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Touching the earth lightly

Jorg Schlaich is one of the pioneers of lightweight structures. Barrie Evans reviews a new monograph that charts his contribution

Titling a book The Art of Structural Engineering* makes a claim for some of the territory traditionally occupied by architects. And for lightweight structures, rightly so. An elemental structural quality, long admired by the Modern movement in industrial structures, is brought into the building mainstream. It is the pioneering engineer as much as the architect that develops the vocabulary of form. But then, when Schlaich began his engineering education in Stuttgart, he also cycled across town to attend the architecture lectures.

Jorg Schlaich's work is by no means confined to buildings. He has designed major road and rail bridges as well as footbridges, described as 'one of the jewels of the art of the structural engineer'. He also designed a 146m-high cable net power-station cooling tower at Schmehausen, built in 1974. Assembled on the ground it was winched up a central mast before the adding of internal aluminium sheet cladding.

In Hamburg in 1967 he built a 272m-high communications tower with pods of up to 39m diameter for high-level equipment and workspaces. Schlaich developed a method of clamping the pods to the mast by prestressing their outer rims, which meant that the mast could be slipformed continuously and the pod cast on the ground and winched up the mast before prestressing.

Though this latter project is a rare case of Schlaich working with concrete, along with some early projects on concrete shells, the above examples are typical both of his ingenuity (the root of the word engineer) and his concern for the practicalities of construction. Lightweight structures have nowhere to hide the way they are made. Alongside Schlaich's schooling his father made him train as a joiner, for which he is retrospectively grateful, it being another echo of the early Modern Movement and Bauhaus tradition of integrating designing and making (Schlaich was born in 1934). He describes the best moment of a job as the start of construction, of actually making something.

Not much of a picture of Schlaich the man emerges from this book, except for his German reserve. Obsessively busy, he admits to having little time for friendship. He is relaxed about the complex mathematics, as people are who enjoy that challenge. He is preoccupied with the lightness of structures, in paring down the sections and joints. His measure of structural honesty is of a structure acting mainly in direct tension or compression, with a minimum of bending.

The book rapidly moves on to case studies on different types of structure - concrete shells, cable nets, glass grid shells, textile membranes (few), bridges, early suspended buildings and solar-power plants. All are well presented. One chapter particularly pertinent to current uk interests is on footbridges. Looking through its case studies provides interesting ideas on producing shallow decks or curves on plan, and on the relative merits of cable-staying and other suspension options. All the book's case studies focus on the issues of engineering and project organisation, but accessibly so.

Glass grid roofs

A key example of Schlaich's pioneering development is the continuing evolution of glass- or acrylic-clad steel grid grid-shell roofs, with the bending mainly eliminated. His hope is to create analytical tools and types of structures that will allow such roofs to be part of the construction mainstream.

His precedents include Italian gallerie and Buckminster Fuller's domes, though he finds Fuller's geometrical possibilities limited and the complexity of their six-way nodes and triangular glazing a problem. Another precedent is the free-form grid shell by Frei Otto and Ted Happold for their 1974 Mannheim garden festival pavilion, in which the two-way continuous timber strips cross, forming the nodes of a quadrilateral grid. Inevitably the continuous strips twist a little in their length as they follow the roof curve, something more appropriate to timber than to metal. And the shells are not good at resisting uneven loads from wind and snow. Schlaich's alternative is to use a quadrilateral grid of equal-length members that span only from node to node, acting as glazing bars. The shell is braced to deal with bending loads by a grid of slender diagonal cables on the inside, clamped at each node.

The first major building to use the system was a dome over a pool at Neckarsulm completed in 1989. While the steelwork worked well, creating a dome meant distorting the grid. All members remained of equal length but the openings were not square and so glazing had to be specially cut.

More successful was the courtyard roof for the History of Hamburg Museum, also in 1989, with architect Volkwin Marg. The courtyard is L-shaped so that the grid mainly forms faceted arches over the two arms using rectangular glazing. Only where the two legs meet at the elbow is the grid developed into a three dimensional form with special-shaped single glazed panels.

Currently Schlaich is exploring double-curved surfaces where the four corners of any quadrilateral of the grid lie in one plane, making flat, say double-glazed, units practicable. The options for overall surface shapes are restricted by this requirement, but have possibilities (see diagram). A recent competition entry (with Marg) proposed using a series of domes with this grid characteristic to cover the platforms at Helsinki station. The search continues.

A power grid

The book's final focus, on solar power, arises partly from Schlaich's interest in developing countries, partly from his expertise in lightweight structures. In one case a 17m-diameter ring with a grid of cables was used as a frame for an experimental solar energy concentrators, motorised to track the sun. The reflective metal surface started as a flat sheet on the frame and was vacuum-formed into a dome. In another case an enormous, circular-plan greenhouse was designed, connected to a central flue, with the stack effect up the flue sufficient to drive a wind turbine at its base and thus generate electricity.

It sounds fanciful, but he did actually build one. The greenhouse was flat-roofed, 1.85m high and 250m in diameter. The central flue was 10m in diameter and 195m high. Sited at Manzanares in Spain, it was completed in 1981 and ran experimentally for eight years, twice as long as planned, before corrosion failure of the guys and collapse in a storm. Peak output was 50kW, close to predictions.

In this as in other projects Schlaich combines detailed analysis with imaginative leaps of aesthetic engineering. It looks to be the recipe for developing lightweight structures.

* The Art of Structural Engineering: The Work of Jorg Schlaich and his Team Alan Holgate, Edition Axel Menges. In English. 294pp. £62.

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