Who's the new kid?

The iconic trio of skyscrapers that shape the skyline of Canary Wharf are about to get a baby brother. To the east of the landmark tower and its younger siblings – the Citigroup and HSBC buildings – three rectangular columns of concrete rise like a cluster of factory chimneys. These edifices, crowned by a cluster of cranes, are the completed service cores for the 33-storey headquarters that Barclays will call home from spring 2005.

For Canary Wharf Contractors, the banking giant's building is a truly significant project: the first tower to be designed and built on the estate since 9/11 and its latest opportunity to refine construction techniques developed over 15 years.

In that time, CWC has built 17 major office buildings, totalling more than 1 million m2 of space, along with retail and leisure schemes.

As the construction arm of Canary Wharf Group, CWC is in a prime position to strive for continuous improvement: it is responsible for constructing the shell and core of all the buildings on the estate. Sometimes, as is the case with Barclays, it takes on fit-out too.

"We have ownership of the whole process from lease agreement to scheme design through to completion and occupation," says Paul Lynchehaun, CWC's senior project manager for Barclays. It is a sentiment echoed by his boss Dean Ricci, CWC's director of construction Ricci says if anything goes wrong, "there would be no one else to blame".

To help ensure that nothing does go wrong, CWC records all its previous construction experiences, good and bad, through a formal feedback programme. "This involves us sitting down with a [specialist] contractor and getting them to tell us what they have learned from this project, what they did right, what we did right and how we performed," says Lynchehaun.

For the Barclays tower – officially known as One Churchill Place – the construction team has had a major input in the concept and detailed design, tackling issues such as the cladding detailing in an attempt to make its installation trouble-free. "Poor detailing costs time and money," says Ricci, chanting what one suspects is a favourite mantra of his. The layout of the building's core was also amended to ensure ease of construction and the ability to release areas of the building to trade contractors early in the construction programme.

The concrete cores, which run from the building's basement car park to its rooftop plant room, are one of its more innovative construction features. Rather than the more common arrangement of a solo core running through the centre of the building and housing services, lifts and escape stairs, this building has three cores. Two satellite cores house escape stairs to speed evacuation from the perimeter of the building's giant floor plates, while the enormous central core contains two additional escape staircases along with the more usual lifts and service risers.

The stairs in the cores are 1400 mm wide, 300 mm wider than standard escape stairs – increasing their capacity and thereby speeding evacuation of the building. The stairs are also large enough to act as a refuge for the building's occupants to retreat into in case of an emergency. Their robustness has been enhanced by increasing the core's wall thickness by 100 mm to 400 mm.

Construction of the huge central core was quite a challenge. When CWC asked the various specialists on its database to tender for the contract, it got a mixed response. One contractor said it could not be done; another proposed that its construction be tackled in two halves; and a third – PC Harrington – said it could do it and, what's more, that it would complete the job in less time than had been estimated in the construction programme.

Can you guess who got the job?
To form the cores, PC Harrington used a slip-form rig – a series of shutters arranged so that when concrete is poured between them, it will set to form the structure. Slip-forming is a continuous process: the rig creeps, or slips, slowly skywards as fresh concrete is added at the top of the shuttering and the walls of the core appear slowly as cured concrete at its base. The central core is the largest slip-formed structure of its kind in the UK.

Originally, it was proposed that the lift shafts would be constructed with slip-formed concrete walls on three sides only, with the fourth wall – the one that contained most of the openings – constructed from dry-wall using a plasterboard-based construction. However CWC, drawing on those 15 years as an office builder, decided to construct the fourth wall in cast concrete to eliminate the need for an additional contractor.

As a project manager, the Barclays building has to be my favourite. It is the most challenging and the most rewarding so far. It will look stunning

Paul Lynchehaun, senior project manager for Barclays

Experience also highlighted the need to start installing the lifts early on. "Strategically, there has often been a problem keeping the lift installation within programme," says Lynchehaun.

Usually, lift installation commences after the cores have been completed and their internal lift-lobby slabs have been constructed. However, to bring lift installation forward, CWC insisted that the concrete contractor build the concrete lift-lobby slabs within the core at the same time as the slip-form rig was forming the core's walls. Once the contractor had completed two lobby slabs, the lift contractor moved into the core. It then chased the concrete contractor up the tower. For safety reasons, the lift installer was protected from the overhead concreting works by two levels of temporary decks.

There was one other notable innovation involving the slip-form rig. To enable the steel staircases to be installed within the building's cores while slip-forming was under way, hatches were installed in the top of the rig so that steel staircase sections and lengths of riser pipework could be craned into the core and lowered into position within it. "It's a fabulous solution – we've now got stairs to access all areas of the site," crows Ricci.

Unusually, the steelwork contractor is not right behind the concrete core contractor as it moves up the building. "We decided – in order to give the concrete contractor an easy run at the core's construction and to maximise its access to the cranes – to delay erection of the steelwork until after the cores have been completed," explains Lynchehaun.

It was not only the concrete contractor that benefited from this move: once PC Harrington had finished, CWC was able to remove its cranes from the site. This was useful because the crane supports, which would have passed through the building's floor slabs, and the ties from the cranes to the cores could also be removed to minimise the cranes' impact on the remaining works. CWC then installed its own cranes on top of the cores, unencumbered by the need for any additional supports. "There was a time penalty in swapping the cranes but this will be recouped by the extra operating efficiency this new arrangement gives," Lynchehaun says.

No more balancing on beams
When steel installation finally did start, CWC was looking to introduce new ways of working to improve safety for the steelwork riggers. CWC wanted all steelwork to be installed from a mechanical platform, rather than have riggers E E balancing on beams to bolt the steel into place. The steelwork package was tendered with this as a requirement. "This hadn't been done before so we were taking a chance on programme," says Lynchehaun.

The steelwork package was won by a Victor Buyck/Hollandia (VBH) joint venture. VBH is using a bespoke platform to carry a standard telescopic access machine, or "cherry-picker". The platform allows the cherry-picker to be lowered into position by crane and the base of the platform is formed from two support rails designed to transfer the weight of the cherry picker on to the steel below. The cherry-picker can then run backwards and forwards on the rails to give the riggers access to the steelwork above. Of course, this innovation has come at a cost: "We paid a premium on this project, but in the future we should get our money back," predicts Lynchehaun.

Strappy number
Just as the construction of the building's cores has evolved after the attack on the World Trade Centre towers, so has the design of the building's structure. Engineer Cantor Seinuk's design allows for any two adjacent structural columns to be removed on any floor without endangering the remainder of the building's structure. This works because VBH has developed what Lynchehaun calls "a strap" to tie the floor's supporting beams together. Usually, floor beams are just bolted to the steel supporting columns. VBH's "strap" is a short steel plate that is pushed through the column to connect the floor beams (see detail, above). On removal of the columns, the strap allows the beams to hang from the remaining structure.

It is safer and, as Lynchehaun explains, "the strap can be added later in the steel erection programme, taking its installation off the programme's critical path".

The steelwork is now complete up to level eight. On level six, installation of the ribbed metal decking is nearing completion while, on level four, the reinforcement is in place and ready to receive its blanket of concrete. The first cladding panels are starting to be installed on the ground floor.