cables supporting the structure's main spans is made up of thirty-six 55 to 75-strand Freyssinet H2000 ultra-compact stay cables. These are parallel multi-strand cables using T15S - 1,860 MPa waxed sheathed galvanised strands housed in an HDPE outer sheath. Freyssinet has been developing and implementing this system for over 30 years.
The Freyssinet patented semi-adherent strands used ensure optimum durability through partial adherence of the corrosion protection system. They are made up of seven hot-dip galvanised wires with a total cross-section of 150 mm2 and a strength class of 1,860 MPa, an exclusive HMP (High Melting Point) microcrystalline wax providing additional corrosion protection and also preventing the occurrence of fretting corrosion, and a high-density polyethylene sheath extruded onto the strand, specially chosen for its strength and durability.
Each strand is individually anchored by an exclusive Freyssinet jaw in a high-tensile steel block.
The front end of the anchor is also fitted with a device for monitoring the bending stresses in the strands. The lower anchors have a 100 mm adjustment range, making it possible to adjust the stay cables during the structure's service life. The anchors are plated to provide corrosion protection. Once the strands have been installed, a galvanised steel cover and injected wax provide continuous corrosion protection.
A pale grey outer sheath performs aerodynamic, mechanical and aesthetic functions. It is equipped with a double helical rib to prevent the occurrence of instability caused by the combined action of the wind and rain. A compact system developed by Freyssinet has been used on the structure in order to significantly reduce the diameter of the sheaths.
The presence of pedestrians on the bridge, as well as the vibrations caused by the metro trains braking and accelerating, required careful consideration during selection of the stay cable dampers. High-strength stay cables have low intrinsic damping. It was therefore important that the longest stay cables (over 80 metres) be fitted with dampers that react efficiently regardless of the vibration amplitude.
Freyssinet IHDs (Internal Hydraulic Damper – figure 5) were chosen following analysis.
Hydraulic dampers react to low-amplitude vibrations, unlike the friction bearings initially specified, which have a fuse effect (they are activated when the stress caused by the vibration exceeds the resistive friction). The viscosity selected makes it possible to optimise the damping in line with the specific properties of the stay cables. The durability of IHDs was validated by one million cycle fatigue testing at maximum amplitudes of ± 15 mm. The four longest pairs of stay cables on the structure were fitted with IHDs during construction. The next four pairs of stay cables are fitted with special guide tubes enabling easy subsequent installation of dampers if they are found to be necessary.
The stay cable system was intensively tested in accordance with the most stringent current criteria. In line with PTI recommendations, 75-strand unit prototypes underwent two million test cycles with a 200 MPa variation, and a 10 mrad skewed wedge under each anchor. A tensile test at 95% of the guaranteed breaking stress was then performed on one prototype and a watertightness test was carried out on another prototype. The watertightness test consists of immersing the anchor, together with all of its components, in 3 metres of water for 96 hours and then checking its overall watertightness (no migration of water into the anchor or on the strands). Intensive quality control inspections take place throughout every phase of the project, including the production of the various components, the calculation of the cable-staying parameters and the stay cable installation process. For example, during the installation phase, all of the cable-staying parameters are recorded automatically by the Isotension system and by hand by Freyssinet's technicians, giving complete traceability.
The stay cable installation system developed by Freyssinet several years ago is light, fast and accurate. The originality of the process lies in the fact that once the sheath is in place, the strands are individually hoisted and then tensioned using a lightweight system specific to Freyssinet, known as Isotension. A force cell is used to measure the load in the first strand hoisted, which is called the reference strand. When the subsequent strands (general strands) are tensioned, an automatic controller controls the single-strand jack and applies exactly the same load as measured in the reference strand. This process means that the same load is applied to all of the strands.
The anchors are installed on their bearing plates and temporarily secured. The HDPE outer sheath is produced by welding several standard length sheaths together (mirror welding). At the top, a short duct with a larger diameter enables the outer duct to expand without introducing any stress. An anti-vandal tube is screwed to the bottom end of the sheath. A reference strand is used for each stay cable, and its length is accurately marked. The strand is threaded into the prepared sheath on the deck. The sheath is hoisted using a jib crane at the top of the tower. The reference sheath is anchored into the tower and onto the deck. It is tensioned to a load calculated so as to obtain the desired load on the stay cable once all of the strands have been installed. The general strands are unwound from the reel using a shuttle winching system. As they are hoisted, the ends of the strands are stripped so that they can be anchored, while ensuring that the stripped area remains inside the stuffing box. Once hoisted, they are tensioned using the Isotension system. After the structure has been stitched together and a general inspection performed, the stay cables are retensioned to fine-tune the geometry of the bridge.
The installation of Freyssinet parallel multi-strand stay cables only requires light loads to be lifted (as the heaviest components are the strand reels), thus avoiding heavy crane costs. The system is an efficient solution for both small structures, on which heavy lifting equipment is not always available, and exceptional structures (for example, those with long spans) as it reduces the construction costs.
4. Construction methods
The two spans were built by successive cantilevering.
The piers, pier segments and towers were lifted using a barge crane. The 17 segments comprising the four spans were hoisted using four gantry cranes. The segments were precast by a workshop in Turkey and transported to site by barge after a short journey on the Sea of Marmara.
Two pairs of gantry cranes were manufactured for the construction of the bridge.
Each gantry crane, weighing approximately 80 tonnes, has a 230-tonne lifting capacity. They are made up of two vertical truss frames connected by braces. A carrier beam rests on the ends of the frames, holding two 140-tonne capacity Hebetec lifting jacks. The carrier beam is used to make longitudinal changes to the position of the lifting jacks and therefore enables fast, accurate adjustment of the longitudinal position and orientation of the segment at the end of lifting. The two lifting beams connected to the segment to be lifted are fitted with two jacks to adjust the angle of the segment. These three adjustment devices meant that the segments could be guided into the desired position while accommodating any construction errors (position of the centre of gravity, etc.).
When the segments were lifted, the gantry crane was secured to the previous segment using eight Ø50 Freyssibar prestressing bars. This made it possible to balance the gantry cranes during lifting, and also to take up any accidental load (such as the effect of the wind, for example). When configured for movement, the gantries run on two rails that are also used as an attachment point for the stabilising system.
The construction cycles for the towers' cantilevers were staggered by half a cycle in order to optimise site resources; the welding teams worked on one tower's cantilever while the lifting and cable staying teams were working on the second tower. The gantry cranes supported the load until the minimum welding specified by the Engineer had been completed, thus saving money and time that would have been spent installing temporary fixings. In order to avoid additional crane handling, the two pedestrian walkways were assembled on the barges and lifted by the gantry cranes at the same time as the segments. This system meant that a full cycle could be completed in 18 days. The structure was stitched together on 7 March 2013.
The very small space (around 30 mm) left for the stitching segment was jacked to keep the structure in its theoretical configuration while the stitching welding was performed.
Article published in the 900th edition of TRAVAUX, October 2013.