General construction details

A variety of issues need to be considered in detailing space grid structures.

The main considerations for the detailed design of bars, members, modules, nodes and joints are material properties, element structural behaviour and dimensional accuracy. Other issues which must be considered include support details, cladding systems, site construction.

The majority of space frame systems for building structures are manufactured from steel or aluminium although timber, concrete and reinforced plastics are also used.

Although the weight of a typical aluminium alloy is only one third that of steel for an equivalent volume, it also has a lower modulus of elasticity. Thus, for an aluminium alloy of equivalent strength to steel the resulting aluminium alloy structure may be lighter unless deflections are critical. In this case, additional material may be required to keep deflections within acceptable limits. As the material cost for aluminium alloy is greater than that of steel the choice of material will depend very much on individual circumstances. Greater care is required to weld aluminium than steel and as many space grid systems involve at least some welding in their manufacture steel, in various strength grades, is the most commonly used material for the members. Many systems use cast steel for end connectors and node joints.

In three-dimensional structures, and long-span structures in particular, dimensional accuracy is of paramount importance as small variations in element dimensions may accumulate to produce gross errors in the dimensions of the final structure.

This property is exploited to produce pre-camber of space grids by controlled variation of element dimensions. Components are cut to length with tolerances of less than 1mm and the members of the Nodus and Mero systems, for instance, have their cast end connectors welded to the tubes in accurately dimensioned jigs. The modules for the Space Deck, CUBIC Space Frame and other modular systems are also welded up from accurately cut components in precisely dimensioned jigs. This ensures overall dimensional accuracy for the modules, in this case, in three dimensions.

The main structural considerations in the design of space truss elements are the buckling of compression chords and web bracing members and the design of joints to effectively and efficiently transmit tension forces between the bars and nodes whilst minimising secondary bending effects.

In space truss structures the bars or members, therefore, tend to utilise square or circular hollow steel sections (RHS or CHS) because of their superior behaviour under axial compression and their aesthetically pleasing appearance. The latter is especially important as space grids are generally left exposed to view so that the grid pattern can be appreciated.

The nodes may be of many forms depending on the system used (e.g. 'ball' joints, hollow spheres, profiled plates etc.) and this tremendous variety of jointing systems demonstrates the difficulty of achieving a simple, aesthetically pleasing tension joint.

Modular systems may use angle, channel, Universal Beam, and Universal Column sections for the bars or members as these are often easier to connect than hollow sections. Space frames (with rigid node joints and no diagonal bracing elements) have to be designed for the bending moments induced by the frame action.

It is difficult to generalise about span/depth ratios for space grid structures as they depend on the method of support, type of loading and to a large extent on the system being considered. Z.S. Makowski suggests that span/depth ratios may vary from 20 to 40 depending on the rigidity of the system used.

High span/depth ratios are appropriate if there are full edge supports but they should be reduced to about 15 to 20 when the supports are only at or near the corners of the grid. Span tables produced by Space Deck indicate that for typical UK loadings roof span/depth ratios of about 30 are possible using their standard modules. For example, a Space Deck roof supported on all edges, with a total imposed, decking and services load of 1.30 kN/m² in addition to its own weight will span up to 39 x 39 m with a module only 1200 mm deep.

As long span structures, space grids require a suitable set of fixed and sliding bearings to enable them to resist lateral loads whilst allowing thermal expansion / contraction to take place.

Space grids form quite rigid plates so it is very important that any potential movements are accommodated in the support details. A major source of movement in metal structures is change of ambient temperature, especially when long spans are involved. It is necessary to install the space grid on a combination of fixed and sliding bearings to permit free thermal movement whilst restraining the lateral movement caused by wind forces and transmitting these forces to the supports. To hold the structure against lateral loads in any direction a minimum of three lateral restraints are required. The position of the lateral restraints will depend on the distribution and rigidity of the supporting structure. A bearing allowing movement in one direction whilst restraining movement in the orthogonal direction is shown here.

Alternatively, the space grid may be rigidly fixed to some or all of its supports. In this case, both the space grid and the sub-structure must be designed to cater for the forces generated by temperature change. This alternative was used for the Boeing 747 maintenance hangar at Stansted Airport were the CUBIC Space Frame was fixed in position at the top of the four main corner columns. Thermal expansion and contraction was considered to occur relative to a notional fixed point at the centre of the roof structure with the tops of the columns being forced to move out or in as dictated by the change in roof dimensions.

As most space grid applications are for roofs it is necessary to provide adequate falls for rainwater run-off and an additional pre-camber may be applied to counteract the expected vertical deflection under load.

In the majority of systems it is possible to achieve this by fractionally shortening the lower chords in one or both directions to form, a shallow barrel vault (or arch), a stepped arch or a ridge as shown here or a double curved surface. It must be realised that very small variations in length may cause large differences in geometry of the space frame thus dimensional accuracy is essential in the manufacture of all systems.

There are several alternative methods of erection for space grids and more than one may be used in the construction of a single grid. To some extent the method chosen will depend on the system being used but overall grid size, site access, and component size will also be determining factors.

The most commonly used methods are:

• Assembly of the individual space grid elements or modules on a temporary scaffold support - this is normally only used when no other method is possible as the scaffolding is expensive; however, it may be required in large grids to establish a structurally stable area for later connection of pre-assembled sections.
• Connection in the air, in which space grid elements or modules are lifted individually by crane for connection to areas of the grid that have already been installed - this is more appropriate for heavy modular systems when the site cannot be obstructed by assembly of the grid at ground level and was primarily used for the erection of the CUBIC Space Frame hangar roof at Stansted Airport.
• Assembly of grid elements or modules into panels before lifting by crane and connection in the air - this is a good compromise where it would be difficult to lift the whole space grid as one piece or where it is not possible to assemble the whole grid on the ground, due to lack of space.
• Assembly of the whole grid on the ground before lifting onto the permanent supports by crane in one lift,
• Assembly of a part or the whole grid on the ground before by jacking or winching into position over temporary or permanent supports.

Of these methods the last two are preferred, especially if the space grid components can be manhandled, as crane use is expensive and should be avoided where possible. The complete 260 x 260 m, 650 ton, double-layer, space grid of the Exhibition Centre, Anhembi Park, Sao Paulo, Brazil was lifted 14 m, using the last method, over 25 temporary supports, in one operation lasting 27 hours.

An important advantage may be gained from assembling the grid at or slightly above ground level prior to lifting it to its final position. The installation of services and/or cladding is facilitated as this can be carried out from the ground, eliminating the need for temporary access scaffolding.

The roof of the new Sant Jordi Sports Palace for the 1992 Olympics in Barcelona (architect Arata Isozaki) was erected using an innovative 'Pantadome' method proposed by engineer Mamoru Kawaguchi. The centre space grid of the domed roof was assembled on the floor of the arena and 16 separate curved space grids were connected to the sides of this frame and to the perimeter columns at the top of the raking seating. After the centre space grid had been jacked to its final elevation the gaps between the individual grids were filled with additional bars to complete the domed roof.