Clear vision

The ancient wall that guards the Tower of London is soon to have a new neighbour. Unlike its crenellated counterpart, however, the newcomer will consist of a single glass skin, suspended above the pavement like an impossibly vast transparent curtain.

The 21st-century wall is part of Foster and Partners' continuing contribution to the design of the City of London's office spaces. It is also the architect's latest attempt to give the UK engineering industry a nervous breakdown.

What Foster asked for was nothing less than a 68 m long, 25 m high glass wall floating 3 m above the pavement, which wouldn't sway in a gale and had as little visible means of support as possible. The firm charged with finding a design solution to this modest proposal was Arup Facade Engineering, and in the end, it came up with something completely new.

The brief for the Tower Place development consisted of two low-rise office buildings that looked from above like two teeth on a saw, with a roughly triangular space between them. "The developer wanted to unite these two buildings with a glazed atrium," says Stuart Clarke, a senior facade engineer at Arup. But the space was crossed by two public footpaths, which meant that the architect had to lift the glazed end-walls 3 m above the plaza to let people pass under. This would create, in effect, a gigantic glass canopy with a vast wall along the northern base of the triangle and a much smaller southern one at the corner – the east and west boundaries being supplied by the stone and glass facades of the office buildings.

What gave this unusual structure an extra twist was its hypersensitive location next to the Tower of London and the historic church of All Hallows by the Tower. To ensure that the scheme did not intrude on this landmark, the facade had to appear to be held up by magic.

Arup set about devising its solution. First, the roof would be supported by 10 slender steel columns. The roof itself would have a primary structure of box beams, and between them would be secondary steelwork arranged on a 4.8 m grid. The glazed walls would be suspended from this structure using stainless steel brackets in the form of inverted Ts. Andy Foster, a director at Arup and the project's structural engineer, refers to these as "coat hangers". A steel bar drops from the underside of each of these hangers to the foot of the glass wall 25 m below. Small cantilevered shelves between the glazing support each of the 3.8 × 2.4 m panels.

Unlike the roof, which is constructed from laminated annealed glass, the walls will be formed from laminated toughened glass. "We had to use toughened glass for the walls because, whereas the roof panels are fully supported, the wall glazing is only supported in the corners, so the glass will have to cope with increased stresses," explains Clarke. A clear plastic interlayer of PVB holds the panes of glass together and a small clamp in the corner of each panel connects it to the rods.

With the facade suspended on a series of hangers, the next challenge was to find a way to brace it against the force of the wind. If it was left to hang freely, the force of the wind would make it swing, to the distress of the public walking beneath. So, to steady the wall, a series of horizontal steel cables were run across the atrium 150 mm behind the facade at each floor level of the office buildings. The bars dropping from the coat hanger brackets are connected to these cables using stainless steel clamps (see inset). The cables are then tied back to the columns that support the roof using glass struts.

These glass struts are the most innovative aspect of the project. The idea of using glass to support the facade originated from early discussions between Arup Facade Engineering, Foster and Partners and American-based designer James Carpenter Design Associates, when the group set about finding ways to, in Clarke's words, "make the space special".

To this end, the team considered using the reflectivity of the glass to create a mirrored volume as well as using glass as part of the structural system. The reflectivity idea was dropped – "to keep the simplicity of the space" – but the concept of the glass tubes remained. "They have never been used in this application before," says Clarke.

Arup and Foster worked with Austrian cladding specialist Waagner Biro to ensure that the 3.3 m long, 150 mm diameter glass struts were strong enough to withstand the wind forces on the facade. Each tube takes the wind load of almost 50 m2 of facade.

The struts are fabricated from bora-silicate glass pipes more commonly used to transport corrosive chemicals in factories. Each strut consists of two layers of glass, one forming an inner structural tube and the other an outer protective layer. The outer layer is actually two half-tubes, like a rainwater gutters, placed around the main structural tube. A PVB interlayer holds the two together.

The tubes were tested to three times their design load. And, because the lowest row of them will be mounted only 3 m above the ground, Waagner Biro reassured itself that they could withstand the force of someone hitting them with a hammer while hanging from them. In fact the tubes were so strong that the weight of three hammers at once had to be used to get them to crack – "We had a plane to catch, and the glass was not breaking fast enough," says Clarke. But even with damaged glass, the tubes could still resist the wind load.

The problem with using glass is this way is that it is strong in compression but weak in tension, which would mean that the tubes would be vulnerable when the wind tugged the facade outwards. To overcome this, the designers developed a method of pre-compressing the tubes. A stainless steel plug joined by a steel cable is fitted to either end. This cable is tensioned so that it holds the plugs tightly in position and subjects the glass tube to a strong compressive force.

When the wind tugs at the facade, the compression in the tube will be reduced, but never enough to pull it into tension. "The pre-stress load is so large that the tube will always remain in compression – whatever the wind load," says Clarke.

But it is not just tension that is the enemy of fragile glass tubes; bending, too, is a problem.

To allow the tubes to move freely with the facade and to ensure that no bending forces are applied, the tube's metal end caps are attached to the structure using a ball-and-socket joint.

The glass tubes are not due to start arriving on site until midsummer, but the first sections of steelwork to support the atrium will start to be erected in April. The atrium is due for completion by the end of the summer – just in time to give the last of the summer's tourists visiting the Tower of London something extra for their money.