General requirements
More detailed issues can be considered in plan, section, and 3 dimensional form.
In assessing the general design requirements of single storey buildings there are three main areas that will require more detailed examination - plan forms, sections and 3-dimensional systems.
Plan shape: Early planning decisions will result in the general plan form. Although structure is most easily integrated into rectilinear forms, steel can accommodate a wide range of plan shapes.
In simple buildings plan shape may be dictated by the likely structural solutions, which will tend toward rectilinear forms with columns located at the perimeter. Generally speaking, straightforward arrangements will be the easiest and most cost-effective. However, steel frames can accommodate a wide range of plan forms.
Simple plan diagram for single storey frame
An important consideration is whether there is a requirement for internal columns, which will simplify the structure and lower costs, but lead to more congested spaces.
This will depend on the nature of the building and its functional requirements. With wide clear spans, the structural costs are higher than where some internal columns are used. The internal columns can be relatively slender as they only resist compression. With large span structures, internal columns may have a different spacing from perimeter columns, where a major function of the latter is to stabilise and support the cladding. On plan, internal columns can lead to a very congested internal space unless the secondary spans are increased to about 9m.
Sectional diagram of multi-bay structure
For rectilinear plans the common arrangement is for the primary structure to be located at the plan edges and to span the shortest distance.
For small buildings, or planar wall structures, these may be small elements closely spaced (e.g. timber joists at 600mm c/c). In a steel frame it is more normal to have primary structure such as columns and beams at larger centres (3-7m) with secondary structural member such as purlins spanning between these.
If the same depth of structure zone is sought for main and secondary beam spans, then the main span should be shorter as it supports more load than the uniformly loaded secondary beams.
Furthermore, the loading to the main beams is in the form of point loads, which in general produce, greater bending effects than uniformly distributed loads. Although it is common to make the primary structure span in the shortest direction, this does not have to be the case. A longer primary span will give deeper main beams and smaller secondary beams, which may be desirable for appearance or service integration. Occasionally, the main structure is deliberately placed in the long span direction for planning or spatial reasons. For larger spans/enclosures a triple form may be effective – a large primary spanning element such as a truss, shorter secondary beams or trusses and tertiary rafters.
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