by Jerry ElengicalJul 28, 2022
As the first fixed crossing over the historically and geopolitically significant Dardanelles Strait in Turkey, the 1915 Çanakkale Bridge is a testament to the ingenuity and logistical limits of contemporary bridge design. With a main span of 2,023 metres - commemorating the upcoming centennial of the founding of the Republic of Turkey - the structure is the new world record holder for the largest suspension bridge in the world, surpassing the Akashi Kaikyo Bridge in Japan. Designed by COWI in association with contractor DLSY, and construction companies Daelim, Limak, SK, and Yapi Merkezi, the structure is settled within a zone that is prone to high winds and seismic activity. Under these conditions, the bridge's elegantly proportioned twin-box girder carriageway is suspended from a pair of bright red steel towers that rise to a height on par with certain skyscrapers. As per the designers, their soaring forms pay tribute to Turkey’s national colours while their 318-metre heights are a reference to a historically important naval victory for the Ottoman Empire on March 18, 1915 during World War I. Expected to carry over 45,000 vehicles across its expanse on a daily basis, the bridge is an integral segment of the planned Kınalı-Balıkesir Motorway, which will be the first route outside Istanbul to connect the European and Asiatic parts of the country. After a nearly five-year long construction process that commenced in 2017, the project was completed an entire year ahead of schedule, opening to the public on March 18, 2022.
In conversation with STIR, Inger Birgitte Kroon, Project Director at COWI, and Henrik Andersen, Senior Director for International Bridge Projects at the company, delve into the structural, logistical, and environmental challenges encountered in realising this landmark endeavour.
Jerry Elengical: Considering how a suspension bridge of this span has never been realised before, what were some of the initial obstacles and considerations that had to be addressed while developing the design?
Henrik Andersen: At COWI, we have been practising on this scale for many years. However, this is a world record span, and we had not aimed for something like this before. Many years ago, we did work on the Great Belt Bridge in Denmark - which was the second longest in the world for a considerable period of time. Since then, we have done a number of other major suspension bridges, including one in Turkey actually, where more or less the same team worked on the final structure.
Inger Birgitte Kroon: Of course, for this particular bridge we had to take it one step further. One of the main challenges with all suspension bridges is to achieve aerodynamic stability in fairly windy conditions as in the case of this bridge. So, a part of the early investigations involved figuring out how to address this. But we also gave a lot of consideration towards finding a solution that was resistant and not too exposed to ship impact because there are a lot of huge container ships and all kinds of vessels passing directly up the Çanakkale Strait, including all the traffic going to the Black Sea, the Sea of Marmara, and number of other harbours. Now Henrik mentioned the other Osmangazi Bridge that we were doing, which was in a more earthquake exposed area. Here it was moderately exposed, but we aimed to find a solution that was resistant and resilient to earthquakes, ship impact, wind loads, and soil conditions. Of course, you want to look for an optimal solution, but first of all, you want to find a feasible one. Besides this, there was also the question of how we could build this in a short time, because there was a wish to have a faster construction timeline which governed a lot of our choices. We signed a contract for the detailed design in May 2017, and the bridge was opened to traffic in less than five years.
Jerry: Could you take us through each stage of the design process with respect to the involvement of various collaborators and their role in the progressive evolution of the structural design?
Inger: We worked on the project in a number of stages. There was a preliminary phase where we took over the tender design in an initiation period where we were adapting and optimising the initial design. Then we had an approximately 15-month long period for the detailed design. This was naturally a very intense time, but once we had finalised the detailed design with the contractor, the construction work had already begun. We started in May and then began the detailed design phase around the 1st of July. By late 2017, they had commenced construction of the dry dock for the caisson foundation, which ran in parallel. To some extent, this pushed us to deliver the first parts of the finalised design - with respect to anchor blocks, tower foundations, and soil improvements. The soil had to be improved at both land sides as well as under the tower foundation. That was also a critical part of the process. Once we finalised the detailed design, we had to get it approved by the authorities, and there was a long period of achieving design approval. It went through an independent design check and then all reports and drawings were reviewed by the authorities.
Jerry: Since certain design guidelines such as the heights of the towers and the span between them were provided beforehand because of their symbolic significance, how did you go about accommodating these specifications while also accounting for aesthetics and the challenges brought on by the context?
Henrik: There were a few parameters that were fixed beforehand. It was a given that the main span should be 2,023 metres, for instance. That was determined in a pre-feasibility study started by the Turkish authorities. The heaviest ship traffic also warrants such a span, and it would have never been around 1,500 metres. It was also a given that the pylons should be 318 metres high. There's a funny story about that because it's the 18th of March, a magical day in Turkish history with its connotations towards the Çanakkale victory in World War I. And 2023 is also the year when the bridge was supposed to be opened since it's the 100-year anniversary of the founding of the Republic of Turkey. Besides that, there was a corridor where the alignment could have been, but I think it was our interactions with various stakeholders that helped to optimise it. We actually ended up moving the alignment within the allowable margin with the aim of getting a better and much more feasible situation quantitatively. We could have placed the anchor blocks using a lot of concrete - which of course, has a big carbon footprint. But by moving the alignment and placing the anchor blocks in much better soil conditions, we were able to achieve major reductions in quantities. This also resulted in a lower carbon footprint and better scheduling, since we had to cast less concrete. COWI and DLSY (the contractor) worked with the concessionaire and independent checkers to get that fixed. But I think that the early decisions you make are where you can really bring about the biggest optimisations. We could also add, that for a bridge of this scale, sometimes you see what I would call ‘crazy architecture.’ And you can always do different things with it. You could design the towers in a different manner, but there's a lot that is specified beforehand - like the proportions that you have to respect because it's the forces of nature that actually drive it. On the other hand, there was a discussion about whether it would be a portal with vertical legs and horizontal cross beams, or a more traditional structural design with X-bracing. Those were our two choices. It was decided in the end, to maintain the layout that we started with. This led to a more flexible response to earthquakes rather than a stiff one, which would draw more forces to it and have a different appearance. Some might prefer one over the other. But we decided at the end of the day to go with horizontal cross beams.
Inger: I would say somebody looked into it before putting out those requirements. The 318 metre-high towers and span of 2,023 metres along with a reasonable navigation and clearance actually generate a sag-to-span ratio that is very close to 1:9 - which is likely to be close to optimal for such a bridge. It's not that we had to suffer to achieve this. You can also say that the main span of 2,023 metres follows the symmetry of the area. If you don't want to make foundations in much deeper water with a lot of trouble, a span of around two kilometres, would probably have been optimal. But ultimately, these are just requirements that had to be fulfilled. We did want to design a bridge with a beautiful view, even though, as designers, we were looking more into the optimisation of the design. We did want for it to have some elegance, but it was not completely our call.
Jerry: With regards to the prevalence of high levels of seismic activity and intense winds at the structure's location, what were some of the other considerations or measures taken to ensure that the bridge is capable of withstanding extreme natural phenomena?
Inger: The towers are steel towers and placed on composite concrete tower foundations and underneath them, there is a very significant soil improvement and a sort of gravel bed reinforced with around 196 steel pipes of diameter two-and-a-half metres each. The prime idea of doing this was to strengthen the structure and soil to withstand ship impacts and earthquakes, working with huge horizontal forces. In principle, at a very high horizontal load, the tower could dissipate energy by moving on the gravel bed without damaging the structure and the soil improvement underneath. We have not found any situations where it will actually move, and hence, it is an extra safety measure. But of course, earthquakes can be extreme enough for you to imagine it in that way. The towers were also tested in a wind tunnel as free standing towers and with regards to the feasibility of their construction. This was done to make sure that the bridge was not unreasonably sensitive to wind throughout construction, even though the site will always be sensitive to wind.
Jerry: What were some of the structural and logistical challenges you faced while transporting, assembling, and erecting these elements along the bed of the strait?
Inger: The construction period extended for just a little more than four years, or four-and-a-half at maximum. We explored methods for reasonably fast erection and worked with the contractor during construction engineering, helping with temporary structures and deck erection for fast and safe assembly sequences in high winds. We delved a lot into how the temporary connections between deck elements would work once they were lifted into place and how they would handle high winds. But that did not concern the wind so much as it did lifting weights and efficiently assembling the structure. In this vein, we looked into how the tower was split into blocks, sizes, and weights to fall within the capacity of the equipment that the contractor possessed.
Henrik: The tower erection particularly, was done in two different ways.
Inger: Yes.The lower parts were lifted in with a heavy lift vessel. The first five blocks were actually lifted whole and the rest were lifted as half blocks, optimised in terms of the height of the block compared to the tower crane and the lifting angles, etc. Then of course, a few of them were split into four: the ones close to the cross beams, which were heavier. We adhered to a principle where the blocks were bolted together initially and then welded afterwards, so that you could safely continue the tower erection and follow up by welding, allowing you to run more processes at the same time. This was done to avoid being stuck waiting for welders to complete their work before lifting the next block.
Jerry: Having been handed the task of creating the first fixed crossing across such a geographically significant waterway, what impact do you hope the bridge will have on movement across the Dardanelles Strait in the long term, particularly with regards to its integration into the planned Kınalı-Balıkesir Motorway?
Henrik: It's clear that it opens up a shortcut from southwestern Turkey towards western Europe - where you don't have to go through the highly congested Istanbul area. I think we could already see as designers when we landed in Istanbul airport and had to go to the construction site - a little bit before the official opening of the new highway - that it definitely made travel a lot easier.
Inger: Also, it's about a hundred kilometres of highway on both the European side as well as the Asian one. It cuts more than an hour of the travelling time too, which was a pleasure in and of itself. Obviously, it's fantastic to be part of the first fixed crossing of the Dardanelles. In that way, the bridge also provides a very important piece of traffic infrastructure to the area. It was quite clear to see after going down there that, for example, if there was ever some fog or some jamming up of ferry traffic, there would be kilometres and kilometres of trucks waiting to pass. In that way, it has eased life in the realm of commercial transport and individual movement. We hope that it has a positive influence on Turkish infrastructure, the economy, and the country’s overall development. I am sure it's a piece of important infrastructure for Turkey that could add a lot of value in the future. It’s a huge landmark in that way and we are quite proud of being the designers behind it.