01 A 58-metre span that has to be able to move
The figures first. The bridge links floors 41 and 42 of the two towers, stands 170 metres above the ground, spans 58.4 metres and weighs 750 tonnes. With numbers like that anyone would assume it is firmly fixed at both ends, like a beam between two walls.
The reality is the opposite, and for an elementary reason: two 452-metre skyscrapers do not sway in unison. Each responds to the wind in its own way, and the distance between them at that height changes. A rigid bridge would have to absorb that difference within its own structure, and it could not.
02 The arch you can see from below is the trick
If you stand directly underneath and look up, the whole solution is visible. Two long struts come down from the flanks of the towers and meet in an inverted V at a point below the centre of the bridge. That is the two-hinged arch holding it up.
The weight, therefore, does not hang from the junctions with the buildings: it travels down the arch. And the ends of the bridge bear on the towers in a way that permits relative movement, so that when the towers shift, the bridge goes with them instead of resisting.
It is a bridge between two buildings that move. The only possible solution was for it to move too.
03 Two towers, two contractors, two separate jobs
That they are two independent structures is not only a matter of calculation: they were also independent on site. The west tower was built by a Japanese consortium led by Hazama and the east tower by a South Korean consortium led by Samsung C&T. Two teams, two contracts, the same design and the same deadline.
Construction was completed in June 1996 and the building was officially opened on 31 August 1999. Each tower has 88 floors and reaches 451.9 metres. Seen that way, the bridge is not an ornament between two halves of one building: it is the piece that had to resolve the meeting of two separate jobs.
04 Not even the ground was where it should have been
There is a second movement, earlier than all the others, that almost nobody knows about. Test boreholes revealed that the chosen plot effectively sat on the edge of a geological step: half the site was decayed limestone and the other half soft rock. Building two towers of that height straddling two different grounds was asking for differential settlement.
The solution was drastic: the entire site was moved 61 metres so that both buildings would rest wholly on the soft rock. Even then the foundations had to be exceptional, with 104 concrete piles between 60 and 114 metres deep, among the deepest ever built for a building.
What all this leaves you with, if you go up, is an anecdote better than the view. You cross a fairly ordinary glazed corridor, with railings and a tiled floor, without noticing anything strange at all. And you are walking on 750 tonnes that are resting rather than fixed, precisely so that the corridor can go on being ordinary when the wind blows.
