Turkey’s newest suspension bridge – and the fourth longest in the world – is set to open to traffic shortly. The structure is a central part of the new 420km-long Gebze-Izmir motorway, and is being built to carry this new highway across the Sea of Marmara at the Bay of Izmit in northern Turkey.

Just two years ago, construction of the tower foundations and the cable anchorage was in full swing at the site in Izmit Bay, Turkey. The concrete caissons for the main towers, each having two 16m-diameter steel shafts, were sunk down to the sea bed in May 2014 by the use of water as ballast (Bd&e issue no 75), and once in place, all chambers were filled with water for stability.

The 1.2m gap between the inner and outer ring of the steel shaft was filled with concrete, and the top of the steel shaft was covered by a 3m-deep concrete slab. On top of the concrete slab, anchor frames supporting 84, 110mm-diameter and 10m-long anchor rods, and a further 8.15m of concrete was cast to complete the plinth for the tower erection. The two plinths were connected by a concrete tie beam above sea level which was constructed by installing the lower half as a precast beam and pouring in situ concrete for the upper half. Above the plinth and the tie beam, a heavy steel working platform 36m wide and 72m long was installed to create sufficient space for the superstructure erection and berthing facilities for service boats. 

Construction of the massive concrete north anchorage, which started in November 2013 below sea level, reached ground level in May 2014. The concrete structure was built using block by block construction divided both vertically and horizontally into 11 lifts, each 2m high, to suit the concrete supply of 1,000m3 per day. Once it reached ground level, PT ducts for post-tensioning strand to anchor the main cable were embedded, and the  triangular cable anchorage was cast leaving the roof and front face open to allow cable erection. 

At the south anchorage, construction of the 16m deep concrete slab that began in September 2013 by the same method as the north anchorage but divided into nine lifts, each 1.6m high, was completed in June 2014. Likewise PT ducts were installed, and the triangular cable anchorage was cast, leaving the roof and front face open for cable erection; both anchorages reached this stage in December 2014.

Construction of the transition pier and the side span pier were carried out in parallel on both sides of the bridge. During construction of mass reinforced concrete, limiting the occurrence of early-age cracking was one of the main tasks intended to protect the reinforced concrete during the 100-year service life.

Specifications stated that only cracks of 0.3mm width or less were allowable, and any larger cracks must be repaired. Prior to construction, a temperature-stress analysis for a typical cross-section was carried out and during the concreting process, internal cooling pipes were used to control the temperature difference using a thermocouple embedded in the concrete. 

During the last two months of 2014, a total of 30, 210mm-thick cross-head slabs to which the main cable strands were to be anchored, were installed on the face of each of the anchorages. Two 6-37 PT strands were used to secure each large slab supporting four main cable strands and two 6-19 PT strands were used for each small slab supporting two main cable strands.

Steel tower erection also began in the summer of 2014 with installation of the base segment of the north-east tower leg on the plinth. This segment was 8m high and weighed 340t; it was transported and installed by a 1,600t-capacity floating crane. The unit was transported from the storage area where inspection ladder, guide rails for elevator and scaffolding for joint welding had been attached, and was lowered down slowly towards the top of the plinth. During this process, 84 anchor bolts had to be inserted into the bolt holes of the base plate and bearing plate of the base segment. Once in place, orientation of the unit and its inclination were adjusted using  separate sets of jacks, the gap between the bottom of the base plate and the top of the plinth was filled with non-shrink mortar, and the segment was connected to the tower foundation by introducing 60% of the full tension of 6MN to the anchor bolts. 

The second tower leg segment was transported by the floating crane, and placed on top of the base segment, being guided by matching pieces which had been welded to the bottom of the second segment and the top of the base segment in the shop, to produce the required orientation and alignment of the tower leg. The two segments were connected as soon as they had been correctly aligned, and maintained using temporary friction resistant bolts on the vertical stiffeners and by butt welding for the perimeter plates followed by friction resistance bolts for the vertical stiffeners. 

Tower erection continued in the same way for the tower leg segments from number three to number 11, which were typically 13m high and up to 260t weight, by using the floating crane, allowing subsequent segments to be placed before completion of butt welding for the previously-erected segments. The lower cross-beam was erected by the floating crane; together with a support frame of a 46t-capacity self-climbing crane which was subsequently used to erect the segments from number 12 to 22. The rest of the self-climbing cane was also erected on the support frame by the floating crane.

Once the self-climbing crane was fully assembled, in late October 2014, erection of tower leg segments 12-22 and the upper cross-beam began. The tower leg segments consist of five stiffened panels, identified as inner, outer, front and rear, and a diaphragm. The inner and outer panels, parallel to the bridge axis, were erected by the crane and placed on top of segment 11, guided by the matching piece, and fixed temporarily. The diaphragm panel was erected and connected to the inner and outer panels and then front and rear panels, perpendicular to the bridge axis, were erected on top of segment 11, guided by the matching piece and connected to the inner and outer panels and the diaphragm. 

Once the vertical joint between the four panels had been made with friction resistant bolts as guided by pilot holes, the two segments were connected in the same way as for the lower segments 2-11. Tower erection continued in the same way for the tower leg segments 13-21, which were typically 9m high and weighed up to 140t (panel weight less than 40t), allowing subsequent segments to be erected without completion of butt welding for the previously-erected segments. The self-climbing crane also erected the upper cross-beam. When the segment 14 was erected, two active mass dampers of 10t mass each were installed together with this segment on each leg. They will mitigate vortex shedding oscillation during construction and in the service condition. The final segment, numbered 22, was erected piece by piece due to its weight; this segment supports the tower saddle and it is much more structurally complex. Tower erection was completed in the middle of December with the installation of the upper cross-beam.

Preparation for cable erection began the following year, with installation of the catwalk taking place once the cable saddles were in place on the anchorages, the side span piers and the tower tops.

The first operation in the installation of the catwalk was to erect two hauling lines between the north and south anchorages, bringing the rope by barge from north to south as it was unwound from winches on the barge, making a loop and laying it down on sea bed. It was lifted up and fixed to top of the tower once it arrived at the tower foundation, and two ends were connected to make the complete loop once it arrived at the south anchorage. Marine traffic only had to be interrupted when the ropes were erected in the main span. The catwalk was made of ten strands, of 29.6mm diameter for the floor, separated at the top of the tower, and two of the same strands for the hand rope, which was continuous right across the bridge. The catwalks for each cable were connected by cross-walks at intervals of approximately 140m, providing access and stability, and there were cross-beams in between for additional stability. 

For the side spans, ten catwalk floor strands were unwound from reels on a barge, pulled to the anchorage and fixed, wound further from reels on the barge moving toward the tower, laid down on sea bed, and lifted up and fixed to the anchor girder at the tower top when they arrived. For the main span, floor strands were erected during a shipping closure, by unwinding from reels on the barge, lifted and fixed to the anchor girder at one tower top, then unwound further from reels on the barge moving towards the other tower foundation, laid down on the sea bed, and lifted up and fixed to the anchor girder at the other tower top when they arrived. 

With the floor strands fixed in place, mesh and timber steps were rolled out across the spans from the tower top, together with the cross-walks and cross-beams. However on 21st March, with the floor completed on both side spans but still in the process of installation on the main span, the south end of the east catwalk on the main span broke away from the support beam to which the anchor girder was connected at the tower top and fell into the sea. No-one was injured in the accident. Subsequently a detailed investigation was carried out and a recovery plan put together, involving fabrication of new support beams, procuring new floor strands and tension rods, and a full check and repair of all catwalk elements. Within five months of the accident, by mid-August, the catwalk was completely rebuilt with new support beams manufactured by a local fabricator under strict quality control and new catwalk strands and tension rods manufactured in Japan. 

The tie-back ropes which had been previously installed were re-tensioned to bring the tower top back to the appropriate position for the main cable strand erection. 
A tramway system was installed on the catwalk, alignment of the hauling lines was confirmed, and plastic rollers to support the main cable strand were placed beside the cable saddles and on the catwalk floor. 

The main cable of the Izmit Bay Bridge was constructed by means of prefabricated parallel wire strand, 127 number of 5.91mm-diameter wire having a breaking strength of 1,770 MPa, 3km long and weighing 90t. The first main cable strand was pulled on 9 September 2015 between the two anchorages. It was pulled out from horizontally-wound steel reels which were set on an unreeling machine behind the north anchorage towards the south anchorage at a hauling speed of 30m/min. On the catwalk, the strand was carefully monitored to ensure no twist occurred and there was minimum breakage of the seizing tape. Once the strand arrived at the south anchorage, it was clamped temporarily at the cable saddle locations, lifted off the rollers and shifted transversely above the cable saddles. The sockets on both ends were connected to the cross-head slab at the anchor face with 64mm-diameter tie rods; the cross-sectional shape of the strand was changed from hexagonal to rectangular over a sufficient length, and the strand was placed into its final position between spacer plates in the cable saddles. During the night, weather permitting, each strand was adjusted to its design sag in each span to allow the subsequent strands to be erected the following day. 

Special care was taken for the first strand, which was used as reference strand for the sag adjustment of the subsequent strands; this was adjusted by measurement of span length, strand sag and strand temperature in each span and by moving it at the cable saddles until the measured sag fell within the allowable tolerance for the design sag calculated for the measured span length and temperature. The subsequent strands were able to be adjusted much more quickly as they were adjusted relative to the sag of the reference strand within a given tolerance. 

Since the bridge has deviation saddles on the side-span pier which make the anchorages rather compact, the strand erection and sag adjustment took longer, and several measures were required to avoid the strand slipping over the deviation saddle. At the initial stage of the strand erection, the alignment of the plastic rollers was adjusted, and the tension introduced into the strand via the unreeling machine was increased, to improve and increase production. Together with increased familiarity with the operation, the production rate was increased to three strands per day per main cable. Consequently, the erection of 112 strands on each main cable was successfully completed in early December.

Once the hauling lines, tramway beams and tie-back ropes had been dismantled, compaction of the main cable using eight compacting machines began, followed by installation of cable clamps. Erection of the hanger ropes followed, with ropes lifted directly from a barge on the sea below and connected to the cable clamp. 

In July 2015 the transition span decks between the side-span pier and the transition pier – the north 114.6m long and weighing 1,800t and the south 99.6m long and weighing 1,500t – were erected segment by segment onto the supporting bent provided between two piers. They were lifted off the barge and placed on the supporting bent by a 1,600t-capacity floating crane, skidded towards the transition pier, adjusted to the design pre-camber geometry, and connected to each other with welding and friction resistant bolts. 
Erection of the suspended deck began in January with use of a 1,600t-capacity floating crane and continued later with 350t-capacity lifting gantries. 

The suspended deck is supported by hanger ropes typically spaced at 25m intervals, thus the deck segments assembled were typically 25m long and 300t; 113 segments in total. However in order to save time, 52 of the typical segments were paired up into segments of twice the length before lifting, as a result reducing the number of main span segment lifts to just 87 and saving time welding segments after lifting. Once lifted, the deck segments were generally connected to the permanent hangers and the deck segment previously erected, but at the tower and deck end, where the hanger spacing extends to 70m and 65.85m, they were also connected to temporary hangers. Connections between deck segments were made with matching pieces to reproduce the deck geometry and the temporary connections to transfer forces between the two segments.

In early January, the 11.8m-long and 365t deck segments transported by a dynamic positioning barge were erected by the floating crane at the north and south towers, followed by the 19.7m-long, 330t deck segments erected on each side by the floating crane in February, and subsequently three deck segments were connected by welding and friction resistance bolts. In the middle of February, typically 25m-long and 300t five deck segments were erected by the floating crane at the middle of the main span. Later the same month three deck segments, 10.4m, 18.8m and 25m long and weighing from 250t to 330t, were erected by the floating crane one after the other at the north and south deck ends, and subsequently were connected by welding and friction resistance bolts.

The lifting gantries were erected by the self-climbing crane and assembled on the main cable near the tower top and sent to the middle of the main span and near the side-span pier, from where the deck erection began; two others were assembled on the main cable at the middle of the main span after being lifted into place by the floating crane to save some time.

In later March, once a set of two lifting gantries was made ready in the main span, the first 50m-long and 600t deck segment transported by a dynamic positioning barge was erected, followed by 23 more 50m-long segments erected by two sets of two lifting gantries each moving from the mid-span towards the tower; some 1,200m of deck in 17 days. In parallel, a total of 16, 25m-long 300t deck segments were erected by the lifting gantry moving towards the tower both in the north and south side spans; 800m of deck in 14 days.

Deck erection then switched to the segments at the tower location toward the closure segment. Four closure segments were lifted one after the other in gaps secured by setting back the deck segments that were already in place. The suspended deck was finally made continuous with lifting of the last closure segment on 19 April. The deck segments were erected with inspection walkway, brackets and trays for electrical cabling already in place where possible, along with scaffolding for joint welding.           

Deck welding started before all the deck segments were in place from where the gap between two segments could be closed with only a small force. This will be followed by road surfacing, traffic barrier installation, electrical and mechanical work and so on once all the deck welding is completed. Waterproofing was due to begin at the end of April and must be applied in just 25 days. Stirling Lloyd’s Eliminator system has been specified for the 96,000m2 steel orthotropic deck. Cable wrapping, painting, hand rope and access way installation on the main cable will also be carried out and all temporary works such as the catwalk, self-climbing crane, construction lift and working platform have to be dismantled. The Izmit Bay suspension bridge will be open for traffic before summer 2016. 

Yasutsugu Yamasaki is deputy project manager and Manabu Inoue is general manager for engineering at IHI Infrastructure System Company. Ali Nebel Ozturk is employer's representative and Fatih Zeybek is project manager for Nomayg JV.

Izmit unveiled

The motorway is being promoted by Otoyol Yatirim Ve Isletme, a joint venture of Nurol, Ozaltin, Makyol, Astaldi and Gocay which is contracted by Turkey’s General Directorate of Highways as build-operate-transfer concessionnaire. Nomayg Joint Venture, which consists of the same five companies, is the engineering procurement construction contractor responsible for building the whole 420km-long road, which includes the Izmit Bay Suspension Bridge.

Construction of the bridge was contracted to IHI Infrastructure System and Itochu consortium. In September 2011 notice to proceed with the detailed design of the bridge was given. This was carried out by Cowi under supervision by IHI. The bridge is situated in a very active seismic area where the 7.6 Kocaeli earthquake occurred on the North Anatolian fault in 1999.
Nomayg set certain criteria for the reference design, in that it should be a suspension bridge, with a total length of between 2.8km and 3km, and a main span of between 1,550m and 1,700m. The size of navigation channel of 1,000m and height of 64m was also set. 

The chosen design has a main span of 1,550m and side spans each 566m long with a total suspended deck length of 2.7km which is continuous between the two side-span piers. The main cables deviate at the side-span piers toward the cable anchorages which are below the deck of the transition spans. The back spans of the main cable between the side-span piers and the splay saddle are 92m at the north and 67m at the south. There are 114m-long transition spans at the north and 99m-long transition spans at the south.