Steel Buildings in Europe

Part 8: Building Envelope 8 - 31 services or suspended ceilings. The structural engineer responsible for specifying the purlins will frequently play little or no part in the specification of the services or ceilings. Nevertheless, it is important that an accurate estimate of these loads is obtained together with the nature of the loading (whether concentrated or distributed), since they could form a significant proportion of the overall gravity loading on the purlins. Particular care should be taken where the purlins are required to support concentrated loads. Gutters and their supporting structure require special attention, as the loads associated with them are often very high. Designers need to consider the weight of the gutters plus that of their contents (water or snow). Specific information on the specified gutter system should be sought from the gutter manufacturers. During the construction stage, the purlins may still be required to carry significant gravity loads, but without the benefit of any restraint provided by the cladding. The magnitude of the construction load will depend largely on the cladding installation procedure and the materials, plant and labour used. The cladding installation sequence, in particular, can have a significant effect on the buckling resistance of a purlin, due to its influence on the unrestrained length of the purlin and the location of the load within the span. It is therefore essential that the designer takes account of the proposed method of working when specifying the purlins. Preferably, this should be achieved by dialogue between the roofing contractor and the designer at the time of the purlin specification. 4.3 Deflections The deflection limits for the purlins and side rails are generally governed by the choice of roof and wall cladding, since the governing factor is the ability of the cladding to deflect without compromising weathertightness, airtightness, non-fragility or any other performance requirement. In general, the greater the flexibility of the cladding, the larger the allowable purlin or side-rail deflection. In this respect, profiled metal cladding systems are far more tolerant of deflections than brittle materials such as masonry. By contrast, windows are often critical and further guidance should be sought from the glazing manufacturers. Excessive deflection under purlin or rail self-weight, or under the action of construction loads prior to the fixing of the cladding, can lead to difficulties for the cladding installation. This should be addressed by careful consideration of the likely construction loading and by specifying a method of cladding installation that avoids overloading the unrestrained purlins. Gutters are especially sensitive to deflections, due to the need to avoid backfalls. 4.4 Purlin and side rail selection The major purlin and cladding rail suppliers have invested heavily over many years in the development and testing of their systems and all publish design guidance and load/span tables for their products. In many cases, design software is also available. Thanks to these design tools, the structural engineer is spared the complexities of the design of light steel members and can simply select the most suitable section from the available range. However, specifiers