Steel Roof Construction - part 2

Typical beam layouts for primary and secondary spans

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

Diagram of rectangular and diagonal structural layout

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.

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