The appearance of piling rigs at London Bridge station last week heralded the start of construction on what will be Britain’s biggest skyscraper, the London Shard of Glass.
From Chicago to Dubai, a raft of iconic record breaking skyscrapers have been axed or mothballed. Nowhere has the global financial crisis been felt more than in the high rise sector where the bottom has fallen out of commercial development market. But despite the credit crunch, a new tower is starting to emerge in central London.
The 80-storey pyramidal Shard of Glass at London Bridge will be the highest skyscraper in the UK at 310m when it is completed in 2012. It will also be one of the highest in Europe. Funding for the project was secured in December last year when Qatari Diar Real Estate Investment Company stepped in to join Qatar Islamic Bank, Qinvest, Qatar National Bank, Barwa International and Sellar Property Group as one of the funders for the development.
The project was also helped by the fact that it had secured a series of major early prelets – Transport for London and hotel company the Shangri-La both signed agreements in 2006 and 2005 respectively.
Over the last few months key contracts have been signed. Mace has been appointed project manager, and work started in earnest on site last month when Stent began piling work. The Shard is finally happening.
As with all major projects, the scheme, which is a mixed residential, hotel and office development, has been in the pipeline for some time. Structures consultant WSP Cantor Seinuk first got involved in the job in 2004, when initial proposals had come in over budget and the developers invited firms to bid for the structural and mechanical design works.
We came up with some alternative proposals for how it could be made more economical. We went unconventional
Kamran Moazimi, WSP Cantor Seinuk
“We came up with some alternative proposals for how it could be made more economical,” says WSP Cantor Seinuk director Kamran Moazimi. “We took out material, we played with it and we used material where it was required. We also had to add floors and add lettable area. The shape did not change”
A pick and mix approach to floor slab materials helped increase the lettable space. “We went unconventional,” says 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 we went concrete in the basement is because we wanted to minimise the excavation and it’s underground so it’s susceptible to corrosion [and concrete is more durable],” explains Moazimi.
We intended to build the core bottom up, but it was taking too long. Instead we’re going for a top down strategy
Kamran Moazimi, WSP Cantor Seinuk
“We used steel in the commercial levels because it’s optimal for commercial construction. More air conditioning is needed and you can put fan cores between beams.” By using concrete in the residential floors, not only can the height per floor be reduced, but the additional mass that it gives the building helps its dynamic properties and means that there was no need for additional dampers to combat sway. “At the top [in the original design] they had a damper and major outriggers,” says Moazimi. “Because of that there were not enough floors. “If you go all the way up in steel, the dynamics aren’t as good, it’s not as heavy. By using a 200mm concrete slab instead of a deep beam and slab with combined depth of 650mm, we’ve added two or three floors and it also gives better performance in terms of vibration and acoustics and it helps the lateral acceleration.”
A central concrete core forms the basis of the Shard’s lateral stability system. The core is linked in to the perimeter column by a “hat” truss which crowns the building. “The core comes up the middle of the building,” says WSP Cantor Seinuk technical director John Parker. “Perimeter columns are linked in by a hat truss. If the building tries to sway due to wind, it stiffens up by engaging perimeter columns “The geometry helps, triangular is better,” adds Moazimi.
Why the Shard is special
What is unusual about the project is that the construction of the core will start at ground level and go downwards into the three storey basement as the superstructure is constructed upwards. This will save time.
The site is in a densely built up area and there is hardly any storage space. To get round this constraint, the ground floor slab will be poured before basement excavation starts. The 16.5m deep basement will then be built top down as superstructure and above ground core construction starts. “We were intending to build the core bottom up, but it was taking too long,” says WSP Cantor Seinuk director Kamran Moazimi. “Instead we’re going for a top down strategy including the core.”
Firstly the piles are sunk. Piles 900mm in diameter are used for the secant basement wall, and the piles that support the basement floor are either 1500mm or 1800mm diameter.
These piles go to 53m depth and are filled with concrete to around 16m below ground floor level. For the core, large plunge columns will be fed down the empty top sections of the shafts used to create the piles. The plunge columns are embedded in the top 5m of concrete in the pile shafts. Foam concrete is then pumped in around the plunge columns to fill in the voids.
These plunge columns will support the core while the basement is being excavated. They are large braced plated columns around 500mm square. The columns weigh around 800kg/m and will be taking around 19MN each.
The slipforming of the core above ground will start at the same time as the basement is excavated, meaning that these columns take the load of the core while it is being constructed. Once the basement excavation reaches the bottom, the concrete walls of the core will be poured.