This Part of this British Standard Code of practice has been prepared under the direction of the Civil Engineering and Building Structures Standards Committee as a new Part to BS 6399 (formerly CP 3: Chapter V). Imposed roof loads were previously included in BS 6399-1. This new Part of BS 6399 now gives more information on imposed roof loads and in particular gives snow loading data separately, allowing account to be taken of the variation of snow in the United Kingdom and the effect of redistribution of snow on roofs due to wind. Use of the uniformly distributed snow loads are subject to an overriding minimum requirement.
This code can be used for design using permissible stresses or partial factors. In the former case the values should be used directly while in the latter case they should be factored by an appropriate value depending upon whether an ultimate or serviceability limit state is being considered. The exception to this is the treatment of the load cases involving local drifting of snow, where it is recommended that these are treated as exceptional loads and used in design with reduced safety factors.
Section two of this Part of BS 6399 is broadly in agreement with ISO 4355-1981 “Bases for design of structures — Determination of snow loads on roofs”, published by the International Organization for Standardization (ISO). However, one difference is that, in general, the uniform snow load condition and the drift snow load condition are treated as independent load cases. This is in recognition of the United Kingdom’s maritime climate which means that for many parts of the country the maximum snow load condition is likely to result from a single fall of snow, rather than an accumulation over several months.
The treatment of snow drifting against obstructions in section two is similar to that given in BRE Digest 290, issued in October 1984, but now withdrawn. However, it should be noted that there are some differences as follows:
a) the notation has changed to conform better to ISO 3898;
b) there are increased restrictions on the amount of snow that can form in the drift;
c) the drift loads are to be treated as exceptional loads.
The last point explains why the upper bound values for the snow load shape coefficients have apparently increased. (Digest 290 was drafted so that the drift loads could be treated as ultimate loads.)
The designer should be aware that the deposition and redistribution of snow on roofs are very complex phenomena. The type and record length of the ground snow data available and the paucity of observational data on roof snow loads make it extremely difficult to estimate snow load distributions reliably. This Code models the actual drift shapes and load intensities by simplified linear distributions, based on assumptions on the amount of snow available to drift and limitations on the drift height. Wherever possible, available observational data have been incorporated in the development of the design models.
In this Part of BS 6399 numerical values have been given in terms of SI units, details of which are to be found in BS 5555. Those concerned with the conversion and renovation of existing structures or buildings designed in terms of imperial units may find it useful to note that 1 N = 0.225 lbf and 1 kN/m2 = 20.89 lbf/ft2.
This Part of BS 6399 gives minimum imposed roof loads for use in designing buildings and building components which are to be constructed and used in the UK and the Channel Islands. It applies to:
a) new buildings and new structures;
b) alterations and additions to existing buildings and existing structures.
Caution is necessary in applying the snow load calculations for sites at altitudes above 500 m and specialist advice should be obtained in such situations (see appendix C).
