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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
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
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.

Beams on columns
Beams on columns

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.

Typical beam layouts for primary and secondary spans
Typical beam layouts for primary and secondary spans

 

Services below shallow secondary span
Services below shallow secondary span

 

Planning grids for organising the layout of spaces should correspond to a suitable structural grid system for compatibility, for example fit of materials, fixed doors and windows.

A planning grid may typically be based on 100 or 300 mm module, while the main structural grid should be based on some multiple of the former, eg.. 4.8m or 6.0m. The spacing of the primary structure will affect the size of the secondary structure.

Where multibay plan forms are used the most obvious solution is a repetition of the form.

Columns which become internal do not need to take lateral forces, but will be taking a larger load from two roofs. Internal valleys of multibay pitched roofs are often troublesome due to build-up of rubbish leading to blocking of rain water pipes. Careful detailing and drainage is required in these situations. The roof disposition can be useful in admitting and controlling natural light

For square plans the choice is less obvious, with choice of orientation of the primary structure being dictated by other considerations, eg. service runs or appearance. Alternatively, diagonal forms can be used, particularly if rising forms such as a pyramid are used, although these will lead to more complex connections, and varying member lengths.

If a sloping roof is required for a square plan, then a portal system is a possibility.

Section through repeating form
Section through repeating form

 

Diagram of rectangular and diagonal structural layout.
Diagram of rectangular and diagonal structural layout

 

Sloped portal frame for square roof
Sloped portal frame for square roof

 

The other choice for square forms is to use space frames.

A sensible solution for a square plan (and up to a ratio of sides of about 1:1.3), might be to use a 2-way spanning "truss" system or "space frame". The principle of a space frame is based on the manufacture of standard small size elements and node joint connections that can be easily transported and erected rapidly. These systems make an interesting internal "spider web" pattern. The standard systems produced make for deliberate grid planning sizes which must be maintained. Internal wall partitions may need to be stabilised at the top against wind loads and their planning must fit the space frame grid. In economic terms, the joints are expensive to produce and resist less force than a fully welded fabricated joint, leading to deeper overall trusses than linear beam/truss systems. Space frames with one layer set on the diagonal have been found to be between 6 - 17% cheaper than square on square. Space frames should rarely be considered if a false ceiling is to be used below, thus hiding the latticework visually.

Square plans can also form the basis of bi-directional multibay forms.

Whilst these can be achieved by repeating square plan forms, careful consideration is required of the joints between bays and between elements, and ways of forming the enclosure. This form will usually require a substantial number of bays to be economic. However, solutions to these problems have proved to be very successful

Circular forms may also have primary and secondary spans, or radial forms in which case edge and ring beams will also be required.

For circular forms, the edge beams can be pre-bent, but must be torsionally restrained. The length should be kept below 6m. The central joint can be made with top and bottom circular cap plates bolted to the main beams. An alternative, which is less elegant when viewed from within, but easier structurally, is to make the edge beams straight and "fudge" the curves with additional small bent angles or channels. If a circular plan with a sloping roof is required the portal approach can be adopted with a direct central joint or a compressive crown ring.

Diagram of circular frame forms
Diagram of circular frame forms

For unsymmetrical shape plans try to arrange the largest area of regular, parallel and almost equal length elements, then support the remainder in the best possible way.

A flat space frame roof may be possible subject to the rigorous planning geometry inspired by this type of structure. An alternative approach is to adopt a regular planning grid for the structure and to cantilever beams to form the asymmetric edge.

Space frame over irregular grid or with cantilevered edge
Space frame over irregular grid or with cantilevered edge

The placement of the structure relative to the building skin is an important consideration. It may be internal, within the thickness of the wall or, with careful detailing, external to the skin.

The structure system for a building is usually arranged within or close to the exterior environmental envelope to reduce intrusion into the usable plan area. The most straightforward arrangement is to have the structure internally, with the water proof and insulation layer externally. Structure may be place within the thickness of the skin, but consideration is required here of the modularity of the infill and the risk of cold bridging. Structure placed externally may allow a clean interior volume and allow external expression of structure, but the detailing required to prevent water penetration and cold bridging required very serious consideration

   

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