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BENDING AND LATTICE STRUCTURES The scaffolding lattice system for roofs was devised to avoid the volume of material that would have been required of a ''bending'' structure. Small diameter compression or tension tubes in a three dimensional lattice transfer the roof loads back to a few columns for the 102m x 52m membrane covered Interbau Buildings Berlin 1961. This was a ''first'' for Mero and in a way the precursor to the many lattice space frame structures by Kenzo Tange at Osaka 1970 (fig 1).
19 ON FREI OTTO'S PHILOSOPHY OF WIDESPAN LIGHTWEIGHT STRUCTURES Michael Dickson This is a text prepared in collaboration with Professor Frei Otto and based on a presentation given on his behalf at the inaugural session of the Bath University Symposium on Widespan Enclosures in April 2000. Frei Otto's long career in lightweight structures includes the development of stressed tensile sails for the Lausanne EXP064, the distinguished German Pavilion membrane and pre-stressed cable nets for EXP067 in Montreal, the Munich Olympic Roofs in 1972, and the Gridshell at Mannheim in 1975. The conceptual design studies for these and many other projects were carried out at the Institut fur LeichteFlachentragwerke (I.L.) which Frei founded at the University of Stuttgart. Between 1967 and 1995 he worked often with Ted Happold, a friend and fellow spirit, on such projects as the 120m x 90m cable net structure for Jeddah University and the Diplomatic Club, Riyadh, (Aga Khan prize for architecture 1998). Professor Otto's current work includes consultancy on the Venezuelan and Japanese pavilions for the EXPO at Hannover 2000, and conceptual design for the new railway station for Stuttgart 21. He is one of the leading innovators in the development of lightweight structures and has recently received the following international prizes: • Honda prize for ecological technology 1990, Tokyo • 77ie* principle prize of the German Institution of 0 Architects and Engineers in Berlin, 1996 • The Wolf Prize for Arts, Jerusalem, 1997 He is an Honorary Fellow of the Institution of Structural Engineers and of the Royal Institute of British Architects, and Dr of Science honoris causa at the University of Bath. INTRODUCTION In both the developed and the developing world, widespan enclosures are increasingly required to house and facilitate many of the collective activities of society. Such enclosures need to do this without drawing down excessive quantities of scarce construction material or drawing upon unnecessary quantities of energy in their operation. To ensure such aims requires an efficacy of construction, a delight in their occupation and appropriateness to their location. Beauty of architecture, efficiency of structural form and appropriateness of material usage are the fundamentals in securing this aim. Yet in the solution of this theme the use of large spans is not just a game to make the Guinness Book of Records but a search for real solutions for mankind. To know about large spans also opens opportunities for advances for shorter spans. In the absence of scale factors on short spans it is possible to use material less effectively. Conversely for the larger spans, it is necessary to seek out fundamental 'absolutes' of performance and to recognise the significance of 'scale' and the problems of enclosure. Part of this search is the recognition of optimal performance and benefits of different structural forms in ascending order of opportunity - so this paper tackles the fundamentals of performance of successive structural types - bending structures for smaller spans, lattice structures, gridshells and compression vaults, tension structures and finally the opportunities for pneumatic structures. These structural systems are discussed and illustrated principally through a wide variety of projects. BENDING AND LATTICE STRUCTURES The scaffolding lattice system for roofs was devised to avoid the volume of material that would have been required of a 'bending' structure. Small diameter compression or tension tubes in a three dimensional lattice transfer the roof loads back to a few columns for the 102m x 52m membrane covered Interbau Buildings Berlin 1961. This was a 'first' for Mero and in a way the precursor to the many lattice space frame structures by Kenzo Tange at Osaka 1970 (fig 1). Fig 1 20 Prefabricated standardised galvanised Delta units and 'bolted' cross nodes made up the 42 cm deep intermediate viewing platforms for the German Pavilion at Montreal (1967). Engineered by Leonhardt and Andra the cruciform head units positioned diagonally across the grid concentrate loading from the floor grid onto the column top, each element as in the human body prepared for its particular duty (fig 2). Fig 2 In the 24m high Bell Tower for Berlin (fig 3), the architectural form of the virendeel truss is retained while the plate thickness is aggregated from 10mm at the top to 50mm at the bottom in order to restrain the drift of the Tower to 16mm under the ringing action of the 3 bells - function following form: Fig 3 DIRECT FORCE STRUCTURES Inescapably, the most efficient way to transfer load back to foundations is by a 'direct' way - an inclined straight spar. Early investigations with the mushroom support 'spars' to the 'humped' tent at Koln 1957 led to studies for radiating 'fan' systems for the Transrapide Maglev viaduct system (figure 4a). It should be noted in passing that the alternate form to the nose of the model capsule 21 on the bridge is itself a holistic proposal to reduce wind resistance at speed, hence the required magnet power and therefore also the weight to be supported by the bridge itself. The purpose of the viaduct design for the Maglev was to reduce the impact of the linear induction Transrapide support system on the countryside of Northern Germany from Hamburg to Berlin. The Transrapide Maglev is a light multicar system capable of travelling at up to 450 km/hr using the technology of aircraft systems. Breaking loads from emergency deceleration are more critical to the support structures than vertical loading. Structural continuity and close accuracy of construction, allowing also for thermal distortions, is essential for ride comfort - hence the structural concept of a minimal triangulated tubular network casting little shade on the ground below and supporting loads onto simple foundations. Fig 4b A further development of these thoughts has led to the fan pedestrian bridge system for Gelsenkirchen 1999 (fig 4b). In line with earlier studies of bamboo structures at the I.L., the various spans of this radiating system are to be made from kit form of solid 70mm galvanised bars and 4-bolt cup-clamp systems. Individual buckling lengths are to be reduced by an internal criss cross of stabilising bars - also 70mm 0. The 1960 study at Yale for a thin roof did so by dividing the individual spars to form a "triangulated" network of stable compression elements (fig 5). For the Council of Ministers project in Riyadh (1978) the loading from the 3'D' hexagonal grid shell for the seating bowl is gathered by irregular triangulated configurations of tubes of successively increasing diameter. These match the buckling length restrictions to the requirements of increasing load back onto a single composite column of 3 individual braced tubes (fig 6). Fig 6 But the aesthetic of a design in its surrounding is also of great importance - the tree fountain on its well in Warmbronn drips its water carefully into the well (fig 7). Sometimes a symbol will be sufficient. For the one day meeting of the Evangelic Church at the Berlin Olympic Stadium (1961), only a single 40m high guyed cross structure was needed to express the enclosure (fig 8). Fig 7 I fey m 1 L^A 1 i 1 ''ii'-tnnMi'' 1 \ \ v, 1 K ' Fig 5 Fig 8 22 FUNDAMENTALS OF MATERIAL AND FORM In terms of the 'absolutes' of measurement, illustrations on a logarithmic scale relate the basic forms of stability of everything from mountains to grasses and hairs - aim high grass has a H/D of 100 or more (fig 9). Of consideration too, for all enclosure tension studies, are the fundamental rupture lengths of different materials under their self-weight - wood is the leader. 1 f ! QB'«a» / «f 1 y /// 1 ' 1 i« m r \ f | * 1 L. ,J » f; 7 i h ? — J s I * 'j -r- ' ' j J s I * 'j -r- i Fig 9 I Other studies have also shed light on fundamentals of performance: • What stable forms does sand create when allowed to run away? (fig 13). • What are the laws of form for spine structures (fig 10) and for hanging vaults? (fig 11). Fig 13 • Based on the historic shells from Harran (fig 12) what crucial forms from local brickwork can resist the lateral forces of an earthquake? - as measured on the tipping table, the cone of make 0.3g (30°). Fig 11 Fig 14 23 The proposals for the naturally light and ventilated forms for Islamic University at Uzbekistan, constructed solely of bricks is the outcome of such studies (fig 14). To optimise bridging, theoretical studies show that you can bridge 10 miles so probably at least 1/10 of that can be achieved in reality. How do vaults really work? In the vousoir model, it is to be noted that the zig-zag string transmits the 'shear' for stability (fig 15). Indeed the study of arch forms led directly to the form of the openings in the supporting walls of the Diplomatic Club, Riyadh [now CasaTuwaiq] (fig 18). Fig 15 On the tipping table, lower arch forms are more stable than high forms. Even arches can be curved in plan, (figures 16 & 17). Fig 16 Fig 18 SHELLS, GRID-SHELLS AND VAULTS In 1958, with the help of students at Washington University, a rubber membrane weighted with nails was used to investigate forms "without bending". Such forms were then stiffened with plaster and inverted into a shell form. There followed the single layer timber gridshells for Essen (1962) (fig 19) and that by students at Berkeley (1962) constructed out of steel rods and washers from the hardware store (fig 20). Fig 20 24 The bending free grid-shell form is really a low cost construction method for creating complex forms for public space. An early example is the auditorium of the German Pavilion, Montreal (1967) prefabricated in Germany and drawn out into its final form on site (fig 21). This was a forerunner to the minimal energy house designed for Ted Happold. Here oak lath gridshell, turf covering, south facing glass wall, pv cells and wind generation are all part of a holistic approach to design (fig 22). Fig 22 With the help of the computer, there are now forms which are difficult/impossible to model physically - the naturally light and ventilated workshop in Dorset for John Makepeace of roundwood spruce trees formed by green bending the tapered green debarked trees is one (fig 23). Another, the Japanese Pavilion at Hannover with Shigeru Ban is in reality only "findable" on the computer even though here physical modelling gets close to the final form (fig 24). An originally flat grid of 12cm diameter paper tubes banded together in a 1 metre grid is pushed up to form a bended amphora form subsequently stiffened by the cable - undertied timber ladders and diagonally braced cable formed honeycomb end walls. In turn these were used to attach the paper membrane. All components including the "sand box" foundations were designed to be easily recyclable and so give an enclosure which specifically "touches both the 'planet' and ground lightly" (fig 25). Fig 25 The double layer gridshells for the Bundesgartenschau, Mannheim designed by Frei with Mutschler, Langer, Happold and Liddell were most courageous enclosures and extremely inexpensive (fig 26). So inexpensive that Kikutake followed them with a much larger 'shell' for the Japanese Silk Road Exhibition in 1988 (fig 27). Fig 26 'Inversion' of the tension eye for Montreal and the I.L. (fig 28) led to the development of the compression forms for the new below-ground naturally light station beneath the Schlossgarten, Stuttgart with Ingenhoven and Buro Happold/LAP (fig 29). The inverted forms modelled in plaster span a grid of 60m x 30m using only a concrete vault 35cm thick at the crown thickened to 65 cms around the eye. Indeed each pier supports of the order of 35,OOOkN of loading from the landforms above. Recent form models for Stuttgart 21 envisage inexpensive construction techniques using propped timber grid shell forms (remember the bending free forms of Mannheim) to create the free vaulted form from the plasticity of wet reinforced concrete. Fig 27 26 HANGING STRUCTURES AND DEAD WEIGHT FORMS Simple hanging forms are able to exploit the effectiveness of the long rupture lengths of tension fibres - especially if they can be stabilised against disturbing loads by self weight, damping or enclosure. Early studies for a pagoda roof 1983 previewed the prototype house at Hooke Park with Richard Burton. The hanging roundwood spruce thinnings curved down under dead weight are opposed by A frame compression spans (fig 30). The elegant Wilkhahn factory with Gestering architects and Speik und Hinkes engineers for timber products in its agricultural landscape uses a similar philosophy but employs square sawn timber (fig 31). Really this was a focussed study in the use of minimum embodied energy and of minimum operational energy in the industrial context - built in the countryside. Fig 31 Both are predated by the aluminium covered, heavily insulated auditorium for Mecca with Gutbrod/Arup/Happold. The 22mm 0 spiral cables hanging from the central steel portal are cross connected by double angles that support and distribute the loads of the insulation and cladding and contain the enclosed air volume (1968) (fig 32). Fig 32 More daringly, the wind tunnel at Teddington was used to investigate the stability of the proposed hanging roof for covering the Berlin Olympic Stadium. Solid steel rods supported on tension cables add sufficient weight to the plexiglass forms (1973) (fig 33). Fig 33 PRESTRESSED TENSIONED ROOFS At the heart any discussion of prestressed tensile roofs are the many studies that contrast tents with a central support point and radial cutting patterns to those with double curved saddle and sail surfaces into which eyes, rings or mushroom supports are introduced. The Riyadh Heart tent (1986) with its radial spider net of stainless steel cables supporting painted glass panels onto a central mast (fig 34) is diametrically different to the 40 x 30m Berlin humped tent of deformed cotton canvas tied down at the edges but supported on a series of mushroom supports (1957) (fig 35). The 55m radial patterned squares of the Hadj tents by Fasler Khan of S.O.M. supported on central cable-hung rings of ptfe glass fabric by Chemfab/Birdair are forerunners to the triple layer central supported tensile enclosure for Storek Furniture in Leonberg (2000) (fig 36). Fig 34 Fig 37 The 36m 0 wave tent for Dance Stage at Koln 1957 is now also a protected structure for its six exceptionally slim supporting batten masts, each externally guyed to separate foundations (fig 38). To save on foundations, the high points of the waveform for the Biennale at Venice (1996) use A-frame masts to transfer loads to foundations shared with the tie down (fig 39). Fig 35 Fig 38 Fig 36 The first wave form system had 3 parallel spans of 15m and was patterned by overlapping cotton canvas to create the enclosure (fig 37) for a flying priest, Pater Schulte. Multipurpose, it doubled by day as a translucent place of worship and at night as a covering to his small aircraft! Fig 39 28 Tension structures offer a huge opportunity for very longspan lightweight structures. The recently restored building of the Institut fur LeichteFlachentragwerke (IL) is now also a listed building (fig 40). Originally this was the prototype eye structure for the many masted free- form translucent white pvc enclosure for the German Pavilion at Montreal (1967). The patterned membrane was hung underneath the cable net of 12mm galvanised cables at 500mm centre on springy bretzels (fig 41). This remarkable cable net construction brought the skills of Gutbrod architect, Fritz Leonhardt engineer, and Peter Stromeyer, tent maker and manufacturer together with Otto to create a longspan building that brought with it a paradigm shift in cable net technology. This technology was then transferred to the 120m x 90m double membrane cable net enclosure on eight masts for the Sports Centre, King Abdul Aziz University, Jeddah (1978) (fig 42). Clamped anchorages and chizel point masts and 'teller'plate membrane supports were introduced here. Fig 41 The plan of the multimasted Voliere at Munich (1980) is reminiscent of Montreal but the doubly-curved snow supporting stainless steel woven mesh gets its form from the earlier humped tents at Berlin and Dyce (Aberdeen) 1975 (fig 43). Here computer visualisation enabled development of the zigzag eye form required to support Fig 42 the mesh over an existing ash tree. The particular form for this Voliere was devised to facilitate flight and resting patterns for the ornithological occupants within a natural landscape. The architect for Miinchen Tierpark was Jorg Gribl. Fig 43 Another protected structure is the Olympic Roof at Munich (1972), by Behnisch, Otto and Leonhardt. These structures with their first use of the flying mast laid the corner stone to the wide-ranging research (SFB64) on long span structures directed by Leonhardt, Otto and others in many departments of the University of Stuttgart (1975-1985) (fig 44) Fig 44 [...]... the swimming pool at Regensburg (1972) demonstrates the importance of obtaining a construction geometry that is capable of delivering a well resolved geometry especially under the extreme load of wind, snow and snow ponding (fig 47) At Regensburg this was made possible by having a mast which was sufficiently high Along the way, the entrance arch tent originally of polyurethane covered glass fabric stabilising... pressure produces a concave spherical surface for external projection (fig 52) Fig 52 The ultimate pneumatic structures are those of man himself - Bone is made as a liquid filled tension structure - a baby, a composite pneumatic structure, is gestated within a protective water sack Fig 50 Fig 51 As a concluding thought such illustrations show that if we are to break open the discussions between architect,... water, helium etc At one level, air supported structures enable economic and safe enclosure of very large public spaces - City in the Arctic (1971) (fig 50) - or the project for "58° North" At the smaller scale for the Academieschiff, Berlin, positive internal pressures generate the stable screen for projection of images onto the largely cylindrical internal surfaces (fig 51) while on an earlier project... executed at Montreal in 1967) This is probably the alternative technique for spanning spaces as large as that of the Dome at Greenwich (1999) In a way, the 80m x 40m study for a pvc covering at Sullom Voe, Shetland Islands (1981) which is supported from a number of masts by arrays of single cables is a scaled down version of the design for Bremen (fig 46) Fig 47 PNEUMATICS Studies and calculations in 1958... that a factory of 3 pneumatic bubbles, made out of aluminium sheet might be able to enclose spans up to 800m (fig 48) Many fundamental pneumatic forms were originally investigated during research using flexible rubber membranes whose forms were "captured" by plaster casts (1960) (fig 49) Fig 46 Fig 48 30 Fig 49 The creation of stable pneumatic enclosures is concerned with the differences of pressure between... thought such illustrations show that if we are to break open the discussions between architect, engineer, user and constructor, we need to attend to some of the 'absolutes of performance' Reference to the underlying form of the many natural structures around us will help address the problem of achieving similar efficiences - or at least coming closer ... glass fabric stabilising a single 150mm diameter tubular arch was a preview of the Otto/Tange/Arup proposal for arch supported cable net forms for the Kuwait Sports City The design proposed only a 1.0m diameter steel pipe stabilised by a cable net roof for the 250m long main Olympic Stadium (fig 45) The 1961 proposal with Leonhardt for a 1800m x 550m covering to Bremen Harbour employed high masts and...29 DEMOUNTABLE STRUCTURES Fig 45 Historical studies at the IL on Roman Vela and Islamic Toldo's resulted in the design and execution of a number of demountable or semi demountable enclosures Early projects at Cannes and Paris led to the demountable covering over the ruined church