One of the most exciting things about the skyscraper construction market in London is the immense desire among developers to produce the capital's next landmark. Swiss Re's Gherkin has done it and there are hopes that British Land's 31storey Broadgate Tower is next in line. But both these buildings will be lost in the crowd once the Shard of Glass, or London Bridge Tower, rears it's conical head.
At 83 storeys, and 310m, it will be the tallest building in Europe and probably called an eyesore and an icon in equal measure when construction starts in April next year. For now though, structural engineer WSP is working on the building's detailed design ahead of client Sellar Property Group announcing the construction manager for the job in the next few weeks.
The £350M tower is claimed to be the first vertical city in Europe, incorporating offices, shops, restaurants, a hotel, leisure facilities and apartments.
All of this will sit within the site of London Bridge railway and underground stations, so residents and visitors can use the building without adding to congestion on surrounding streets and pavements.
Signature buildings need signature architects, and Renzo Piano is the Shard's mastermind. But keeping both Piano's vision and the client's expectation for the maximum lettable space has been a major challenge for WSP project design directors Ron Slade and Kamran Moazami.
'The architect wanted the columns in the building to work like the branches of a tree, ' says Moazami. 'This means fewer large diameter [trunklike] columns at the base of the building, and thinner, more numerous, columns as you rise up, until they disappear altogether into the sky.' But with the building tapering, the tree analogy has proven troublesome.
'The geometry of the building meant that internal columns were all over the place. But we wanted them all to line up eventually so that there was no bending in them, ' says Moazami.
Coordinating lines of columns through the building to meet with core walls lower down ensured that no transfer beams were required - they would have taken up more oor-to-ceiling space.
There was also the problem of horizontal forces from the inclined cladding. The solution involved WSP designing a steel diaphragm structure between the perimeter columns and the concrete core across floors four to seven. This also meant that below level four, the perimeter columns could take on the trunklike form so desired by Piano.
Moazami is no stranger to skyscraper design, having designed World Trade Center 7 and Trump Riverside in New York, before setting up Cantor Seinuk's London Ofce in 1989. In the UK, his portfolio includes the Beetham Tower in Birmingham and the Manchester Hilton, as well as Castle House and One Blackfriars, which are yet to start on site in London.
However, the challenges on the Shard differ wildly from these UK projects, which are not nearly as tall nor do they incorporate mixed use.
Early on, WSP realised that different structural systems were needed for the various types of occupancy: ofces need long column-free spans; hotels need regular floor plates and room for lots of services; and apartments need high levels of sound insulation.
The latter requirement is being met with 200mm thick post tensioned slabs spanning 9m on the residential oors.
'This ensures the vibration and acoustic requirements are met, and keeps the oor-to-ceiling height as big as possible, ' says Moazami. The added mass to these residential floors, which are located high up the building (see diagram), would also have a damping effect on any sway that might develop in the building.
For the open plan ofce oors lower down the building, composite steel construction proved more economical. These will be made up from 510mm deep steel beams, which will accommodate services through their webs, and 130mm thick concrete slabs. Since these are the largest floor plates in the building, they are also able to cost effectively accommodate the largest area of concrete core, which must be robust enough to support the weight of the building. They will also accommodate lifts, stairs and toilets.
Above floor 66, the limited oor area means that it is uneconomical for the lift cores to continue up, so these levels will be used to store plant, and house a viewing gallery.
Final stability will be provided at the top of the building. A steel outrigger structure will connect the top of the concrete core to the perimeter frame of the building.