Twin peaks

A hundred metres away, on an equally tall building opposite, a gang is wrestling with a hose spewing concrete pumped from 200 m below as they hurry to finish the final section of the deck that will form the 43rd floor of a second 44-storey tower. After last year's crane collapse in which three people died, the structure on this scheme is less advanced. This will be the shiny new world headquarters for another banking titan, HSBC.

Amazingly, both of these 210 m towers stand beside the 240 m high, pyramid-capped icon of One Canada Square, Britain's tallest building. They will be the joint holders of the title of Britain's second tallest building. They were both started, and will both be finished, at almost exactly the same time. Aside from height similarities, both towers also have a common developer in Canary Wharf Group and contractor in Canary Wharf Contractors.

You might assume that the two towers have identical project teams as well, but these have been kept entirely separate. Similarly, the towers have different architects. UK-based Foster and Partners designed the HSBC tower, and the North American team of Cesar Pelli & Associates designed the Citigroup building.

The rising distinctions
The two teams work separately, but not in isolation. Every two weeks, the managers visit each other's site. This allows them to discuss construction problems, while casting a critical eye over each other's work. "We certainly don't visit to tell them how well they're doing," says John Spencer, senior construction manager on the HSBC building.

The race to be the first to finish started in February 1999 when piling was begun on the HSBC tower. A month later, piling started on the Citigroup building. The project managers on both schemes stress that they are not in competition with each other. Even so, both teams give the impression that professional pride is at stake to be the first to finish.

The transatlantic nationalities of the design team reflect the origins of each of the architects: UK-based structural and services consultant Arup is working with Foster, and the Canadian practices of Yolles Partnership and Mitchell Partnership are responsible for, respectively, structural design and services on the Pelli tower – the same design team that worked on the original Canary Wharf tower. "The design team are all located within a couple of blocks of each other in Toronto," says Dean Ricci, project director on the Citigroup tower.

Like the design teams, the specialist contractors are different for each building.

This is a deliberate decision by Canary Wharf Contractors to spread the work because of the size of the projects. "We didn't want to overcook the market," says Winston Huth-Wallis, senior project manager on HSBC.

Both towers have a concrete central core, from which the steel frame is supported. Steelwork contractor Cleveland Bridge led the construction team on the HSBC scheme; the concrete contractor was Byrne Brothers. "Construction of the building's frame was let as an integrated package to overcome any interface problems between the different trades," explains Huth-Wallis. The specialists were encouraged to take the initiative in deciding on the method of the building's construction.

For the Citigroup project, roles were reversed: construction was led by concrete specialist O'Rourke Civil Engineering, with steelwork supplied and installed by the Dutch-Belgian joint venture Buyck/Hollandia.

Behind the scenes of creation
Work on construction of the towers' reinforced concrete cores started in August 1999, following completion of the piling and construction of a 3 m thick concrete raft foundation. The central core runs the full height of each tower, decreasing in section towards the top of the tower, their layout dictated by the arrangement of four banks of six passenger lifts. The cores provide the building's lateral stability and houses the service risers.

Both buildings used a jump-form technique to construct their central cores. This is a climbing shuttering system that "jumps" the core in stages as the concrete is cast. This is a different form of construction from that used on the Canary Wharf tower, which was an all-steel structure.

Once the cores had started to rise up out of the drained cofferdams constructed in the docks, installation of the steelwork followed. "We were keen to get the steelwork under way," says Paul Lynchehaun, senior construction manager on the Citigroup scheme, "because it lets the other trades follow swiftly behind." With the concrete contractor setting the pace on both buildings, the skyward procession of trades began. Following close behind the jump-form rig was the steelwork contractor. Steel beams were connected to the core using embedded steel plates cast into the core during the jump-forming process. As soon as the steelwork was complete on each floor, profiled metal decking was laid between the beams and the floor-laying team started work.

Lightweight concrete was used to construct the 55 × 55 m floors. This was mixed in a batching plant on site using 12 mm Lytag aggregate shipped from Poland and delivered to site by barge. One problem was that the stone had to be soaked to prevent it taking water from the concrete mix during pumping. A lack of space on site meant this was done in a series of water-filled barges moored adjacent to the batching plant. Because of the buildings' height, a pumping specialist had to be flown in from Australia to adjust the concrete mix as the pumping height increased.

Once the floors were cast, fire protection of the steelwork could commence. The protective coating was pumped up and sprayed onto the steel, which Huth-Wallis admits is a messy option, but it reduced the number of men needed and did not take up valuable hoist time. "You design your process to suit the conditions," says Huth-Wallis.

Next, the cladding on both towers was installed while in the risers and services cores, the M&E specialists started installing services and lifts. The cladding, fitted from inside the buildings, was delivered on palettes and distributed to each floor using a goods hoist. This meant that the cladding did not take up hook time on the cranes and its delivery could continue uninterrupted by wind. Huth-Wallis says using the hoist meant the size of the cladding had to be decided early in the project to fix the dimensions of the goods hoist.

Time, space and motion
By April 2000, the contractors on both towers were ahead of schedule. The problem now was transporting the materials and the 1200 people working on each tower to their lofty stations. "High-rise construction places a big emphasis on logistics, sequence and rhythm," explains Huth-Wallis. "Vertical movement using cranes and hoists is vital." Each tower is fitted with five hoists and four tower cranes. During the day, the hoists carry men and materials. Once the day shift finished, the cranes and hoists worked all night ferrying materials ready for the following day's work. "Even so, contractors still had to book a hoist three weeks in advance," says Citigroup's Ricci.

Getting materials to the towers was another headache. A cramped site surrounded by water, other construction work and a road created what Huth-Wallis calls a "logistics nightmare". The problem was worsened on the HSBC job by the construction that started on the site adjacent to the tower that Huth-Wallis had been using as a storage yard. To rectify this, a holding yard was created several miles down the Thames at Dagenham, and materials were brought to site by barge.

In this atmosphere of feverish activity, disaster struck. On 21 May, one of the tower cranes on the HSBC site crashed to the ground while its height was being increased, killing three men. "It was a real blow to morale; we worked as part of a close team," says project manager Huth-Wallis. "When something like that happens it scythes your legs from under you." The accident delayed work for three months. "But you have to get moving again," says Huth-Wallis. The team have picked themselves up after the tragedy and are rushing to get the project back on track. Things were going well until high winds stopped work on the site for days on end between December and February. "It played havoc with the schedule," says Huth-Wallis. "In the last month we've lost three out of four weeks of crane time." Despite these trials, the structure of the Citigroup tower stands complete, the HSBC tower will follow shortly, and both are on target to open in February 2002. Meanwhile, they have already brought the lofty isolation of Britain's tallest building to an end.