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Loading and Load Cases
It is important that all truss loadings are specified before quotation to ensure correct design.
Unless otherwise advised, trusses will normally be assumed to be for normal domestic use.
Loadings for Domestic Use

The great majority of trusses fall into this category. The relevant document, BS 5268-3 describes the minimum loadings which should be taken into account.
The following data provides a useful guide to typical loading factors in roof design:
TOP CHORD (Rafter)

Weight to be as laid. Nearly all commonly used interlocking concrete tiles are within 0.575kN/m2, which is regarded as the standard loading. It is
important that the actual tile weight to be used is notified to the Trussed Rafter Designer. This loading is specified as a long term loading on slope; i.e. applied along the length of the sloping rafter.
Felt, Battens, Self Weight
The allowance usually made for felt, battens and self weight of trusses is 0.11kN/m2. As are the tiles, this is regarded as a long term loading slope.
Except in the case of vertical and near vertical chords, wind loading is not often a critical criterion in the design of fully triangulated trusses.  All trusses should be designed for wind loading in accordance with BS 6399: Part 2 code of practice for wind loads. Wind load data should be provided by the Building Designer to the Trussed Rafter Designer.  Wind loading is treated as a very short term loading, applied at right angles to the relevant members. Snow Designs for snow loadings are in accordance with BS
6399: Part 3: Actual design loads are dependant upon several factors, such as building location, altitude and roof plane geometry. The loadings imposed by snow are regarded as medium term loadings, on slope. Where appropriate, snow drifting should be considered.
Man Load on Rafter
This is specified as 0.75 x 0.9kN in any position. Test have shown that, in normal circumstances, tiles and battens provide sufficient transverse load distribution for this loading not to be a critical criterion in design. However it can dictate the design of a long overhang. This loading is treated as short term loading.
BOTTOM CHORD (Ceiling Tie)

Plasterboard, Self Weight etc
The standard ceiling construction of one layer of 12.5mm plasterboard and skim coat is taken as giving a load of 0.25kN/m2 (including truss self
weight). This load is treated as a long term loading on slope (although generally bottom chords will have no slope).

Light Storage
For normal domestic applications, the specified allowance for storage over the length of the bottom chord (ceiling tie) is given as 0.25kN/m2 (on slope).
For anything other than this condition, the Building Designer should inform the Trussed Rafter Designer of the required storage loads to be used.  This load, as for the ceiling construction load, is treated as a long term loading on slope.
Man Load on Ceiling Tie
To allow for loadings imposed by a person working in the roof void, an allowance of 0.75 x 0.9kN at any location on the bottom chord, either in the bays or at the node points (joints) should be made. This loading is treated as short term loading.