EXPO-DACH

Architectural Review, The, Sept, 2000 by Layla Dawson

Symbolically, the permanent roof over the main performance space at Expo is mostly made of timber: the renewable building material.

Timber, and its by-products, are politically correct New Age materials. For a World Fair with the motto 'Man, Nature, Technology', wood is the perfect construction material; 'natural', grown from sustainable sources, and with engineering traditions and building codes. Munich based architects Herzog Partner, specialists in timber research and experimentation, are, not surprisingly, much in evidence at Hanover this year. Along with their Trade Fair administration tower, and exhibition Hall 26, their sculptural timber Expo-Roof will remain as a permanent feature long after Expo 2000 finishes.

Trees are nature's air conditioners. Photosynthesis, the method by which green foliage captures carbon dioxide and produces oxygen, gives us the air we breathe. So long as timber is not burnt, or allowed to rot, this carbon dioxide remains captive. Forestry, like farming, is essential for human health.

In 1958 the Brussels Atomium optimistically heralded in the then emerging atomic industries. Hanover's Expo-Dach (Expo-Roof) symbolizes, hopefully, a more benign partnership between Man, Nature and Technology, in the twenty-first century. As a weather-proof, open sided shelter in the uncertainty of a Northern European summer, it protects a stage area for musicians and artists, small pavilions and restaurants, and spectators taking time out between events, in the central piazza.

Each of the 10 upturned umbrellas, over 20m high, consists of a central four-legged tapering pylon of timber and steel supporting a 40 x 40m square crown, which is further divided into four identical square leaves. At the base of each pylon, steel feet are anchored in a 15m deep concrete ring foundation. The leaves of the crown are double curved surfaces made up of a skeletal net of laminated timber struts clad in a weatherproof membrane. Precipitation is conducted to the centre of each umbrella, down a rainwater pipe in the centre of each pylon, and feeds into a grid of 5m wide canals on the piazza which complement the roof geometry. The illusion is of pontoons and pavilions, floating on water, under a substantial billowing cloud. Changing daylight, the aspect of the sun and sky colour, is filtered through the millimetre thin translucent roofing membrane which is fireproof, self cleaning and, if necessary, recyclable.

The timber originated in the Black Forest, in south-west Germany. Here grows the biggest concentration of ancient white oaks in western Europe. Used for centuries as ships' masts, the standard girth of the oaks chosen for the Expo-Roof columns was 1.4m, and some were up to 250 years old. For a forest, the culling of these older trees, up to 50m high, is essential for new growth.

Suitable trees were selected using ultrasonic equipment to reveal any internal structural weaknesses.

Thomas Herzog has said that the aim was not to produce a filigree transparent construction, as would have been the case with glass and steel, but to reflect the solidity of a tree with its structural strength visible in a progression from large supporting trunk through to branches and twigs. The Expo-Roof tests the limits of present day timber engineering. Computer technology saved wasteful felling of trees. The double curved roof deck and supporting structure were coordinated using a computer program which steered CNC-robot cutting machines in the manufacture of partially factory made sections, small enough for transport, which were then assembled on site. This precision saved time and resources.

The design process was not linear but a ping-pong ball discussion between initiator, engineers and contractors, testing structure and construction feasibility using real models and computer simulations, wind tunnel and loading tests. Form and materials inevitably developed and changed accordingly. Steel shafts were unavoidable at critical points but this is the first time that timber alone, in the form of large area plate collars, solid and laminated sections, has been used for structural wind bracing. Like great-grandmother's embroidered alphabet the Expo-Roof could be viewed as the sampler for a new generation of timber buildings.

Ten years ago the decision to hold Expo 2000 in Hanover met with much opposition from environmentalists who argued on both ecological and economic fronts. Some of their fears may still be realized. But more than in any earlier Expo buildings, landscaping and public transport systems have been designed for the city and the permanent Trade Fair site, and many experimental constructions have been sponsored and given media coverage. Germany's industrial strengths: systematic precision, an intact system of apprenticeships and master builders, a predominance of middle-sized, family operated manufacturing firms, codes which sometimes exasperate clients and architects, and a thriving economy, have all given Hanover the opportunity to show how we might deal with old problems in new ways, or even new problems in old ways.