By Zou Bin
From May 1-October 31, 2010, more than 50 international organizations and representatives from more than 170 countries will gather in Shanghai under the theme “Better City, Better Life” at the 2010 World Expo. The event showcases the latest developments in sustainable urban development, highlighting innovations from China and elsewhere in Asia. Pudong Expo Park is located in the heart of Shanghai’s newly developed district, spans both sides of the Huangpu River, and covers 5.28 square kilometers.
Taking center stage at the event is Expo Boulevard, a one-kilometer long, 110-meter wide, north-south transportation axis extending from Riverside Expo Park to Yaohua Road. As the main entrance to the Expo complex’s main pavilions, it has a critical traffic and transportation function, but the three-level boulevard also provides food, entertainment and exhibition services.
Shade above, light below
Expo Boulevard features an extended plane and membrane-structure roof attached to six steel and glass ‘sun valleys’—distinctive structures that cover the ground-level thoroughfare and serve as a landmark for the Expo site. There is a practical function for the entire complex as well; it provides the two underground levels of the boulevard with natural light, saving energy costs for both lighting and temperature control, and it channels rainwater underground where it is then filtered and circulated for use in the Expo pavilions.
To accomplish this purpose, a membrane-structure roof and six sun valleys composed of steel and glass were designed by Knippers Helbig Advanced Engineering, Stuttgart, Germany and New York, N.Y. The roof and its supporting framework consist of interconnected membrane, mast, and cable sections of varying heights (10, 4.5, 1.0, and 6.5 meters), widths, and angles, together stretching across the 1,045-by-80 meter above-ground boulevard. The giant sun valleys, standing 41.5 meters high and measuring 90 meters in diameter across the top and 20 meters across the bottom, are composed of a single-layer shell structure of net frame in triangular grids in welded, box-type sections. Five of the sun valleys use axial symmetry while Sun Valley 4, at the center of the boulevard, employs rotation symmetry.
The membrane roof
The continuous tension membrane roof, which extends between each sun valley, is made of fiberglass fabric with a polytetrafluoroethylene (PTFE) coating, a material that is relatively new to the global market and not yet used in residential applications. Its mechanical properties meet China’s requirements for Grade A membrane. A pivot system and a framework of cables and masts accommodate the irregular dimensions and angles of the roof and maintain the required tension.
With the sun valleys, the masts support the membrane roof along the length of the boulevard. Outer masts line the periphery of the thoroughfare, while central masts resting on pedestals on a 10-meter above-ground platform follow a zigzag pattern at varying distances and heights along the boulevard.
The membrane roof attaches to the masts and sun valleys in a series of cable-membrane units of varying sizes. Two types of membrane units were designed: one is cone-shaped with a central mast; the other type attaches to a sun valley rather than a central mast and is smaller and varies in shape. The cone-shaped membrane units are formed by three smaller triangular sections of membrane bordered by valley cable. Ridge cables, which take on a continuous ‘Z’ shape on plane, define the periphery of the membrane units. The circular center of the membrane unit attaches to the base of the central mast.
Pivots and tension
The ridge, edge, and valley cables that form the membrane units are the primary load transfer transmission components, transferring the load from membrane to pivot. An elaborate system of pivots maintains the roof’s tensile property. There are four types of pivots: pivots for the outer masts on the periphery of the cable-membrane unit, for the drop-down ring of the central masts, for the sun valleys, and for the suspension cables.
On the cable-membrane unit’s periphery, pivots attach to the outer masts, which tilt outward and range in height from 18 to 39 meters. Pivots for the drop-down ring attach to the bottom of the central masts. A third type of pivot connects the roof membrane to the periphery of each sun valley. A fourth pivot prevents displacement by connecting suspension cables between the top of the central masts and to the valley cables in the membrane’s surface.
Connections within the pivots system create a self-supporting tensile structure independent of the entire membrane surface system. It is formed by the central masts; their corresponding horizontal cables that connect to the outer masts; ridge cables; and a small number of essential temporary cables, which prevent pivot displacement.
The construction companies, Shanghai Mechanized Construction Co. Ltd. and Shanghai Taiyo Kogyo, faced a number of challenges related to stretching continuous flexible cambers of irregular sizes with a high preload stress:
- The sun valleys’ single-layer shell surface has poor outer rigidity, making it difficult to control and stabilize its shape during installation
- There are no typical or representative structural components, yet numerous cable strengths needed to be tested for construction control
- Limited experience working with the new type of membrane material
- The large dimensions of the cone-shaped cable-membrane units (1717 meters squared when stretched) require more operations for cutting, packaging, transportation and installation
- The relative size of the cable-membrane units: The sun valley membrane units are smaller in area than the cone-shaped membrane units with central masts, so the reaction force between the cone-shaped units on either side of the sun valley membrane unit is less
- The membrane structure roof needs to be installed and stretched only after its corresponding sun valley has been constructed
Given these challenges for hanging and stretching the roof, the membrane structure was divided into five regions and a nine-step process was designed for constructing the sun valleys, cable-membrane units, and their infrastructure. Construction began at the center of Expo Boulevard with Sun Valleys (SV) 4 and 5, moved next to SV3, SV2, SV1, and finally SV6. The cable-membrane units in region 4, between SV4 and SV5, were erected first, followed by regions 3, 1, and 5, with region 2, between SV2 and SV3, the final section to go up.
Construction of the sun valleys and membrane-roof structure began in December 2006 and was completed in September 2009.
Additional design specifications and drawings were provided by Shanghai East China Architectural Design Institute Co. Ltd, Zhejiang University, Hangzhou, China; Shanghai Taiyo Kogyo Co. Ltd; Shanghai Pujiang Cable Co. Ltd.; and Beijing Research Institute of Electrical and Mechanical Services.
Zou Bin is sales manager with Shanghai Taiyo Kyogo Co. Ltd.