Brief history and development of systems

Alexander Graham Bell may be considered the first to construct modular space grids.

It is not well known that the inventor of the telephone, Alexander Graham Bell (1847 -1922), experimented with space truss structures made of octahedral and tetrahedral units early in this century. He appreciated the dual properties of high strength and lightweight exhibited by these rigid structural forms and incorporated them into several of his projects.

The MERO system was the first commercial space grid system.

Despite Bell's construction of modular space trusses early this century, the MERO system was the first space grid system widely available commercially. This was developed in Germany by Dr. Ing. Max Mengeringhausen (1903 - 1988) but was not introduced until the early 1940s.

The MERO system is probably the most elegant concept for the construction of space grid structures, the connection of circular tube members to 'ball' joints at the nodes by a single concealed bolt. It developed from Mengeringhausen's study of natural structures such as wheat stalks and bamboo canes. The name MERO derives from an abbreviation of the original name Mengeringhausen Rohrbauweise.

Usually the members are circular hollow steel or aluminium tubes that have tapered cone sections welded to each end (complete with connection bolt and sleeve) and the nodes are hot forged solid steel or drop forged aluminium spheres with drilled and tapped holes and profiled to receive the tube ends. It is also possible to use laminated timber members that have short tubular steel inserts at the ends for connection to the nodes.

Originally the MERO system had only one type of standard joint, a sphere with 18 threaded holes and machined bearing surfaces at angles of 45, 60 and 90° to each other. A series of standard bar lengths were also produced such that the node to node lengths were increased by a factor of Ö 2 with each increment in the series. However, with modern numerically controlled precision drilling techniques, holes may now be at almost any required angle and members are manufactured to the appropriate length thus allowing great flexibility in geometry.

There are, of course, many examples of MERO space frames worldwide as one would expect for a system nearly 60 years old and there are many alternative node joints and member cross sections. However, one of the most dramatic uses of the MERO system was for the grandstand roofs at the stadium in Split, Yugoslavia. These roofs are segments of a 452 m diameter cylinder inclined at 11.2° and the free edge spans 215 m with an arc length of 220 m. An impressive use of space grids and glazing can be seen in the atrium wall and roof at the Presidential Circle Office Building in Hollywood, Florida.

The MERO system uses the most common method of space truss construction consisting of individual bars connected at 'ball' shaped node joints. The popularity of this system has endured to the present day and there are now many alternatives based on the MERO ball joint concept, for instance, the 'TM Truss' produced by Taiyo Kogyo Co. in Japan, and 'Abba Space' produced by ABBA Space Structures in South Africa. Because of its elegance, the MERO system it is not only used for building structures but also for shop displays and exhibition stands using light weight materials.

In the UK in the 1950s Denings of Chard developed the Space Deck modular system and with the former Ministry of Public Building and Works also developed the similar (now defunct) Nenk system.

Both systems were based on prefabricated steel pyramidal modules (1.22 m x 1.22 m in plan and 1.05 m or 0.61 m deep respectively). The Nenk system was used for roof and floor construction in barrack blocks where it could span 40 feet (12.2 m) with normal floor loads and 88 feet (26.8 m) with normal roof load. Space Deck has been widely used for roof and floor structures with only slight modification to module dimensions and materials ever since.

The Space Deck system consists of pyramidal units constructed from a square frame of steel angles connected by circular steel tube bracing members to a cast steel boss. All elements of the pyramids are welded together in a jig to ensure dimensional accuracy.

The boss at the base of the pyramids has two threaded holes in one horizontal direction and two threaded studs in the other to receive connecting steel tie bars. Modules are produced to standard plan dimensions of 1200 x 1200 mm, either 750 or 1200 mm deep, and 1500 x 1500 mm with depths of 1200 and 1500 mm. Different strength modules are available within the same overall dimensions so that the high shear forces around column supports can be accommodated. In addition to these standard modules, edge frames are available to suit alternative edge profiles and special pyramid modules are made to order.

Assembly is achieved by bolting the angle frames of the pyramids together and then connecting the cast steel bosses with tie bars. The tie bars have left-handed threads at one end and right-handed threads at the other as do the holes and studs on each side of the cast bosses. Rotation of the tie bar, therefore, screws the bar into the boss at each end simultaneously. By varying the length of the tie bars the node spacing can be altered to produce a one- or two-way camber in the space frame.

There are many examples of Space Deck structures as it has been available for almost 50 years in essentially the same form with only minor changes in materials and metrication of module dimensions. Some examples of Space Deck projects: a pitch-roofed atrium at the Hyatt Hotel, Birmingham; a barrel vault at the Bentall Centre, Kingston-upon-Thames and the Pearl Hotel, Libya.

Elsewhere in the world, during the 1950s and 60s, space frame systems were proliferating as architects explored the new aesthetic and engineers experimented with alternative jointing systems, materials and configurations.

For instance, in the USA, Richard Buckminster Fuller (1895 - 1981) developed the Octetruss system using tubular steel members and a bolted node joint and Konrad Wachsmann (1901 - 1980) developed a space grid system for large span aircraft hangars for the U.S. Air Force.

Wachsmann's system incorporated a relatively complicated universal connector made from a combination of four standard die forged elements which allowed up to 20 tubular members to be connected at each joint. Stephane du Chateau, in France, developed Tridirectionelle S.D.C., Tridimatic and Pyramitec forerunner of the Unibat system. In Canada, the Triodetic system, predominantly using aluminium as the material for the bars and joints, was introduced by Fentiman Bros. of Ottawa.

Notable examples of long span space grids constructed in 1970 and 1973 were the British Airways maintenance hangars (formerly owned by BOAC) at Heathrow Airport, designed by Z.S. Makowski and Associates.

The hangar roofs are diagonal, double-layer grids 3.66 m deep and provide a column-free covered area 67 x 138 m in plan. In this case, the space grids were not constructed from a proprietary system but were manufactured from tubular steel prefabricated elements joined on site with bolted grid connectors.

Advances in analytical capabilities led to further developments of new systems and the use of space grids for longer spans.

Around the same time, the introduction of electronic computers and the development of programs to enable space grid structures to be analysed more accurately increased confidence in their use for longer span structures. In the late 1960s and early 1970s many of the pioneering space grid systems were superseded by second-generation systems such as Nodus, Orona, TM Truss etc.

The Nodus system was developed during the late 1960s by the Tubes Division of the British Steel Corporation (now Corus) and was introduced in the early 1970s. After 1985 the system was owned by Space Decks Ltd. of Chard but is no longer available.

The Nodus system was developed with a sophisticated node joint in a range of different sizes and load capacities appropriate for their tubular section products. All of the joints were tested to failure in a special rig, at their Research Centre in Corby, to prove their effectiveness and a full size 30.5 x 30.5 x 1.52 m deep space grid was also tested. This structure was dismantled after testing and re-erected as the Space Structures Research Laboratory at the University of Surrey.

The Nodus joint uses cast steel connectors which are butt welded to the chord or bracing members in jigs to ensure dimensional accuracy. Chord members are clamped together between the halves of the node castings using a high strength friction grip bolt and the bracing members are joined to lugs on the node plate by steel pins through the forked end connectors (see exploded view of the joint).

The joint configuration with pinned connection of the bracing members permits variation of the space grid depth, limited only by the requirements of structural efficiency or interference between members at the joint. However, the standard joints are only produced in two forms with lugs for the connection of bracing members either in line with the chord members or alternatively at 45° to the chords when viewed in plan. This limits the possible grid configurations to variations of the square on square or square on diagonal layouts.

Some examples of the use of Nodus are for a canopy at the Hyatt Hotel, Birmingham, the refurbishment of Cwmbran Town Centre and in the construction of Terminal 2 at Manchester Airport.

The CUBIC space frame system is a 'true' space frame that was developed during the late 1970s by Leszek Kubik and his son Leslie and is now marketed by ASW-CUBIC Structures a subsidiary of ASW Construction Systems. It is formed from rigid-jointed modules with no diagonal web members.

As this is a modular system containing no triangulation the chords and vertical members are subjected to bending and shear, in addition to axial forces, as they resist the applied loads by frame action. If an orthogonal grid is formed from Vierendeel girders in both directions and joints are introduced in the top and bottom chords midway between each chord intersection, the grid can be constructed of modules 'X', 'T' or 'L' shaped in plan. These are the basic modules of the CUBIC Space Frame system. No special components are required in fabricating the modules as they are made from standard hot-rolled steel sections and plates welded together in a jig using standard fabrication techniques. Final assembly of the space frame is by site bolting of the lap joints in the chords.

The CUBIC Space Frame system was first used for re-roofing a rehearsal theatre 12 m x 20 m at Trent Polytechnic (now Nottingham Trent University) in 1979. It has since been used successfully to roof many building types including factory units and supermarkets where the absence of bracing members has allowed installation of plant, services and even offices within the depth of the space frame.

Recently, Hall 3 at the International Convention Centre in Birmingham, approximately octagonal in plan and around 55 m in span, was covered by a CUBIC space frame capable of carrying a point load of 30 tonnes at any of the nodes.

A modification of the system has been developed for composite floor construction in office buildings to exploit the facility for service provision within the structural depth.

The 1980s saw the development of lightweight space truss systems using cold-rolled sections such as the Conder Harley System 80 and Space Deck "Multiframe".

The Harley system was introduced into Europe by Conder Group plc in 1989. It was manufactured under licence from the patent holders in Australia where it has been available since 1980. This system, which is no longer available in the UK, was used for re-roofing 8500 m² at the Eagle Centre Market in Derby in 1991. Space Decks Ltd. have also developed a lightweight space truss system called Multiframe which has only been used for a limited number of small projects despite its competitive price.

Generally lightweight space trusses are manufactured from pre-galvanised steel coil up to 8.0 mm in thickness which is cold-formed, using a continuous rolling process, to produce light weight channel sections and thin walled tubes.

The chords of the space trusses are continuous over several bays (up to about 12 m long) and do not intersect in the same plane. This makes construction easier but means that bending moments are introduced into the chords due to the slight eccentricity of forces at the nodes caused by the channels in the two orthogonal directions being connected back-to back and not in the same plane (see photograph of Harley system and diagram of Multiframe node).