The advantages and disadvantages of using space grids

There are many benefits to be gained from the use of space grid structures, including structural efficiency, reduced deflections, integration of services, resilience, ease of construction, and regularity.

The principal advantages of space grids are as follows:

• All elements of the space grid contribute to the load carrying capacity. This is particularly useful when point loads are applied to the structure. Whereas when planar beams or trusses are employed they must be, individually, capable of carrying any possible point load.
• Loads are distributed more evenly to the supports. This can reduce the cost of the supporting structures especially when heavy moving loads may be applied to the space grid (e.g. overhead cranes).
• 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 three-dimensional structure may be used resulting in deflections no greater than those of the planar structure.
• The 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. Their fixing is simplified as there is a regular grid of supports available thus reducing or eliminating the need for secondary steelwork.
• The statical redundancy of space grids means that, in general, failure of one or a limited number of elements, for instance, the buckling of a compression member, does not lead to overall collapse of the structure. There have been exceptions to this, notably, the collapse of the Hartford Civic Centre, Coliseum, space truss roof in January 1978. It depends which members fail and whether an adequate alternative load path exists.
• Space grid structures are resistant to damage caused by fire, explosion or seismic activity. Unless critical elements (e.g. those adjacent to individual column supports) are removed or weakened by explosion or fire collapse is unlikely for the same reason as above.
• Modular space grids are usually factory fabricated (thus producing accurate components) easily transportable and simple to assemble on site. Because of their modular nature they may be extended without difficulty and even taken down and re-erected elsewhere.
• Within reason supports can be located almost anywhere in plan. This give the architect considerable freedom in space planning although approximately square structural bays are preferable. Support location is discussed in a separate section.
• For ease of construction, most space grids have a regular grid pattern which may be exploited architecturally to great effect. If the colour chosen for the structure contrasts with the colour of the cladding, or against the sky in fully glazed applications, particularly striking effects can be achieved. In fact the colour chosen for the grid as well as the grid pattern can influence the perceived weight of the structure even more than the actual member sizes.

The disadvantages to using space grids are associated with cost, complexity, problems of fire protection, and standardised layout.

There are few disadvantages to using space grids but some are described below:

• The main criticism of space grids is their cost, which can be high when compared with alternative structural systems. This is particularly true when space grids are used for short spans. The definition of a short span is very dependent on the system under consideration but less than 20 to 30m can probably be considered short. Often a direct comparison is not made like with like. For instance, in a portal framed structure additional purlins will be required to support roof decking and secondary steelwork may be necessary to carry services and equipment, neither of which may be needed if a space grid were used instead.
• Visually, space grid structures are very 'busy'. They are rarely seen in plan or in true elevation and at some viewing angles the lightweight structure can appear to be very dense. Grid size and depth as well as the grid configuration can have considerable influence on the perceived density of the structure.
• The number and complexity of joints can lead to longer erection times on site. This is obviously very dependent on the system being used and the grid module chosen.
• When space grids are used to support floors some form of fire protection may be required. This is difficult to achieve economically due to the high number and relatively large surface area of the space grid elements.
• The standardised modular nature of most space grids can impose a geometric discipline of their own. This sometimes makes difficult the use of irregular plan shapes and imposes control on the location of supports.