Cantilevered glass canopy, Tokyo

• Novel laminated glass and acrylic bladed structure

• Linear static self weight, thermal and wind loading assessment

• Seismic analysis

The plaza of Tokyo International Forum in Japan features a unique 10.6m long cantilevered glass canopy designed by Dewhurst Macfarlane and Partners with the aid of LUSAS Civil & Structural

The canopy shelters an 8m x 4.8m wide staircase well leading to the Yurakucho underground station. The supporting structure comprises cantilevered beams each made up of 4 component beams pinned at their middle and end points to form an arch. These component beams are made up from both laminated glass and acrylic blades that reduce in number from 4 blades at the base of the cantilever beam to 1 blade at the tip. The blades are connected by 40mm diameter stainless steel pins to T-shaped brackets which in turn support the glass panels forming the canopy roof. At the base of the canopy V-shaped stainless steel brackets connect each cantilever to a horizontal beam running the full width of the canopy.

LUSAS Civil & Structural was used to investigate the response of the canopy to both static and dynamic loadings. The effects of self weight, differential thermal effects, and wind loading, both from the front and the rear of the structure were studied. Thin shell elements were used to model the cantilever beam blades, roof panels and support brackets with thick beam elements being used to model the base connections. Joint elements modelled the connections between the glass blades and stainless steel pins.

To assess the response of the canopy to seismic loading a natural frequency analysis was done followed by two spectral response analyses using expected local ground accelerations. Two different levels of structural damping were also allowed for. Initial attempts to perform the natural frequency analysis revealed a large number of mode shapes in the glass roof panels. This prevented extraction of the higher frequency global modes which have a significant influence on overall response. To overcome this problem, a useful procedure known as Guyan Reduction was used to filter out the low-energy modes to leave only the dominant global modes in the solution.

By using the extensive graphing, contouring and plotting features in LUSAS, structural deflections, forces and moments in the joints, and stresses in the roof panels and cantilever members were obtained to help optimise the design. The peak joint forces and corresponding stresses from the seismic analysis were generally found to be greater from the spectral response analysis than those from the static loadcases. This was mainly due to the CQC values used applying ground accelerations in all 3 directions, inducing lateral loads which were not applied to the static loadcases.

Tim Macfarlane, responsible for the design of the canopy was pleased with the results obtained. He said: "By using LUSAS Civil & Structural for this project we were able to determine with confidence the response of the structure to earthquake loading and to check the analysis for wind and dead loads against more conventional computer models and our preliminary hand analysis".

Since completion the canopy has already withstood two typhoons and an earth tremor measuring 6 on the Richter scale. At the 1997 British Construction Industry Awards, the Yurakucho canopy received a special commendation for demonstrating British expertise in the design and construction of glass structures.

"By using LUSAS Civil & Structural for this project we were able to determine with confidence the response of the structure to earthquake loading and to check the analysis for wind and dead loads against more conventional computer models and our preliminary hand analysis".

Tim Macfarlane, Partner, Dewhurst MacFarlane and Partners

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