SPACE FRAME A space frame is a structure system assembled of linear elements so arranged that forces are transferred in a three-dimensional manner. In some cases, the constituent element may be two dimensional. Deflection of a system of individual trusses Deflection of a system of individual trusses Deflection of a two-way spanning double-layer grid of intersecting trusses demonstrating the load distribution advantage of the latter Load sharing • THE advantage of space grid structures is that generally all elements contribute to the load carrying capacity. Planar beams or trusses must be capable, individually, of carrying any possible concentrated or heavy moving loads (e.g. overhead cranes). However, in space grids such concentrated loads are distributed more evenly throughout the structure and to all the supports. This can also reduce the cost of the supporting structures as maximum column and foundation loads may be less. Maximum deflections are reduced compared to plane structures of equivalent span, depth and applied loading, assuming that the structural elements are of similar size. Alternatively, a lighter or shallower threedimensional structure may be used to carry the same loads, resulting in maximum deflections no greater than those of a planar structure. Advantages of using space grids • Load sharing • Installation of services In space grids, the reasonably open nature of the structure between the two plane grids allows easy installation of mechanical and electrical services and air-handling ducts within the structural depth. • Robustness Space grids are highly redundant structures, which means that, in general, failure of one or a limited number of elements – for instance, the buckling of a compression member under excessive loading – does not necessarily lead to overall collapse of the structure. • Modular components Space grids are highly modular structures assembled from components that are almost exclusively factory fabricated. The components therefore, are usually produced with high dimensional accuracy, with a high quality of surface finish and they are generally easily transportable, requiring little further work except assembly on site. Because of their modular nature, space grids may be xtended without difficulty and even taken down and reassembled elsewhere. Advantages of using space grids • Freedom of choice of support locations Great choice in the location of supports is offered by space grid structures. Within reason, space grids can be supported at any node of the grid and at practically any location in plan. • Regular geometry For ease of construction, most space grids have a regular pattern that may be exploited architecturally to some effect. Particularly striking effects can be achieved if the colour chosen for the structural members contrasts with the colour of the decking, or with the sky in the case of unclad grids or fully glazed applications. • Ease of erection A further advantage of the use of space grids is the efficiency of erection for large-span roof structures and especially on sites with limited access. In the former case, the whole roof can be assembled safely at or near ground level, complete with decking and services, and then jacked into its final position. In the latter case, the small components of a space grid can be assembled at almost any location using only manual labour and simple lightweight tools, even inside existing buildings. Disadvantages Cost cost, which can sometimes be high when compared with alternative structural systems such as portal frames. This disparity in cost is particularly evident when space grids are used for relatively short spans, although the definition of a short span is very dependent on the system under consideration. However, spans of less than 20 to 30 m can probably be considered short Regular geometry Although the regular geometry of space grids is generally cited as one of their appealing features, to some eyes they can appear very ‘busy’. Erection time Again, this is a topic that also appears under the list of advantages. However, another common criticism of space grids is that the number and complexity of joints can lead to longer erection times on site. Fire protection Space grids are mainly used in roof construction where, depending on the materials involved, nominal or no fire resistance is normally required. However, when they are used to support floors, some form of protection is usually demanded to provide the necessary fire resistance, if they are exposed. This protection is difficult to achieve economically due to the high number and relatively large surface area of the space grid elements, but intumescent coatings can be applied. Space truss stability • The stability of rigid jointed space frames depends on the bending resistance of the joints for its structural integrity. However, space truss structures depend on their geometrical configuration to ensure stability. A fully triangulated structure must be formed. • In a three-dimensional pin-jointed space truss structure, it is a necessary condition for stability, (variously known as Maxwell’s Equation or Foppl’s Principle), that, TYPES OF SPACE FRAME 1) Two and three-way grids • Characterized as two way or three way 2) • • • Single, Double and Triple Layered Single layer frame has to be singly or doubly curved. Commonly used space frames are double layered and flat. Triple layered is practically used for a large span building. Double layer grid type Two-way and three way lattice grid Two-way space grid with square and three way space grid with triangular pyramid Two-way space grid with square and three way space grid with tetrahedral unit SECTIONS (a) Uniform curved camber formed by slightly shortening all of the lower chord members in one direction, (b) ridge camber formed by shortening one lower chord member in one direction under the ridge, (c) stepped curved camber formed by slightly shortening the lower chord members in one direction at regular bay intervals and (d) freeform curve generated by suitable manipulation of top and bottom chord lengths in one direction SUPPORT LOCATIONS SUPPORT LOCATIONS EDGE PROFILES Top-chord supported Four-point cruciform Interior wall Bottom-chord supported Frame capital Exterior wall Materials and systems • STEEL • ALUMINUM • CONCRETE • REINFORCED PLASTIC • BAMBOO (EXPERIMENTAL PHASE) • GLASS (SCULPTURE) STEEL • HIGH YIELD MILD STEEL ELEMENT , COLD-FORMED FROM STEEL STRIP • CASTINGS FROM SPHEROIDAL GRAPHITE IRON KNOWN AS DUPLEX SYSTEM IF BOTH THE MATERIALS ARE USED THESE MEMBERS ARE GALVANISED OR PAINTED Space frames may be constructed of structural steel pipe, tubing, channels, tees, or Wshapes. Welded connection Bolted connection Threaded connections • Fabricated connectors join the members. • Consult manufacturer for details, module size, and allowable spans. TYPES OF STEEL SPACE GRID SYSTEMS • ASSEMBLED USING ‘PIECE-SMALL’ SYSTEMS • ASSEMBLED USING CONTINUOUS CHORDS • ASSEMBLED USING FACTORY PREFABRICATED MODULES ‘PIECE-SMALL’ SYSTEMS • The ‘piece-small’ systems primarily differ in the jointing method. Most use circular or square hollow tube members because of their better performance in resisting the forces present in space trusses. 1 Spherical nodes: (a) solid (b) hollow. 2 Cylindrical. 3 Prismatic. 4 Plates. 5 ‘Nodeless’. Spherical nodes • Solid cast steel spheres are drilled with threaded holes at the appropriate angle for connection of the adjacent members and are machined to provide appropriate bearing surfaces. The attachment of each member is usually achieved with a single bolt on its central axis. In some systems the ends of the members come into direct contact with the nodes, whilst in others the axial forces are transmitted solely through the connecting bolts. Mero KK space truss system, FIRST COMMERCIALLY AVAILABLE PRODUCT FOR SPACE TRUSS The original ‘ball’ joint, now known as the KK system, is a hot-forged solid steel sphere finished with 18 threaded holes and machined bearing surfaces at angles of 45°, 60° and 90° relative to each other. With modern computer numerically controlled precision drilling techniques, holes may now be drilled at almost any required angle rather than only at the standard angles, although there is a minimum angle of 35° specified between adjacent holes. SEO SPACE FRAME SYSTEM This connection system allows any bar to be removed easily from a completed grid at any time. By unscrewing the sleeves at both ends of a member, the bolts retract inside the tube sufficiently to enable its removal and replacement. Hollow spherical nodes Nodus Connector • It can accept both rectangular and circular hollow sections and that the cladding can be fixed directly to the chords. • Chord connectors have to be welded to the ends of the hollow members on site. Triodetic Connector • It consists of a hub, usually an aluminium extrusion, that has slots or key ways, which the ends of members are pressed or coined to match the slots. Moduspan (Unistrut) node showing the coldformed members bolted to the node plate ‘Nodeless’ joints Because the special separate node components usually represent a considerable proportion of the cost of a 40 Space Grid Structures space grid, some systems eliminate these completely, relying instead on direct connection between the ends of the grid members. Although this saves in overall cost, it tends to limit the possible configurations of the grid as the end connections of members are often designed to accommodate standard angles between the parts. CONTINUOUS CHORDS A typical Harley Type 80 node joint OTHER SYSTEM INCLUDES Mai Sky System Catrus MODULAR SPACE TRUSS The system is based on pyramidal units that are assembled from a square frame of steel angles connected by circular steel tube bracing members to a central cast steel boss. All elements of the pyramids are welded together in a fabrication jig to ensure consistent dimensional accuracy.
