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Tracking the arch enemy - the Shard skyscraper

One of the largest finite element models in the UK is analysing the effect of building the Shard skyscraper on the brick vaults beneath London bridge railway station.

London Bridge railway station is one of the oldest in the capital, not to mention one of the busiest, handling more than 42M passengers a year. Its tracks and concourses are supported by a network of brick arch vaults sitting on approximately 3m deep strip foundations.

The 310m tall Shard of Glass skyscraper, incorporating a 16.5m deep basement is being built within a few metres of these vaults. Some vaults are in use, including one occupied by the London Dungeons tourist attraction. Station owner Network Rail appointed consultants Gifford and TGP to assess the condition of the vaults and advise on how they would be affected by the demolition of the previous building, Southwark Towers, and construction of the Shard. The Shard’s designer is WSP Cantor Seinuk.

“It was important for us to find out the potential for a brick to fall on someone while the work for the Shard took place,” says Gifford engineering analysis technical director Carl Brookes. The process has involved one of the most complicated computer analyses in the UK, according to Brookes and Gifford geotechnics technical director
Stephen West. Sophisticated surveying techniques and three-dimensional (3D) finite element modelling have been vital.

The 3D survey of the entire 5,000m2 site took just three weeks, creating nearly 25bn x, y and z coordinates which were accurate to 5mm. The work was carried out by surveying specialist ABA Surveying. It determined the exact position of the vaults and, coupled with a visual inspection, a model with different material strengths relating to
weaker or stronger conditions was created.

“It was essential to model the entire network of vaults since practically every arch is different,” says Brookes, and in this way, he adds, the way they interact could also be modelled. “Two teams surveyed the site. They created, what we call a ‘point cloud’, which was then converted to a mathematical surface in 3D using Rhinoceros [software]. We then applied the finite element mesh using Ansys modelling software.”

Ground movement

Fellow consultant TGP carried out ground movement analysis and used Plaxis 3D to predict the movements which could affect the vaults. This made use of various data, including measurements of movements during the construction of Guy’s Hospital nearby.

“The final model has 13.6M degrees of freedom,” says Brookes proudly.

“We did look into simplifying things − you wouldn’t leap into carrying out this amount of work,” says West. But creating a simplified model of the vaults was never an option because of the inconsistent geometry of the vaults, varying loads applied and varying material strengths of the brick.

The model takes into account the following stages of construction of the shard: initial state; completion of demolition; establishment of new site levels (involving some excavation); installation of the new secant basement wall; staged excavation of the new basement (with propping); construction of piles; building loads as the superstructure is built; as well as looking 10 years ahead at the effect of ground consolidation.

“It was important for us to find out the potential for a brick to fall on someone while the work for the Shard took place.”

Carl Brookes, Gifford

The upshot has been that 27mm of movement is predicted in the vaults. “It’s taken 18 months to get here and prove that the movement wasn’t in the order of 50mm to 60mm, as previously feared,” says West.

But constant monitoring around the vaults is the key indicator as to whether these predictions hold true. “Crack widths and absolute positions are being monitored to check that the predictions are correct,” says Brookes. “And if the movements are larger than predicted, then we can apply them to the model to see how the structure would be affected.”

So how sure can Brookes and West be that their predictions are realistic? “We are a million miles away from believing everything we see [on a computer],” says Brookes. “We’re both engineers so we understand
the dangers of having this sort of software and breezing over what is actually happening.”

There are still ongoing discussions regarding whether arches need to be strengthened, tied or will require propping in the short or long term.

Construction of the Shard is ongoing until 2012 and monitoring of the movement of these vaults will continue beyond this date.

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