Mace aiming high as work starts on the Shard - VIDEO

The news that Mace is to start building the 310m-high Shard at London Bridge later this month gives us hope: an iconic skyscraper going ahead against the odds in a development market which has virtually ground to a halt. Kristina Smith reports.

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Mace has put four years of hard graft into winning the Shard contract, seen the form switch from construction management to design and build, and seen off a late challenge from Laing O'Rourke. The final price is reported to be £425m, up from the original £350m, but lower than it was last autumn at £435m.

It isn't just the timing which makes this 310m building remarkable. The Shard will uniquely feature a concrete frame sitting on top of a steel one (it's usually the other way round). This, combined with the building's distinctive form, make it an engineer's dream. "With many tall buildings, you get a lot of repetitive structure," says Richard Mawer, a senior engineer with WSP, which is structural engineer on the scheme. "The fascinating thing about this building is that nothing is the same, nothing is typical."

English Heritage first mocked the tower as a 'Shard of Glass', but the insult stuck and became an affectionate nickname in the style of 'The Gherkin'. Even developer Sellar Property refers to it as the Shard now. The Renzo Piano-designed skyscraper is one of two buildings that will be known as London Bridge Quarter, the second being a 56,670m² office building, the contract for which has yet to be let. Altogether the £2bn development will be packing 185,805m² of space.

Although the tower is lower than Renzo Piano's original 390m, 88-storey design, it will still break some records: the first skyscraper in Western Europe to reach and exceed 1,000ft and Western Europe's tallest habitable building.

The Shard is also supposed to be setting new sustainability standards for tall buildings. M&E engineer Arup's concept design includes some innovative features, the most prominent of which is a radiator that would form the top of the building, dissipating excess heat in place of energy-hungry air con units. However, the M&E element of the build has reportedly come in for some robust value engineering and it is now not certain that the radiator will appear.

You can't get much more mixed use than the Shard, with wallet weight increasing as the floor plate decreases. At the bottom, the redevelopment of London Bridge Station's concourse is followed by public areas on floors 1 to 3. Transport for London has already spoken for floors 4 to 10, with a further 20 office floors above that, which Sellar hopes will attract top city rents. Then comes a luxury hotel on floors 34 to 52, which will be run by Asian operator Shangri-La, and finally 10 floors of flats with the most amazing views over London.

It is this mixed-use combination that has dictated the frame construction. WSP tried every permutation before it came to the solution of concrete over steel, says Mawer.

Longer spans

Steel for the office floors allows services to pass through the beams and gives longer spans and higher floor-to-ceiling heights. For the hotel and flats, concrete gives better noise insulation and the flat soffit means that the services can be distributed anywhere, allowing the layout of the hotel to be finalised much later in the programme. The mass of the concrete at the high level has an added benefit of reducing acceleration of the building in high winds.

Engineers at WSP have been toiling tirelessly to create the most efficient structure possible. For the steel frame section of the building this means they have taken the highly unusual step of specifying custom-made sections. "Because it's such a big job - there are about 100 custom sections - the steel works are quite happy to provide them," says Mawer.

In addition to assembling these 100 custom sections, steel contractor Cleveland Bridge, which is reported to have won the £50m steel package, also has the challenge of producing architectural steelwork. The main steel columns at ground level will be visible, and at the corners of the building that will house indoor gardens on each floor, the structural steel beams will be on display. "The steel has to be fabricated in a very high quality environment, visible welds have to be nice and smooth. The architects will be very fussy about that," explains Mawer. "That always creates a bit of a challenge for the fabricators."

For the concrete frame contractor - rumoured to be John Doyle - the challenge could be pumping concrete up to the heights required, although Dubai has already shown it can be done.

Before construction of the frame begins, Mace has some pretty tough challenges to contend with at ground level. This building takes up the entire site: there is nowhere to store materials or handling deliveries. For this reason, the three basement levels will be built using top-down construction, which will create space to work from but is a slower and more costly exercise than bottom-up because of the difficulty of getting plant in and muck out. Add all that to the fact that you are working virtually on top of a busy bus station and London Bridge railway station and you have a mighty logistics puzzle to solve.

A complex project like this is an ideal candidate for construction management - which is what Mace was bidding for when it began the process back in 2004. But since then, the economic tide has turned. Banks, which are always wary of construction management, wouldn't even consider it now. So Mace finds itself in a very different position, carrying the risk with all those innovations, variations and architectural aspirations.

It may be an icon of hope. It's certainly an engineer's dream. But the Shard has the potential to become a contractor's nightmare.

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Shards of glass and balls of steel

It seems incredible that at a time when businesses are failing and rents are heading downwards, the City of London will see three new skyscrapers rising from its street: the Shard, due for completion in May 2012 which will have 53,585m² of offices, will sit alongside Heron Tower (40,836m², due 2011) and the Pinnacle (88,000m², due early 2013).

"It takes a very confident developer to start anything now," comments Jones Lang Lasalle's head of UK offices research Bill Page.

Confidence is one trait that is not lacking in the men behind the Shard and the Heron Tower: Irvine Sellar and Gerald Ronson respectively. Both are giants of the property world who have been through recessions before and know how to play the game. Less is known about Pinnacle developer Arab Investments.

Track record is key for anyone trying to finance a development in the current economic climate. In January, when Sellar announced that four Qatari banks were to provide funding for the project, some talked of the Qatari investors 'bailing out' Sellar. But we should really be impressed that he secured the funding at all. "There is virtually no speculative development finance available now," says Page. "And if it is available, it will be very expensive."

Finance apart, the other great challenge is making a development stack up financially. Jones Laing Lasalle predicts that the best city space will go for £47.50 per square metre by the end of this year, down from £58.50 at the end of 2008. But to make a building work economically, you would need to achieve rents of over £60 per square foot when you let the scheme, says Page. "What that means for the general market is that new starts have ground to a halt and the tap has pretty much been turned off with one or two exceptions."

It's a case of he who dares wins. There will be a surplus of office space in the city which will peak in 2010, but after that experts are predicting that businesses will start letting space again and demand will start to grow again. "Anyone delivering space in 2012 and 2013 will be delivering into a growing market," says Page. So it looks like all three developments could be onto a winner.

They still have to secure top rents though. Ronson knows this only too well, referring to his tower as 'six star' office space. The Shard already has two tenants in place: Transport for London and hotel operator Shangri-La. Critics have questioned whether such extreme mixed use can work: high rollers and civil servants in the same building, but Sellar remains bullish that the building will work.

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Fact File

• Height: 310m
• Number of storeys: 70
• Floor space: 129,134m²
• Office space: 53,585m²
• Depth of basement: 13.3m
• Plot ratio: 1:32
• Area of glass façade: 56000m²
• Weight of steel frame: 12,000t.
• Volume of concrete: 54000m³
• Weight of reinforcement: 4500t-5000t

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Who's who

• Developer: Sellar Property Group
• Investors: LBQ Ltd (Qatar Islamic Bank, Qinvest, Qatar National Bank, Barwar International, Sellar Propety)
• Architect: Renzo Piano
• Alliance architect: Adamson Associates
• Structural engineer: WSP
• M&E engineer: Arup
• QS: Davis Langdon
• Project manager: Turner & Townsend
• Planning supervisor: PCM Safety Management
• Fire engineer: Arup Fire
• BREEAM consultant: Rickaby Thompson Associates
• Vertical Transportation: Lerch Bates Associates
• Furniture: Vitra
• Contractor: Mace
• Piling: Stent
• Steel frame: Corus/ CBUK
• Cladding: Scheldebouw
• Lifts: Kone