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Loading and Load Cases
Loadings - Water Tank
Water tanks in trussed rafter roofs should be supported by a system of bearers and cross-bearers in such a fashion that the loadings imposed on the trusses are transferred to a position as close as possible to the node points (joints) of the trusses. The standard 230 litre water tank is usually supported over three individual trusses, or 300 litre tank over four trusses. The long term loading from this arrangement is taken as 0.9kN/truss  (0.45kN per node).
Loadings - Agricultural Buildings
Loadings for agricultural buildings are described in BS 5502 and are based on weight of the actual materials in the fabric of the building. Snow and
wind loading criteria depend on occupancy classification determining the acceptability of collapse and expected life of the building. 
Compliance with BS 5502 has been a condition of obtaining certain capital grants and an up-to-date briefing on the matter should be obtained before specification.
Trussed rafters are generally used in conjunction with tiling battens fixed to the upper edge of the top chords and this provides an excellent method of outof-plane restraint to the top chords. If tiling battens are not to be used, it is vital to specify the maximum purlin spacing to be used for two reasons:

1.To allow the Trussed Rafter Designer to apply the loads in the correct way.
2.To allow the Trussed Rafter Designer to apply correct top chord restraints.

The Trussed Rafter Designer will require this information in order to obtain a correct design.
Load Duration (load cases)
The load-carrying characteristics of timber are such that it can sustain heavier loading for a short time than it can for a long time. 
This effect is used in establishing the allowable structural properties of a particular timber grade (or Strength Class). 
Trussed rafters and other structural timber components are then designed taking into account the differing durations of the various loadings which they are required to carry. 
The main loadings encountered in dealing with trussed rafters (see earlier in this section) are:
1.Roof Coverings:
Tiles, slates etc. are considered as long terms loads, as they will be present throughout the life of the building.

2.Ceiling Construction:
Plasterboard etc at ceiling level is, like the roof covering, considered as long term.

3.Ceiling Storage:
The allowance for storage in the roof void at ceiling level is also treated as an ever-present, long term load.

4.Water Tanks:
As these will also be present throughout the building's life loads applied by water tanks are treated as long term loads.

5.Snow Loadings:
The design allowances for loadings due to snow on the roof are treated as medium term loads, i.e. these loads will not be present at all times, but will affect the roof structure only for a period of weeks or months at a time.

6.Man Load on Rafter and Ceiling:
Where this is applicable, this load is treated as a short term load, ie this load will be present within the structure for a period of minutes or hours only.

7.Wind Loadings:
Always considered for design, the loadings due to wind are treated as very short term loads. These loads will be present on the structure for a period of minutes or seconds only.  The above loadings cover the most usual types of load carried by trussed rafters.  Other loads may be present within the roof in special circumstances These may include air conditioning equipment, patient hoists, climbing ropes etc and must be allowed for in the design, in the appropriate load case.