A part of the Northern Marmara Highway project, the Ihsaniye viaduct connects the roads of Yavuz Sultan Selim bridge with the main highway route to access the New Istanbul Airport.
The viaduct is made of two parallel decks of respectively 856 et 867 m. with 5 traffic lanes.
Original design consisted of a 45m span deck with pre-tensioned I-girders and a link slab as well as large foundations. Erection of the deck was planned by using a beam launching gantry.
The alternative design proposed by Freyssinet was based on post-tensioned single cell box girder that could be erected by incremental launching. The deck had been divided into 14 spans, with typical spans of 66 m, which made the project outstanding in terms of maximum launching span without temporary support.
The Incremental Launching Method (ILM) would also facilitate the crossing of two existing highways. Above one of them, an 80 m span was proposed to avoid the construction of a permanent pier and foundation in the median. Hence a temporary pier was placed between two carriageways so that traffic would not be disrupted during works execution.
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The seismic design was the major focus of optimization – with seismic devices like Fluid Viscous Dampers and isolation through flexible portal frames at pier top – and a key advantage over the original design.
Moreover, preferring the incremental launching method allowed significant acceleration of construction time, and equipment reuse; hence the launching nose was upgraded from the previous Northern Marmara Highway project including a modification of the 1st segment (3.75m longer), strengthening of the other segments and of connections between 4 segments, and adaptation of the deck-launching nose connection with 12 Freyssibars D66 per main girder. The launching device was adapted to the deck geometry and calculated after measuring the friction coefficient on sliding bearings on a few launching tryouts. On the precast yard, sliding bearings were completely reused and with regards to piers and abutments, bearings were adapted and fitted out with guiding devices to both guide the deck and transversally restrain it during launch and stop phases.
Design of the substructure was contingent on earthquake combinations, given the levels of ground accelerations. Therefore, the use of ILM did not lead to increase of piers and launching abutment as it could be the case in a non-seismic region: adopted seismic strategy enabled to significantly decrease the foundations size.
While the deck geometry was straight in plan and in elevation, it was launched up the 0.5% slope using two 600-tons strand jacks. Such a launching force was high and rather unusual for this type of construction method, however necessary for 66 m long spans, therefore pushing the limits of maximum span length to be launched without resorting to temporary support.
Another specific aspect of the project was the precasting yard, arranged in three work areas, independent in space but parallel in time: casting of U-section, casting of top slab and prestressing works. This organization resulted in accelerated work cycles and improved safety conditions.
Controls performed during concreting (stop phases) would lead to regular checking of the casting bed and bottom slab, which were important parameters to monitor in case of settlement. Cracking of the deck was avoided, due to the adjustment of the vertical position of the casting bed.
At launching phases, measurement of the longitudinal displacement of piers was checked as well, considering their slenderness.
Launching forces were also subject to regular controls, to prevent any sliding pad or jack issue.
Ihsaniye has been a significant achievement with the completion of a 1,732-meter-long Viaduct in a 13-month time frame, including the design. In Turkey, it also opens a new way for constructing bridges with far less concrete, which means a reduced carbon footprint. Since the successful completion of Ihsaniye, 6 other bridges have been completed using the Incremental Launching Method in the country.
