14 May 2013
Building a new metro bridge in the middle of one of Istanbul’s most sensitive historic locations has created numerous challenges. Mario Mancini, Osman Sari, Gökhan Asçioglu, Erdem Kula and Dogan Demir report on how these were overcome.
The three bridges that cross the Golden Horn in Istanbul, Turkey are soon to be joined by another link, this time a combined bridge and metro station. The new cable-stayed Haliç Bridge, which is due to open later this year, is being built in a very prominent location between the Galata and Ataturk bridges.
Its position also places it between two of the city’s most famous buildings, the Galata Tower and Suleymaniye Mosque. The purpose of the 1km-long Golden Horn Metro Crossing project is to create a link between two tunnels on opposite banks of the Golden Horn. This will enable the metro line to be extended beyond the Yenikapi terminal to Taksim Square and increase the present ridership of 200,000 passengers per day to an estimated 750,000.
After the alignment of the metro line was established between Yenikapi and Taksim station, several construction alternatives were reviewed before the present scheme was developed based on an original concept by Michel Virlogeux for client Istanbul Metropolitan Municipality. Architectural and structural drawings were completed by Hakan Kiran and Wiecon respectively, and the contract was awarded to Astaldi Gülermak in December 2009.
The crossing is formed of threee main parts, the most significant of which is the 387m-long cable-stayed span over the Golden Horn. This bridge has a 180m-long central span and two 90m-long side spans.
A 120m-long swing bridge provides access for marine traffic on the Golden Horn, and the two bridges are flanked by approach viaducts at each end, linking to the tunnels. The cable-supported structure has two 65m-tall steel towers between which the station platform and canopy are located, on the main span of the bridge.
The station extends across the full 180m length of the main span, and it is covered with a 90m-long canopy, the design of which is still in the process of being finalised. Titanium cladding is proposed for the lower half of the oval-shaped cross-section, with the upper part being transparent. The deck is a box girder in S355 steel which is fabricated in 17m-long segments which each weigh around 200t and are welded together. The cross-section varies on each part of the structure; on the viaducts it is narrower but has a similar geometry. The main deck of the swing bridge has the same section as the cable-stayed bridge but with an additional haunch on the top of the deck for structural stability, since the swing bridge is essentially a cantilever structure.
Steel piles of 2.5m diameter were driven down to the rock formation level – to depths ranging from 30m to 89m – and the soil extracted from within them before excavation continued into the rock to create a socket.
Reinforced concrete anchors the piles into the rock layer. The steel for piles was produced in Germany and transported to Portugal where subcontractor Martifer fabricated the piles to full length. Because of handling and storage restrictions at the work site in Istanbul, both ends of the piles were temporarily closed and they were delivered to Golden Horn, unloaded onto the water where they were stored floating. Before being driven, they were taken onto a loading ramp on the 10,000t capacity barge where the 800t-capacity lattice boom crawler crane was prepared and equipped for this purpose. The closures at both ends were removed, and the pile was tilted to a vertical position, set out and driven to the required depth using an hydraulic Menck MHU500 hammer, with continuous monitoring of verticality and position.
Under each of the main towers, nine piles were driven, compared with just four or five for the side supports. The piles were connected in their groups by structural steel pile caps with piers built on top to support the deck segments. The crossing also includes a 4.4m-wide steel pedestrian bridge which is located below the main deck and is attached to it via a diagonal strut and a horizontal beam which is also welded. This part of the crossing was assembled with the main deck on the ground and the two were installed together.
Each segment was brought to site on barges with a Liebherr LR1800 crane, from which they were lifted directly and secured in position by the use of several lugs on each segment. Using the balanced cantilever process, adjoining segments were also lifted up and after dimensional adjustments, the welding procedure started. Once half of the welds were completed, the segment was detached from the lifting gantry. The full process of lifting and welding one segment took an average of 15 days to complete.
Four lifting gantries were used on the bridge, with two working simultaneously at each tower. Once the segment was detached from the lifting gantry, with 50% of the welding complete, the gantry was moved and reassembled at the next segment. After full completion of welding and cable stressing on the previous segment the gantry was allowed to lift the next one. The two main towers support the deck of the bridge; these towers, manufactured in steel with thickness varying from 70mm to 50mm are connected to the core segments on the piers by full penetration welds and support the deck by means of a total of 36 stay cables in a single plane along the centreline.
Nine cables are connected to each side of the two towers; these cables are formed of bundles of between 55 and 73, 15.7mm-diameter low relaxation prestressing strands. The second main part of the crossing is a swing bridge with 50m and 70m spans, resting on a central pier and designed to be able to lift and rotate by 90 degrees when necessary to provide an opening.
The specially-designed pier is on a pivot, supported by a structural steel pile cap on five piles similar to those used for the main towers. The box girder continuous steel deck is 13.7m wide and 4.45m deep. Two sets of four horizontal hydraulic jacks are fixed on the structural pier at one end and on the bracket of the central pivot at the other. These jacks enable the swing bridge to rotate 90° about the vertical axis within four to six minutes and provide a free clearance of about 40m width for vessels to enter or leave the Golden Horn. The same procedure is reversed to close the bridge.
On the Unkapani side of the Golden Horn, a concrete platform has been built to support the control room for the swing bridge. This platform is situated between the existing shoreline and the swing bridge pivot and is founded on eight 1.8m-diameter piles which are 84m long and made of 30mm-thick steel. A new sheet pile wall has also been built along the Unkapani shoreline, replacing the old protection system and improving the stability around the swing bridge pier. It consists of 46, 1m-diameter piles each 25m long, which are connected by a pile cap and anchored back by tie-rods.
Post-tensioned reinforced concrete approach viaducts have been built on both sides of the Golden Horn; one between the southern metro tunnel portal and the swing bridge on Unkapani bank, and the other one between the northern metro tunnel portals and the cable-stayed bridge on the Beyoglu bank. The former approach viaduct is 172m long and has six spans which vary from 17m to 44.1m long, while the latter approach viaduct is 241m long and formed of seven spans varying from 23.5m to 45m long. The cable-stayed bridge is somewhat unusual in that it involves the construction of a metro station on the main span of the structure.
This arrangement was chosen in order to avoid the need to expropriate buildings that were in the way, to minimise the archaeological excavations and to eliminate the need to build a large structure close to the shore. Hence the contractor had to construct station entrance buildings on each bank as part of the contract – although this did not include finishing works and fit-out. The station extends across the entire main span.
The station platforms along each side of the bridge are 7.3m wide and are divided by a line of the turnstiles which enable paying passengers to access the trains. Site handover took place at the start of 2009, with preparation of the construction drawings, geotechnical investigations, expropriation and demolition works and archaeological investigations beginning immediately. Because the site is in a very sensitive historical location, the alignment of the metro line, and in particular of the viaducts, was very carefully designed in order not to damage any archaeological remains.
All 15 foundations for the viaduct piers were hand-excavated, layer by layer, to the so-called ‘culture zone’ depth, and the excavated soil was washed and screened for anything of historical value. The excavation team worked under the continuous surveillance of archaeologists authorised by the Archaeological Museum & Preservation Committee. A Byzantine-era vault was discovered during excavations at pier foundations on the Unkapani bank, so the initial work was abandoned and the project was redesigned to accommodate this.
Additionally the wall of a Byzantine-era basilica and a graveyard were discovered on the same bank, requiring the revision of the design of the operator’s building for the swing bridge. The archaeological investigations continued until March this year. Pile foundation design for the stay cable and swing bridges had to be revised to accommodate the soil conditions that were encountered; approval for the new design was given in July 2009 and then a test pile was installed in October the same year. Piling work started in February of 2010 and continued for two years.
Two changes to the elevation of the tower tops had to be accommodated during the course of the project, a result of the bridge’s location in the protected World Heritage area of Haliç (Golden Horn). World Heritage body Unesco’s main concern was to minimise the impact of the bridge on the historical silhouette, particularly in terms of maintaining a clear view of the Süleymaniye Mosque. In November 2009 the Istanbul Metropolitan Municipality instructed that the towers be reduced from 82m to 65m with a reduction in the top level of cables from 63m to 55m.
A second revision of this height, further reducing the top level of cable stays to 47m was instructed in July 2011 and the revised design was not approved until February 2012. Construction of the bridge substructure and towers was completed in late September, since which time the deck erection, welding and cable installation has been progressing.
Approach bridge construction followed on from the archeological investigations with foundation works starting at the end of 2010. The approach viaducts were constructed using prestressed, cast in situ reinforced concrete by the post-tensioning method. Specially designed and fabricated structural steel frames together with a Peri system were used simultaneously to support plywood-faced formwork.
The Freyssinet system and equipment was used with prestressing steel imported from Portugal. The first concrete was poured in March 2012 with deck concrete works beginning in August the same year. By March this year, the project had reached 83% completion, with completion of construction and handover to the M&E contractor due to take place in August.
Fabrication of the pile caps, piers and deck segments for the swing bridge and cable-stayed bridge was completed at a shipyard in the Turkish city of Yalova, and all deck segments were delivered to site. Shipping of the hydraulic and electrical equipment was in progress at the time of writing, and due to arrive on site in accordance with the schedule.
The main towers for the cable-stayed bridge were each constructed in three parts and are now complete, excluding the tops, and the final deck segment for the cable-stayed bridge was erected in early March.
On the swing bridge, the hydraulic cylinders that drive the lifting and turning operation have been assembled and nine of the 11 deck segments have been erected. Construction of the two approach viaducts is almost complete – on the Beyoglu viaduct; all 18, 2.5m-diameter bored piles, eight pier foundations and and the seven sections of viaduct deck have been completed.
On the Unkapani approach viaduct all 16, 2.5m-diameter bored piles, eight foundations and eight piers are complete, as are the three sections of the deck. There are bearings in the both ends of the cable-stayed bridge together with the pendulum bearings which absorb the uplift force. Also there are bearings on approach bridges. In the middle piers there are elastomeric bearings and in the side piers there are pot bearings allowing movement in longitudinal axis. The swing bridge has two pot bearings.
Testing of the swing bridge mechanism is due to begin in July, and the construction of the approach viaducts is scheduled to be finished the month before. Once track laying, mechanical and electrical works and testing are complete, the crossing is due to be brought into service before the end of 2013.
Client: Istanbul Metropolitan Municipality
Conceptual design: Michel Virlogeux
Architect: Hakan Kiran Architecture
Structural engineer: Wiecon Consulting Engineers & Architects
Main contractor: Astaldi/Gülermak JV
Construction supervisor (engineer): Hakan Kiran Architecture
Cable, PT supply and heavy-lifting: Freyssinet
Formwork & scaffolding: Peri
Swing bridge hydraulic equipment: Waagner Biro
Fabrication & erection of steel superstructure: Gülermak
Fabrication of offshore piles: Martifer
Driving & concreting of offshore piles: UDI
Mario Mancini is technical communication project engineer; Osman Sari is construction manager; Gökhan Asçioglu is planning & project control manager; Erdem Kula is coordination & steel structures engineer; Dogan Demir is project director; they all work for Astaldi/Gülermak JV