A glass canopy supported by cantilevered glass beams
The glass canopy stands in the plaza of the recently completed $1.5 billion Tokyo International Forum, by Raphael Viñoly Architects (AR November 1996 pp38-45), where it shelters the staircase leading to the Yurakucho underground station.
The story of its design, from first inspiration to last-minute compromise, makes an intriguing diary.
July 1995. Tokyo. A glass canopy with steel beams is on the drawing board.
September 1995. Tokyo. Kunio Watanabe, structural engineer to the Forum, suggests supporting the canopy on laminated glass beams. His office faxes Dewhurst Macfarlane in London to seek advice. Tim Macfarlane had worked on several innovative glass structures using laminated glass beams, but none had been more than 6m long, as laminates in longer beams suffer creep deformation leading to excessive deflection. Tim keeps the fax in his pocket.
29 October 1995. London. Tokyo gives Tim seven days to develop a scheme and fly in with it. This acts as a trigger. That night in a restaurant, Tim sketches a possible solution on a paper napkin - a glass beam made of a series of glass blades connected at their ends and at their mid-points to form a rigid cantilever. The blades would be connected by a steel pin passing through drilled holes in each blade, and the loads would be evenly distributed by a metal bezel fitted in the hole.
Later that evening. Tim is calculating load factors when Jon Corpe - architect, friend and guru - phones. Tim outlines his idea, Jon listens and tells him to bring the sketches round to his place. That same night Tim, Jon, and his architect partner, Sheila Bull, work out the feasibility of the revolutionary beam design. It takes shape with the dawn.
The next day. Despite lack of sleep, Jon draws out the structure to scale and Sheila starts work on a 1:20 model. Tim has a problem . Although the calculations are straightforward and the beam dimensions based on known values for strength and elasticity of glass, the critical unknown is the viability of using a bezel to transmit the load evenly into the glass blade. Tim faxes a rough sketch to Steven Ball of F.A.Firman in Romford who starts to make a bezel and glass beam for testing.
4 November. Romford. Firman's relatively crude but effective rig proves that the bezel andglass beam system could carry a load of over 6 tonnes.
5 November. Tokyo. Tim flies to Tokyo. The drawings, the model and his calculations are received with great enthusiasm.
20 November. Imperial College, London. Small 400 x 200 x 19mm glass samples with bezels and pins are tested; breaking loads of between 7 and 10 tonnes are measured.
10 December. Tokyo. After intense study, the canopy with its glass structure is approved by the chief executive of the Tokyo Metropolitan Government.
November. London. A complex computer model tests the structure's capacity to withstand earthquakes and typhoons (wind pressures in excess of 5 Kn/m²). Firman starts producing a full-size prototype glass blade.
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February 1996. City University, London. The Japanese participants arrive and join the English
team to witness the test of a full-size glass blade and bezel assembly. The academic supervisors, assuming the superiority of steel over glass, had located the glass in the test rig with 20mm diameter mild steel bolts. As the load creeps towards ten tonnes, a loud bang announces sudden failure. The crew rush forward to find the glass intact and a rig bolt sheared through. After a new high-strength bolt is fitted on the rig, the glass is tested again and breaks at a load of 11 tonnes.
March 1996. Suddenly the Japanese get cold feet. They insist that the structure be reinforced with acrylic beams. After much argument Tim bows to the inevitable. His elegant compromise pairs acrylic and glass blades at the root of the cantilever beam.
April - July 1996. Firman works day and night to produce the beams. Compared to the design process, its construction, by teams of white-clad Japanese workers, was without incident. Since completion the canopy has withstood two typhoons and a slight tremor (6 on the Richter scale).
Working Details
The glass canopy shelters an 8m long x 4.8m wide staircase well leading to Yurakucho underground station. It rests on three composite beams of glass and acrylic which cantilever fron Y-shaped steel brackets at ground level to a height of 4.6m at the stair entrance.
Each beam is formed of sets of glass and acrylic blades which interlock at their ends and mid-points to form a rigid structure. At each connection point the ends of at lest two paired blades are pinned to the mid-point of a third blade.
Each blade is made of two 19mm sheets of glass laminated together. For strength they are approximately triangular in shape. They vary in length from 3.8 to 4.8 metres.
Each connection is made by a 40mm diameter stainless steel pin which passes through 68mm diameter holes in the glass blades; loads are evenly distributed by a metal bezel fitted into the hole.
For ease of fitting together on site, the radius of the bezel is offset by 1mm; inside it fits a bush with an offset hole, allowing a tolerance of 2mm for the bolt.
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Each blade is made of two 19mm sheets of glass laminated together. For strength they are approximately triangular in shape. They vary in length from 3.8 to 4.8 metres.