Site visit: Lusail City suspension bridges
A pair of suspension bridges being built by Samsung C&T will become the focal points of the $50bn Lusail City project in Qatar
There was a dramatic moment in contractor Samsung C&T’s delivery of a unique engineering package at the new $50bn Lusail City project in Qatar.
The project features a pair of suspension bridges with huge circular structure as the pylons. These are the first of their kind in the world. The pylons are 50m in height and 30m wide, dissecting the bridge’s main girder
Given their sheer size and weight, the pylons were each fabricated in nine separate sections in South Korea and shipped to Doha Port, before being delivered to the site.
The project’s construction manager, Dr Wonjin Yu, explains that putting these sections together was not an easy process.
“We cannot bring it in as one, so we divided it into nine pieces. The the biggest piece is 141 tons,” he tells Construction Week.
“Then it was installed one by one. And the steel shape was changing every day.”
He explains that a number of factors – the heat, movement of the bridge’s girder below, the wind and the increasing weight of the central pylon – all meant that structures assembled in factory conditions in South Korea had been altered slightly.
Given that eight sections of this pylon had already been put into place, making a perfect circle with the ninth when it was being craned into position in September last year was far from guaranteed.
“We did all of the calculations and we estimated how it would react,” Yu says. “Then, finally, we installed the key segment.
“At that time, if it had been a success with the construction on both sides, this key segment would be inserted smoothly. But if we failed, there would have been a big problem.”
Thankfully, he says, the installation on both bridges went smoothly.
“We inserted it without any adjustments inside,” he says.
After this key section was finished, Yu said he was later asked by a senior figure from the company’s headquarters how much of the steel had to be cut in order to install this last section.
“I told him nothing. He said to me, ‘you are lying!’. But there was nothing cut – we dropped it in. I am very proud of this.”
These two suspension bridges are the centrepiece of the rather long-winded Lusail Development Primary Infrastructure Construction Package 3A phase 2 (CP3A) – one of a couple of contracts being carried out by Samsung C&T ahead of the widespread property development work expected to take place in Lusail over the next five years or so.
The CP3A works involves the construction of seven bridges in total which will link four newly-created offshore islands both to the mainland and each other.
This group of islands, Qetaifan Islands, form a curve off the coast of the northern part of the city (see Lusail masterplan).
They have been formed by excavating a huge section of land, which currently has breakwaters in place and a series of pumping stations keeping it dry. However, once further excavation works are complete, the project’s client, Lusail Real Estate Development Company, plans to flood the channel between the islands and the new shoreline.
Lusail Real Estate Development Company is part of Qatari Diar – the real estate arm of the country’s sovereign wealth fund.
When completed, Qetaifan Islands will have a land area of 2,566,000m2 and will house thousands of luxury residential villas. The eventual population on the islands is expected to be 42,688.
The bulk of these will be on North Island, which is the biggest of the four by some distance. It will be joined to the mainland by the smaller of the two suspension bridges – the 186m-long bridge 9. The other suspension bridge, bridge 8, is 204m long and joins the mainland to South Island 1.
North Island and South Island 1 are themselves both connected by the longest of the concrete bridges – the 221.5m-long bridge 7.
Bridges 5 and 6 run parallel connecting South Islands 1 and 2, and are 123m long, while bridges 3 and 4 connect South Islands 2 and 3 and are 116.5m.
The bridge designs were carried out by design consultants Halcrow and COWI. Parsons is project manager and Dorsch Qatar is contract supervision consultant.
Samsung C&T was appointed as main contractor on 15 November, 2011 and under the terms of the original contract was due to finish the works by 28 February, 2015.
However, deputy project manager Byonghoon Lee said there had been some variations to the initial contract. Enhancements have been made to include some spectacular lighting features, with sequenced lighting displays.
The $97m contract is almost complete, though, with the bulk of work on the five concrete bridges expected to be closed out by 28 February.
In total, around 1,090.5m of bridges have been built. The curved central pylons of two steel suspension bridges have some other unique technology.
Both bridges have a short side span and a longer main span – on bridge 8, the short span is 60m, the pylon width 30m and the long span 114m. Yet unlike typical tower suspension bridges, most of the cable strands are anchored to the bridge’s own girder as opposed to the earth at either side. This is to accommodate both horizontal and vertical movement from the pylon ring.
The main cable itself is also a one-off. It comprises seven locked-coil cable strands that are each 140mm wide. One central strand is surrounded by six outer strands of the same width, with fill strips between them and an outer sheath. This brings the main cable size to 424mm.
Dr Yu explains that use of these locked coil strands is rare outside Europe, and an arrangement of cables built to this thickness for the bridge’s main cables is another one-off. Clamps are then placed on this and 70mm-wide locked coil strands are hung from it as hanger cables.
They also have a girder which is 29.4m wide, which is enough for three car lanes on either side of the carriageway, although only two of these will be used on opening. A crash barrier will separate the third carriageway, which will be used by pedestrians on opening.
The girder itself is huge, with around 2,198 tons of steel used for its structure on bridge 8 and 1,991 on bridge 9. The amount of steel used for the pylons on bridges 8 and 9 is 893 tons and 797 tons respectively, meaning a total of 5,879 tons was used on the two suspension bridges.
The girder has a steel base, and steel cross sections that are 6m apart. These sections are infilled with precast concrete slabs. Like the pylons, the girders had to be shipped to Doha New Port in sections, and then transferred to the site.
“At Doha Port, it was amazing. People were asking ‘what is this? Why is it so long? Why is it so heavy?” Yu says.
And even though it was transported in sections, at 21m x 6m it required both a police escort and a test run with a mock frame to ensure its delivery from downtown Doha. The cables were also shipped in from the UK.
The other seven concrete bridges are of more typical construction, with a series of precast concrete piers between bridge spans. The length of bridge 7, however, has required some extra support.
It is 221.5m in length, and has two, 41.25m spans between anchor points and the first two piers and three, 45m spans between those and the two central piers.
To make sure the bridge retained its strength in its central sections, steel strands were fed through ducts and post-tensioned along its full length once all of the shoring was removed.
Once work completes, more excavation will take place beneath the bridges which will expose sections of the suspension bridge rings which currently remain covered. Of the 50m pylon, just over 41m appears above the bridge’s girder while the bottom 5m or so protrudes underneath the bridge.
Byonghoon Lee said that once the area is flooded, part of the ring will eventually be submerged.
At the peak of the project’s activity, some 1,656 people were working at the site. By mid-January, just over 7.64m man-hours had been completed, and the contractor had achieved over 6.26m man-hours since the last lost-time incident.