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SPACE FRAMES The traditional MERO System, the Ball Node System (KK) is the first prefabricated space frame system developed by Dr. Ing. Max Mengeringhausen during the second worldwar. Loads are applied via the nodes, the members distribute the compression and tension forces. The original idea was to build a space frame with uniform length of members and regular nodes with 18 holes with angels of 45 and 60 degrees. Nowadays the angles between the members may be freely chosen and also the length and diameter of the members is not uniform. The members are round hollow sections, because they have the best resistance against buckling. Diameters from 30355 mm with different wall thickness are standard. The length differs i.g. 1,55,0 m but is not fixed. The nodes are to give a free choice for the connections holes. The standard ball nodes have diameters from 49,5350 mm.
189 TENSILE SPACE STRUCTURES Wolfgang Renner MERO Systeme GmbH&Co.KG, Max-Mengeringhausen-Str.5, 97084 Wurzburg,/Germany ABSTRACT With the rapid advance of technology in engineering, the creation of sound yet innovative construction has evolved into a sophisticated global challenge. For more than fifty years MERO has been a part of world-wide architectural success, in its creation of ingenious modular construction systems, paying particular attention to steel, aluminium and glass structures. MERO's path led from structures made of their classic spaceframe of tubes and nodes to structures of profiles in combination with tensile cables, integrating the cladding as supporting element. In the following this path is described. SPACE FRAMES The traditional MERO System, the Ball Node System (KK) is the first prefabricated space frame system developed by Dr. Ing. Max Mengeringhausen during the second world-war. Loads are applied via the nodes, the members distribute the compression and tension forces. The original idea was to build a space frame with uniform length of members and regular nodes with 18 holes with angels of 45 and 60 degrees. Nowadays the angles between the members may be freely chosen and also the length and diameter of the members is not uniform. The members are round hollow sections, because they have the best resistance against buckling. Diameters from 30-355 mm with different wall thickness are standard. The length differs i.g. 1,5-5,0 m but is not fixed. The nodes are to give a free choice for the connections holes. The standard ball nodes have diameters from 49,5-350 mm. I. Krolsliohiprofii (KHP) 2 Kegel 3. Gevnndebolzen A. Scnliisseliriutte 5. Knebelkerb&lill 3. Threaded boll 5. Dowoipin Fig 1 Fig 3 To connect several members to a relatively small node the members have conical ends. The member is connected to the node via a high tensile bolt. The bolt has a dowel pin and is screwed into the node via a sleeve. Compression force is transmitted via the sleeve, tensions via the bolt (fig.3). With this system single and multilayer space frames can be designed. Examples for Ball Node system ( KK ) are the Split Stadium cover and the Music Pavilion in the Fig 2 Grugapark Essen. 190 Fig 4 Fig 7 The MERO Bowl Node System (NK) is a further development of the MERO space frame system developed from the KK-Ball Node-System. The advantage is that cladding or glazing can directly be fixed on the top chord without additional purlins. Thus a higher transparency of glazed structures is achieved. The standard bowl nodes have diameter from 160 to 200m. The members of the top grid are rectangular hollow profiles. The members are pin jointed to the nodes. The screws are fixed invisible via the bolt insertion hole. Members and nodes are flush fitting and torsion proof. The nodes and the members of the bottom grid such as the diagonals correspond to the KK-system. Fig 5 The biaxial load transfer of space frames often leads to a very low dead weight. They are therefore specially suited for wide span constructions. Loads from cladding or suspended loads should be applied via nodes. Membranes are directly fixed to the nodes, while sheets are fixed to a purlin system. These purlins are fixed on stools of different height to achieve a sloped roof on a horizontal spaceframe. NK Knoten / node / noeud Fig 6 f •¥ Fig 8 Fig 9 191 Fig 10 The pyramid shaped greenhouses in Essen (fig. 9) with a lateral length up to 30 m are a perfect example for this system. These new glasshouses were designed as a replacement for the former ones. Three fully glazed pyramid structures of different sizes together with a shed hall and other flat connecting buildings form a square around a totally enclosed garden courtyard. The loads from the glazing are applied directly to the rectangular hollow profiles of the top chord. These members and the circular hollow section diagonals are interconnected via bowl nodes. The steel weight of the structure is 15 kg/sqm surface area. Fig 12 Also the largest hemisphere in the world, the Stockholm Globe Arena, was built with the MERO bowl node system (fig. 11). The Arena designed as a multipurpose- hall for international events is in the center of the developing district of Hovet. 5.000 to 16.000 seats can be offered according to the different events The geometrical optimization resulted in a dome with 96 meridians and 19 horizontal rings. Thus a very reasonable wide-meshed net with max. field sizes of 3.6 x 4.4m was produced. For reasons of stability a double layered space frame construction with a max. depth of 2.1 m had to be chosen. For cladding an Alucopan® Sandwichpanel was chosen. Due to the requirements of steam tightness a perimeter support of each panel was needed. Thus the MERO Bowl Node System with rectangular hollow profiles in the top grid was used together with an extra secondary member by which the surface was now divided into max. sized panels of 3.6 x 2.2 m. This size was ideal for manufacturing and transport of the panels. With a construction height of 85 m and a globe diameter of 110 m the steel structure has a self-weight of only 32 kg/sqm surface area. Because of their dimensions such large projects are not only a challenge in view of their huge space but also acoustic aspects have to be considered. However the globe shape itself is rather advantageous. It has the highest volume compared with the surface and provides an optimal distribution of temperature as far as the athletes and/or spectators level is concerned and thus creates a comfortable atmosphere. Fig 11 192 A further development is the MERO-Cylinder Node System (ZK). The cylinder node provides a bend resistant connection. Thus plane and curved single layer constructions can be executed with triangular or quadrangular grid. The quadrangular grid is very economical in respect of the cost for the cladding and/or glazing. Rectangular hollow profiles are connected to the cylinder node by two or four bolts per member (fig. 14). The connection is flush fitting and torsion proof. The standard nodes have diameter of 200 mm, the member sections are from 100x60 - 160x80 mm. The members are screwed invisible. ZK Knaten / node / noeud Fig 13 Fig 14 Fig 16 193 Disc Node System (TK) The disc node is the most specialized connector. This system can only be used for biaxially curved structures with a triangular grid, ie domes with a diameter up to 50 m covered with prefabricated triangular panels or glass The Rectangular hollow profiles are pin jointed with disc nodes. The glazing is placed directly onto the rectangular tube profiles and sealed on all sides. Fig. 19, shows a dome structure with 30 m Diameter, creating a recreation area for the Rh'n-Klinikum. TK Knoten / node / noeud Fig 17 Fig 19 CURTAIN WALL SYSTEMS - POINT SUPPORTED GLASS (PSG) Pursuing the request of architects and owners for more transparency and translucency, glass increasingly becomes the center of MERCTs project. Ongoing research has resulted in the creation of MEROPlus system, developed for modern glass structures, and the advanced MERO PSG which uses point supported glass for maximum transparency in design. At the New Leipzig Fair the multi-functional glass hall as an entrance area, is an expressive example for this development. Fig 21 The glazed barrel vault is 244m long with a span of 80 m. The characteristics of the design are (fig.21): • clear hierarchy glass, point fixing devices, single layer barrel vault structure, trussed arches • welded nodes, axial member connection through hidden bolts, • separation of front walls (fig23) and barrel vault, • single-layer shell consisting of bending resistant square grid, without wind bracings, • stress free, point-fixed suspended glazing, two safety glass panes with a thickness of 8mm each laminated with 1.5mm PVB foil were used for the glazing • surface flush sealing, the glass panes are sealed with extremely elastic silicon strips which are glued to the glass edges. 194 The exhibition halls are connected to another via glazed pedestrian bridges (fig.24). The walkways are covered by a point-fixed glazing of 12 mm curved safety glass. A further example for glazed roofs is the project "Underground Station Canary Wharf, London" designed by Foster &Partners. As part of the work on the Jubilee Line extension in London, three Underground stations in the new office district Canary Wharf had to be roofed in. The structures are covered with glass panels curved around one axis, with dimensions up to 3.1 x 1.1 m, which are point fixed with six rotules. The glass panels consist of laminated safety glass. The general layout of the roof is elliptical in shape. This made the geometry of the connecting details a particularly tricky and daunting task Fig 25 To cover the foyer area and the winter garden of the Musee des Beaux Arts in Montreal the Bowl Node system is combined with stainless steel rods and cables (fig.26+27). The top chord consists of rectangular hollow sections with bending resistant connections to bowl nodes. Rafters divide the grid into dimensions of approx. 1.5 x 3.0 m. Diagonal and bottom chord members are of high tensile stainless steel rods 16-20mm, directly screwed into the nodes. Struts of circular hollow section 88.9 mm connect the bowl node with the bottom chord ball node. Fig 22 Fig 24 195 Fig 27 Stainless steel cables act as tie down cables against wind uplift forces. The double glazing is fixed on the top chord as "Structural glazing". TENSILE STRUCTURES In the development of pure tensile structures, pre- stressed structural elements subject to only tensile loads can be used as a plane surface with great deformation and as an extremely stiff structure with counteracting tension members. An example of a pure tensile structure with point supported glass is seen at Tampines Plaza in Singapore (fig28). The challenge was to create a high-tech glass enclosure supported by stainless steel cables and stainless steel fittings. Fig 29 196 Fig 30 The innovative design is based on a tennis raquet, with glass panels supported by pre-stressed cables. The glass is connected to the cables by double hinged connectors which allows movement and rotation in all directions, without breakage to the glass or ruptures to the sealant. An example for a counteracting biaxial curved tensile structure is the cable net structure for the Rhon Klinikum medical facility covering two promenades connecting different buildings. This allows the visitors and patients to move and communicate protected from weather influences. The cladding of the cable net consists of glass shingles connected to the cable net with special clamps (fig.29+30). COMBINATION SPACE FRAME, MEMBRANE AND CABLE-NET The tensile roof to cover the existing Open Air Theatre of the Bilkent University in Ankara is an example of the combination of MERO space frames, membranes and cable net. Fig 32 The roof is designed to cover a ground plan of approx. 4000 m 2 . The surface of the cable net rain screen is about 400 m 2 . The primary steel structure, the MERO space frame, consists of the main trussed arch with a span of about 118 metres and 6 smaller curved trussed girders with variable spans from 34 metres up to 46 meters which are connected to the main arch (fig.32). The trussed girders are rigidly connected with the main arch and pin jointed at the foundation; the main arch is pin jointed at the base points. The membrane roof consists of 7 individual membrane panels spanning between the curved girder trusses. The surface of the membrane is anticlastically curved to take up wind and snow loads. 197 Fig 34 The cable net facade is tensioned between the main arch and the existing building with nearly no curvature. The cable net is covered with glass shingles to protect the stage area from rain but to allow a view as transparent as possible (fig.33). The shapes of the membranes are designed by means of a form-finding process, considering structural and aesthetical aspects, as well as the boundary conditions. The curvature of the membranes is anticlastic (fig 35) with the main load carrying directions following the lines of principal curvature. In this case the wind suction loads are carried by the hogging traverses (fig 35 A-A) and wind pressure and/or snow loads in the sagging direction (fig 35-B-B). All membranes are pretensioned with tension rods to the Mero nodes of the upper chords of the trussed girders (fig.36). The design of the space frame components, tubular members and spherical nodes, was performed by means of a specialized design program, based on the general approval for the MERO space frame system. This program is covers all design steps - beginning with the geometry of the structural model, the analysis and the evaluation of section sizes and diameters of the spherical nodes - and continuing with the evaluation of parts lists and drawings. Fig 37 198 The key task for the design of space frames in general and especially for the wide span front arch of this structure, is the appropriate choice of the module size and structural height. These design parameters are critical especially for tube sizes for compression members and the angles of the interconnecting members at the nodes which are mainly determining the node diameters and consequently the economical efficiency of the structure. After a number of tests, it was decided to use a section of 4,5 m height and 3,5 m width at the apex, decreasing towards the base support points to 3,0m height and 2,3 m width, resulting in max. member sizes of 300mm diameter with up to 90mm diameter high strength bolts and max. node diameters of 350mm (fig.37) A further challenge in terms of membrane structures is the Eden Project near St. Austell in Cornwall, designed by Nicholas Grimshaw & Partners. The buildings will contain different climatic conditions to support a diverse range of plant life. The buildings consist of a number of hemispherical domes which are set against the quarry walls. The diameters of the domes vary from 40 m to 120m and are constructed from two layer reticulated steel members. The domes will be covered with inflated three layer ET -foil-cushions which are hexagonal in plan and the edge lengths vary from 2.1m to 5.3 m. The cushions themselves are held with aluminium frames. CONCLUSION Using the advance of technology in engineering and the development of innovative materials it is possible to create new sophisticated high tech quality solutions. The integration of cladding materials such as aluminium and/or titanium sheet metal, glass and membranes covers a very wide spectrum. So it becomes more and more important to consider the structure and the cladding as one package.