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Step-change comes with good vision

The original staircase linking the south side of London Bridge with the riverside path below probably seemed like a great idea in the 1970s.

Neat, efficient and making good use of the space just on the north-east side of London Bridge.

In reality, it made for a dark descent. Sandwiched between bridge and building, it was a great place for anti-social behaviour. Difficult to negotiate during the day, and to be avoided at night.

To address the problem and open up the area, the City of London Corporation decided to build a new stairway which could be accessed further south along the bridge. This location allowed the structure to be lighter and more open and with views out over the Thames.

Connecting the stair to the existing structure

Connecting the stair to the existing structure

The connection to the bridge at the top of the stair was also tricky. Having established that no vertical load could be taken by the bridge, the team also established that at the point of contact, there were post tensioned cables in the bridge deck. To ensure that these were not affected, the team limited the depth of the connection to 230mm.

The decision to move the staircase brought about the new challenge of how it could be supported. The new structure would now be situated over the river and so could not have foundations directly underneath it. In addition it is directly above a Northern Line Tube tunnel.

Structural engineers Capita together with Bere architects and ground engineering specialists Keller were appointed for the design.

Initially it was thought the top of the new steel structure could be supported by London Bridge. But site investigations showed that the bridge was not capable of carrying the extra vertical loads and a new solution had to be found.

The solution lay in cantilevering the stair 10m from the walkway below, using the bridge above as a horizontal restraint.

To support the structure, Keller designed and constructed a total of 27, 13.5m long mini piles underneath the riverside walkway below. This in itself was challenging as the team had to bore through the 5m thick mass concrete embankment which supports it.

“It’s held from three points,” explains Capita director of engineering, civil and structural Jean-Philippe Cartz. “One point is the biggest pile cap on the east embankment and that has 20 mini-piles. The structure is fully anchored in that position and in particular that’s taking a lot of uplift because the stair wants to naturally fall into the river. So that base is resisting an overturning moment and the twisting of the structure.

“Then on the other side of the walkway, directly underneath London Bridge, this is where the remaining seven piles were threaded through the tidal chamber to not to obstruct any of the existing drainage runs.”

The biggest challenge was dynamics and the model behaviour was not intuitive

Jean-Philippe Cartz, Capita

In conjunction with the tension capacity of the piles and weight of the large pile cap, the connection between the top of the stair and the bridge structure above provided some of the horizontal restraint to stop it tipping over.

“The whole structure wants to tip and twist around the walkway and fall in the river,” he said.

“If you imagine you have a tree and if you cut it at the base that tree is going to fall – if you look at the very top of the tree as it starts to fall, it doesn’t fall vertically, it falls horizontally. The initial movement at the top is purely a horizontal movement.”

The new stainless steel staircase

The new stainless steel staircase

Source: C. Totman

Louvres wrap around the outermost side of the balustrade to stop the public from seeing into the building behind.

The stainless steel structure of the stair, curves around and changes direction three times as it descends from the bridge to the walkway. Louvres wrap around the outermost side of the balustrade to stop the public from seeing into the building behind.

As with most lightweight structures subject to pedestrian loading, dynamics were a huge influence on the design. On this stair though, they were exacerbated by the effects of the cantilever, and Cartz says that the combination of these two factors was one of the most challenging aspects of the design.

“The biggest challenge was dynamics, and the model behaviour was not intuitive,” says Cartz.

Dynamic analysis of the structure showed that the natural frequency of the staircase was around 1.5Hz, which is well below the suggested minimum of around 6Hz. Left like this, pedestrians walking down the stairs would soon have felt them moving, potentially leading it to resonate – a phenomenon which causes structures to vibrate with far larger amplitudes at their natural frequency. In this case resonance would have rendered the structure unusable. So the team consulted with German damper supplier Gerb, which suggested installing tuned mass dampers in the stairway’s landing platform to combat the vibration.

“It doesn’t alter the natural frequencies,” says Cartz. “As the movement starts, the dampers stop the movement happening, it doesn’t change the behaviour of that stair but it means you never enter resonance.”

We walked down, we ran down, we jumped on it, it’s on that basis that the mass dampers were tuned

Jean-Philippe Cartz, Capita

Cartz says that the team carried out a series of tests when the structure was built, to fine tune the dampers.

“Once the bridge was constructed, we conducted tests on it with about 20 people from the team. We walked down, we ran down, we jumped on it and Gerb was there taking measurements of it. It’s on that basis that the mass dampers were tuned. They were designed for comfort.”

The connection to the bridge at the top of the stair was also tricky. Having established that no vertical load could be taken by the bridge, the team also established that at the point of contact, there were post tensioned cables in the bridge deck. To ensure that these were not affected, the team limited the depth of the connection to 230mm.

“For the movement north to south, we have a shear key and then for movement east to west we have a hook device which can side up and down but it stops it falling away from the bridge.”

 

The new bridge gives open access and beautiful views over the river

The new bridge gives open access and beautiful views over the river

Source: C.Totman

The new stair cantilevers 10m from the walkway below, using the bridge above as a horizontal restraint

Space constraints on the bridge on weekdays meant that staircase structure had to be fabricated offsite by Littlehampton Welding in four sections and then lifted by contractor Mackley over seven weekends.

“They [Mackley] lifted the pieces and dropped them from London Bridge. There was a temporary support during construction which held the stair in position and each piece was added on and connected,” said Cartz. “Once the top connection was made, the prop was removed and the whole thing moved as it was supposed to work.”

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