黑洞社区

If ever there was a symbol of confidence for an industry on its uppers, the Shard must be it. It strode confidently from ground level to be the UK鈥檚 tallest building in just 11 months - at a time when many other central London offices were being put on hold. Now a bold new feature on the London skyline, its developer, Sellar, is so confident of the building鈥檚 success, it got anchor tenant Transport for London out from the building because it knows it can get far more rent when the Shard is finished in 2012. The whole construction programme is geared for speed so Sellar can capitalise on a market hungry for prime office space.

The cladding is 60% complete and the services are about to be commissioned. 鈥淭he project below level 31 is largely complete,鈥� says Flan McNamara, project direct for Sellar, adding that the office fit-out is starting on the lower floors.

This quick turnaround is even more impressive considering the project start was delayed by four months. The switch from construction management to a fixed price contract, coupled with protracted negotiations with Network Rail and a trickier demolition job than originally envisaged, meant the job didn鈥檛 get going until March 2009. Keeping the same completion date with a new 38-week programme has driven three key innovations on this project in a bid to claw the time back. The first was 鈥渏ump starting鈥� the core, a technique where core construction starts before the basement is finished (see 黑洞社区, 30 April 2010). This saved between six and eight weeks on the programme but this still left a lot of time to be made up. The core is finished and the floors are racing towards the top so the team is on track to hit its completion date of May next year. How have they done it?

The core

The core has been built using slipform construction. A rig attached to the top of the core is used to form the core walls as a continual concrete pour. The rig is moved upwards at a constant rate to keep up with the pour. A tower crane is needed to build the core so this too is attached to the top. As the core is built, additional sections are added to the crane so it isn鈥檛 overtaken by the core. The slipform on the Shard moves at 150mm an hour which means the crane needs to be jumped twice a week. According to Gareth Lewis, contractor Mace鈥檚 chief operating officer for construction, keeping the core moving upwards was the key to the rest of the project. 鈥淲e recognised early on that moving the core along faster helped with the critical path of fixing the frame to the core,鈥� he says.

But this process is at the mercy of the weather - wind speed must be below 25mph when the crane is jumped up a section. 鈥淭here was a stage last year with a crane when we were winded off for four weeks in a row,鈥� says Lewis.

At low temperatures, we pull people down into the area protected by the cladding because of the risk of frostbite

Tony Palgrave, Mace

To get around the risk to the programme posed by the weather the team eliminated the need to add sections to the crane. Instead of being fixed to the core, it is attached to the slipform rig so it moves up at the same rate. The tail of the crane still needs bracing against the walls inside the core so is attached to a 鈥渢he sledge鈥� - a frame featuring hydraulic jacks that fit between the sledge and the core wall. When the core moves, the jacks are slackened off slightly so the sledge and crane can move too. Then pressure is applied to the jacks, locking everything together. Sophisticated GPS positioning was used to ensure the rig went up perfectly vertical, a traditional downside of slipform construction. 鈥淣ot only has this saved time, it de-risked the whole programme,鈥� beams Lewis, adding that the core was completed bang on its new target date of 10 December 2010.

The frame

The first 40 floors of the Shard are constructed from steel which was relatively straightforward to build. At level 41 the frame switches to concrete with post-tensioned slabs which presented the team with another challenge. The tendons used for the post-tensioning have to be tensioned at the edge of the slab. Traditionally a flying scaffold is used for this purpose - cantilevering out beyond the building boundary to provide a platform. 鈥淚f we had scaffolding hanging over the side of the building, not only do you have to keep dismantling and erecting it, but you have the risk of something falling off the building which is a major risk to us with the station below,鈥� says Tony Palgrave, Mace鈥檚 operation director.

As with the core construction, wind is a potential problem. It could prevent the scaffolding being moved as well as stopping work on the labour-intensive frame construction. Wind chill is an added problem. 鈥淎t low temperatures, we pull people down into the area protected by the cladding because of the risk of frostbite,鈥� explains Palgrave.

Cue another groundbreaking innovation. The team has developed a platform that moves up with the post-tensioning work. It has screens at the side that keep the wind off, enabling the team to work safely and with wind-speeds of up to 50mph. When conditions have been bad the only zone that is out of bounds is the section of completed frame between the post-tensioning platform and the clad area below.

The rig is attached to rails fixed to the side of the building so it can winch itself up. But there is an added complication. 鈥淚t would have been much easier if the building was straight but of course it tapers,鈥� sighs McNamara. This means the size of the rig has to be adjusted as it progresses upwards. The rig is made up from a series of sections that can be taken out as the building narrows and this is done by tower crane at night. The rig was trialled in a site in Docklands and fitted to the building last Christmas to give the post-tensioning team 鈥渁 flying start鈥� at the beginning of this year. 鈥淚t鈥檚 been a saving grace from the health and safety and programme point of view,鈥� says McNamara. The rig encloses four levels which equates to an output of one completed floor a week. McNamara says the post-tensioning work has gone 鈥渋ncredibly well鈥� despite the cold snap at the beginning of the year which meant concrete couldn鈥檛 be poured for a time. 鈥淒espite the bad weather, we鈥檝e lost virtually no time,鈥� McNamara says. The team is now at level 56.

Below all this work, the cladding is being installed, so far up to level 42. Installation is simple as it is a unitised system installed from inside the building using mini cranes.

The M&E installation

Further down again and the M&E installation is in full swing. Peter Savoy, Mace鈥檚 operations director, says the M&E is 75% complete because the bulk of it is concentrated on the office floors where commissioning is about to start. 鈥淭here鈥檚 16 months to practical completion and we are about to commission the services for the first 28 floors,鈥� he says. 鈥淚鈥檝e never been on a job where there is such a long time between commissioning and practical completion.鈥�

The reason the service installation has been so quick is down to prefabrication. 鈥淭he strategy is to reduce the number of people onsite - this is a job you couldn鈥檛 have done without a prefab approach,鈥� says McNamara. 鈥淲hat鈥檚 new here is the extent of prefabrication,鈥� explains Palgrave. 鈥淲e haven鈥檛 had any welding or pipe threading on site,鈥� - apparently cutting the number of M&E workers by half.

The three basements are stuffed full of plant. The strategy was to bring in service modules and install these from the edge of the basement towards the middle. Blockwork walls were built as the modules were installed to speed the job up. There is no first and second electric fixing on this job, instead cables are installed, terminated and labelled in one go. 鈥淭his was a big cultural change and took quite a bit of driving through,鈥� says Palgrave adding that the job was made easier as the M&E installation is being done by Mace鈥檚 in-house services arm.

The spire

Prefabrication is also the answer to the most potentially challenging part of the project - the top 15 floors, otherwise known as the spire. Work will start on this in September and it will extend from level 72 up to 87 鈥� and beyond, as the spire has to support the cladding 鈥渟hards鈥� which will extend above the uppermost level.

The steel spire will be made by specialist contractor Severfield-Reeve with work starting this June. It will consist of three-storey high sections with eight of these making up the full floor plate. 鈥淚t鈥檚 more akin to building an offshore oil rig,鈥� comments McNamara. Severfield-Reeve will make the sections and assemble them at its Yorkshire factory. This will include staircases, floorplates and cladding brackets. 鈥淚t means we know it will fit together,鈥� says Lewis.

They chose to outsource the spire because: 鈥淚t is going to be done over the winter and the last thing you want is guys with their hands frozen 70 storeys up,鈥� remarks Lewis.

The spire will be broken down into lorry-friendly pieces and taken down to London. Here the pieces will be re-assembled, lifted into position and bolted up at height. The team says the plan is to finish it before spring next year - depending on the weather. By then the Shard hopefully should be a bold symbol for a resurgent office market.