030 RCI 0517

RCI May 2017

RCI TECHNICAL NOTE NUMBER 245 030 MAY 2017 RCIMAG.COM Keith Roberts, who compiled this Technical Note exclusively for RCI, is an independent roofing consultant. Contact him at: Roberts Consulting 2 Gardiner Close, Abingdon, Oxon, OX14 3YA Tel/fax: 01235 529 693 www.robertsconsulting.co.uk Limit state design Last year a limit state approach was adopted by BS 5427 as the basis for structural design. This RCI Technical Note offers further information about this significant change Over the past couple of years the British Standards for Slating and Tiling (BS 5534) and Profiled Sheet Cladding (BS 5427) have been revised. One major change that has not been widely discussed has been that the structural design is now required to be verified using limit state principles in accordance with the Eurocode, BS EN 1990. This significant change from previous editions of the well-used roofing standards means that partial factors of safety are now applied to both the applied loads and to the strength of the roof assembly. Previously only a single factor of safety was applied to determine the permissible strength. This ‘new’ approach has been used by structural engineers in the UK for the design of steel and reinforced concrete structures for more than twenty five years. The design standards for structural timber and masonry have also adopted a limit state approach in the past decade. As roofing and cladding standards have been reviewed and updated it makes sense for the structural design checks on these elements to adopt current structural practice. A limit state is a condition of a structure beyond which it no longer fulfills the relevant design criteria. Limit state design requires the structure to satisfy two principal criteria: the ultimate limit state (ULS) and the serviceability limit state (SLS). Ultimate limit state: strength The ultimate limit state concerns the safety of people and the safety of the structure. Essentially: ‘the design load must be less than the design strength’. A simplified basic equation is given in the table below, together with the partial factors commonly used and now referred to in the current British Standards for roofing and cladding. This is a simplified approach and more detailed guidance is given in the Eurocode BS EN 1990: Actions on Structures. Within the Slating and Tiling Standard an additional consequence factor KFI for actions relating to reliability class of the structure has been added with a value of 0.9. This effectively reduces the design load. When there are multiple variable loads acting at the same time then additional factors should be considered, taking into account a combination value and a reduction factor. Such load combinations would be unusual for most roofing applications. Further guidance is given in the Manual for the Design of Building Structures to Eurocode 1, published by the Institution of Structural Engineers. This useful book also gives information about calculating the applied loads, including wind, snow and thermal actions. Serviceability limit state: deflection The serviceability limit state concerns the functioning of the structure under normal use, the comfort of people and the appearance of the construction works. When calculating deflection for the serviceability state, partial factors of 1.0 are used throughout. In this way the actual magnitude of the deflection or movement in the roof and cladding assembly is determined and checked against an allowable limit. This is the same as previous practice. Conclusion The roofing and cladding industry in the UK is not alone in going through the change in the basis of structural design away from a permissible state approach. In the United States and Canada similar changes are afoot, following the introduction this summer of a revised wind loading code. It is recognised that the preparation of structural calculations with their many Greek symbols and multiple factors is remote from the world of the roofing tradesman. However, these equations and factors are now written into published British Standards for roofing and there remains the need for technicians within the industry to have a basic understanding of limit state theory when checking the stability of roof structures. References 1. BS EN 1990: 2002, ‘Eurocode – Basis of structural design’. 2. BS 5534: 2014, ‘Slating and Tiling for Pitched Roofs and Vertical Cladding – Code of Practice’. 3. BS 5427: 2016, ‘Code of Practice for the Use of Profiled Sheet for Roof and Wall Cladding on Buildings’. 4. ‘Manual for the Design of Building Structures to Eurocode 1 and Basis of Structural Design’, April 2010, Institution of Structural Engineers. Above: Simplified ultimate limit state equation and associated partial factors “This significant change from previous editions of the well-used roofing standards means that partial factors of safety are now applied to both the applied loads and to the strength of the roof assembly” “The roofing and cladding industry in the UK is not alone in going through the change in the basis of structural design away from a permissible state approach”


RCI May 2017
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