Braila Bridge in Romania (Webuild)
On Tuesday 28 June, the last closure segment was installed on the 1,975m-long Braila Bridge between the town of Braila and the village of Jijila in eastern Romania, around 50km west of the Ukrainian border. The project was commissioned by state company CNAIR on behalf of Romania’s Ministry of Infrastructure and is being built by a joint-venture formed by Italy’s Webuild (60%) and Japanese company IHI. The contract is worth approximately US$570 million and includes the design and construction of the suspension bridge as well as access viaducts and connecting roads. When completed, the bridge will provide a 180m-wide navigational channel and a clearance of at least 38m. Its main span will be the longest for a suspension bridge in Romania and the second longest in continental Europe.
The suspension bridge has two 192m-high concrete towers, a main span of 1,120m and lateral spans of 490m and 365m. The deck is formed by 86 steel segments, 72 of which are 25m long, 31.7m wide and 3.2m high, weighing around 260t. The remaining segments are shorter but heavier, up to 350t, due to the extra reinforcement they contain to withstand the loads at their locations around the towers and anchorage blocks.
It took four months to install the suspension cable (Webuild)
The 56cm and 58cm-diameter suspension cable (respectively for the main span and the side spans) consists of 16 strands, each containing 554 galvanised wires. It was installed over the course of four months using the air spinning method and, after wire wrapping and painting, will be protected from corrosion with a dehumidification system. The cables are anchored in massive 45m-diameter anchorage blocks consisting of 100,000t of concrete.
The cables are anchored in concrete blocks 45m in diameter (Webuild)
At a time when virtually all building-related costs are high, the project has benefitted from a short supply chain – something almost unheard of in bridge construction. The deck segments were fabricated at a shipyard situated 7km along the river in Braila and were made using steel supplied from a steel factory in Galati, also in Romania. “It is a huge benefit for everyone. Everything is very easy and there is no need to travel round the world to carry out inspections. We have had full control of production on a day-by-day basis,” says Alessandro Minniti, contractor representative of the project and project manager at Webuild.
Minniti has been working on the project in Romania for four years and was previously construction manager on the skytrain cable-stayed bridge for Sydney Metro Northwest, the first curved bridge of its kind in Australia. He explains that IHI is leading the cable installation and segment lifts due to the similarities between the Braila Bridge and the Izmit Bay Bridge in Turkey, which had Webuild as part of the JV leading the project and IHI as sub-contractor. Aside from the Izmit Bay Bridge having a slightly longer main span of 1,550m, the two are virtually identical. “The only differences are that this is over the Danube River instead of the Marmara Sea, and the towers here are made of reinforced concrete and in Turkey they were made of steel. The shape and geometry of the deck are practically identical so the work method statement is very similar,” he says.
The barge brought two steel deck sections at a time for installation in a land/river span sequence (Webuild)
Accordingly, the initial construction method consisted of transporting the steel segments to site by barge and then directly lifting them into position using devices on the suspension cables. For the land sections, the segments would first be lifted off the barge by crane onto SPMTs and then the same procedure followed. One difference between the two projects lay in the lifting devices, which in Romania consisted of strand jacks linked by a connecting beam across the suspension cables, while lifting devices with winches were used in Turkey. The connecting beam’s function was to facilitate the movement of the strand jacks onto the next set of hangers, using a winch installed at the top of the tower.
While this method plan was followed for the river sections, it was significantly altered for the deck segments over land, where the plan to use a crawler crane to lift these onto awaiting SPMTs was reviewed to avoid the construction on the banks of the River Danube of an additional sizeable dock with associated piles and mooring equipment, as would be required for a crane capable of lifting 260t-heavy steel segments. This was considered a costly option and one with associated environmental implications, which would result in further procedural constraints.
The ‘Tarzan System’ was used to shift the deck segment from river to land (Webuild)
Instead, the land-side segments were lifted off the barge onto the SPMTs using a combination of strand jacks in an ingenious sequence dubbed by Minniti as the ‘Tarzan System’. “When the barge arrives, we use two sets of strand jacks, one positioned vertically over the segment and one diagonally, over land. The vertical pair are in tension, and the other pair loose. So we lift up in the vertical direction over the barge, and then we start to loosen the vertical strand jack and start to engage the second, diagonal strand jacks, shifting the segments to the land side. Our partner IHI had used a similar system 20 years ago in a project in the USA.”
The Tarzan System avoided the construction of an additional river dock (Webuild)
With all segments now in place, the Tarzan System has been successful, even if some teething problems were experienced that required some modification. “Due to strong winds the wires in the strand became twisted and the jack could not strike onto the free length of the strands. So we had to stop for a couple of days. We dismantled the strand jack, re-straightened the wires and installed steel clamps every 5m to have the wires in a fixed bundle. So we lost a little time to make the adjustments but finally the result is there and we did it.”
The segments were installed in a particular sequence in order to avoid excessive bending moments at the cable towers. As such, each typical barge load consisted of two segments that were installed in a river/land sequence. The effect of the load on the cables was especially visible during the lifting of the first segment on the main span, says Minniti, when the cable could drop down by as much as 3m until the corresponding second segment had been installed on the side span, thereby balancing the first. For the final standard lifts, when the cables were supporting 22,000t of steel deck, such movements became negligible.
Once fixed onto the permanent hangers, the segments were coupled only at the top plate to allow the deck to follow the movement of the cable, “So the deck opens up when the cable lifts up and closes when it lowers, otherwise the tension on the bottom plate would be huge. We say that the bridge has to breathe,” says Minniti.
The segments around the tower are up to 44.8m in width, wider than the standard segments, because the pedestrian path separates from the main deck and goes around the outside of the tower legs. To install these, the five segments that make up each tower section were assembled on the ground on temporary supports, welded, and then lifted up with strand jacks over the course of five hours, with a total load of 1,000t. The final section comprising the exterior walkway will also be lifted.
The deck sections at the anchorage blocks were welded on the ground and lifted into place (Webuild)
The sections at the anchorage blocks are also wider, where the pedestrian walkways veer around the cables as these enter the blocks. These sections are 60m long and comprise three parts welded together on temporary towers before being lifted into their final position with one end connected to hangers and the other resting on the permanent bearing at the anchorage block.
The last infill segments to be lifted were near the river bank, at locations where non-Tarzan, purely vertical lifts could be carried out. Welding of the deck is now under way, an operation that is expected to take two months, after which waterproofing and asphalt works will take place.