London’s latest iconic landmark, the Shard, set another new record in September, as the UK’s highest crane was raised into place. When fully extended the crane will sit a whopping 317m above ground level — and 7m taller than the Shard’s eventual highest point.
The 111m tall crane, supplied by plant hire firm Select, has been “jumped” into place so as to enable construction of the Shard’s 500t steel spire to get underway and take the building to its ultimate 310m height from the top of concrete core, 244m above ground level. The spire will form the top 23 floors of the Shard and is effectively a tower within a tower starting at level 72’s viewing gallery.
But it is not just the sheer height of the crane that is unique. The crane cantilevers off the side of the building from level 55, a first outside the United States, where it is believed the methodology has only been tried once - on World Trade Center 7 in New York. It’s an innovative approach driven by the need for speed.
“The pace of construction demanded always meant we were going to have to rethink the usual craneage strategy,” says Adrian Thomson, project director for Shard project manager and main contractor Mace. The cantiever solution was developed by Mace with Shard structural engineer WSP carrying out third party checks.
“The pace of construction demanded always meant we were going to have to rethink the usual craneage strategy”
Adrian Thomson Mace
Strictly speaking it’s a propped cantilever, with two connections back into the tower’s concrete frame at 20m centres. But it’s still a pretty spectacular site, as the crane moves further and further away from the building which tapers as it soars ever skywards.
The crane will be busy, with around 100 lifts expected to be needed to raise all the large floor and facade sections. But this is itself a much reduced operation thanks to another UK first.
Due to the limited space on site, Mace has had the steel sections manufactured off site and the crane will be used to lift complete sections up to three storeys deep rather than separately lift the 800 pieces of steel needed.
Full scale preassembly
But the team went further than just knocking a few sections together. The entire spire was pre-assembled in three storey sections on an airfield in Yorkshire prior to partial dismantling and delivery tosite. The move means the Shard is setting a new benchmark in health and safety standards.
The full pre-assembly trial programme of the spire components was carried out by steel subcontractor Severfield Rowen and observed by the Health & Safety Executive.
This focused familiarisation with the process - trial lifting operations of pre-assembled components and optimisation of the number of components to be lifted - a result of the pre-assembly design - was invaluable for the specialist erection team. It ultimately created a safer, more efficient environment on site.
This dedicated, collaborative practice, coupled with the advantages of pre-fabrication and pre-assembly also created new method statement standards.
Employing 3D extracts from the structural model the team could visualise the daily tasks of the erection process to ultimately create detailed briefings about the day-to-day operations. It also means assembly could be done in a much more controlled environment.
“We are now at the high risk stage of the project,” explains Mace chief operating officer for construction Gareth Lewis. “The tolerances on the Shard are very tight with visible steel on every level. Pre-assembling sections of the spire has reduced the amount of work at height, making the installation as efficient as possible and providing greater certainty prior to lifting that the correct fit will be achieved.”
Developer Sellar has reaped the rewards in the form of project certainty.
“The single biggest determining factor on whether this project is a success or failure is wind”
Flan McNamara, Sellar
“It completely de-risked the operation,” says Sellar project director Flan McNamara. “The single biggest determining factor on whether this project is a success or failure is wind. We are on the bank of the River Thames, London is a low city and we are very high up, and we are very exposed to high winds.”
So lifting 800 pieces of steel and assembling them 300m above ground would not have been good. “But it has all gone incredibly well,” he says. Wind related down-time is currently running at about 30%.
“It was factored in at the start, but you couldn’t factor in which day of the week the wild would blow,” says Thomson. “The weather forecast is huge for us as we have got deliveries coming in from all over the UK and the world and we have nowhere on site to store anything.”
Project remains ahead of schedule
The last steel section was being delivered to site as NCE visited and, remarkably, the project remains ahead of programme.
Getting to this position has been thanks to excellent progress on slipforming the tower’s concrete core - “a huge success”, says Thomson with typical progress of 3m per day - and yet more crane and lift related innovation.
This includes a world first, explains McNamara. “Before we switched to the cantilever crane we had a crane slipforming up through the core. That’s the first time that’s been tried outside of North Korea - and in North Korea it hadn’t worked - they couldn’t stabilise it.”
It worked fine on the Shard, as has another first - the installation of the UK’s first “jump lift”. Developed by Kone, the self-climbing elevator system provides an alternative to notoriously slow exterior hoists.
The lift functions by using the building’s permanent shaft during the construction phase and moves higher or “jumps” in the shaft as the building gets taller. It allows shaft construction and lift installations to continue at the higher levels while the lift is operating in the same shaft at the lower levels below a protection deck.
“Without the jump lift technology the Shard programme would have gone out”
Adrian Thomson, Mace
This innovative approach enables the lift shafts to be used to aid construction at the same time as they are fitted out with permanent cars and put into service.
“Without the jump lift technology the Shard programme would have gone out,” says Thomson. “We would have had to use hoist lifts and we wouldn’t have been able to transport people and materials quickly and efficiently enough to the top of the building and down again.”
Jumps take place at weekends when demand is lower, with three storeys added in each move. There are five jump lifts in operation in total, the largest of which can transport 45 people or up to 3,500kg of materials.
Lessons learned from 9/11
The generous lift capacity is in a way thanks to lessons learned from the 9/11 attacks on the World Trade Center in New York. WSP’s structural design took on board many lessons from 9/11, not least the need for structural redundancy and redundancy in evacuation routes.
It designed a hybrid form of structure, with composite steel floor slabs and perimeter columns at commercial levels and post-tensioned slab construction at hotel and residential levels further up the tower.
The design provides a high level of structural robustness, with a number of safety-related innovations including state-of-the-art blast protection and structural fire engineering design.
The building’s unique lateral bracing system consists of an efficient core and a high level outrigger truss to mobilize the entire building perimeter. It also uses the concrete mass at the upper levels of the building to dampen the building’s acceleration during high winds.
“We came up with some alternative proposals for how it could be made more economical,” says WSP Cantor Seinuk director Kamran Moazimi.
“We went unconventional,” adds Moazimi. “We went with a concrete basement, steel to level 40, concrete from levels 40 to 72 and steel above. If you change from composite steel and concrete to just concrete, you can save a 100mm off each floor. If done over 40 floors, it means you can add a floor to the building.”
A composite steel and concrete floor slab was used on the office floors as more air conditioning is needed and the additional pipework could be hidden between the steel beams.
On the residential floors where not so much pipework is needed, post tensioned concrete was specified. The basement floors will be reinforced concrete. The reason the team went for concrete in the basement is because it wanted to minimise the excavation and it’s underground so it is susceptible to corrosion.
Core construction started in 2009 at ground level and went down into the three storey basement as the superstructure was constructed upwards. This also saved time.
Piles 900mm in diameter formed the secant basement wall, and the piles that support the basement floor are 1500mm or 1800mm diameter. For the core, large plunge columns were fed down the empty top sections of the shafts used to create the piles. They are large braced plated columns around 500mm square. The columns weigh around 800kg/m and will be taking around 19MN each.
The Shard is due for completion by May 2012, in time for the London Olympics when it is expected to form a stunning backdrop.