Imposed floor loading for offices: a re-appraisal
This re-appraisal of office floor loading has first required a statistical definition of characteristic loading, taken as the loading having a probability of 0.98 of not being exceeded in any year over a 50-year period. For offices, this is interpreted as 12 separate occupations. The comprehensive Building Research Establishment survey of office floor loading reported by Mitchell and Woodgate has been re-analysed, with the load concentration factor established as 1.4 and additional data on partitions estimated. This has enabled the characteristic imposed floor loading for offices (including partition loading) to be evaluated as 3.5kN/m2 for upper floors and 4.25kN/m2 for floors at or below ground floor. These are modified for areas greater than 10m2 by a reduction factor ƒÑA = 0.5 + 1.6/„©A, where A is the whole area affecting the load on the member concerned (not the tributary area), or for slabs spanning less than 2.7m by an amplification factor ƒÑA = 0.75 + 0.8/L, where L is the span. These characteristic imposed loadings include a characteristic partition loading of 0.75kN/m2 (0.8kN/m2 for floors at or below ground level). Exceptionally heavy partitions can be accommodated by an additional uniformly distributed loading (in kN/m2) calculated as one-half of the excess of the linear loading over 1.5kN/m run. This additional loading is not subject to amplification or reduction. Edge loading, e.g. from cladding, can be reduced by one-half of the partition linear loading, minimum 0.75kN/m run. The characteristic imposed loading for columns and walls supporting more than one floor can be reduced by 10% for two floors, 20% for three floors, 30% for four floors or 40% for five or more floors. A floor supported by a transfer beam can be included in the same way. These reductions can be combined with the loaded area reductions.
The sustained loading for deflection of slabs and beams is proposed as 2.7kN/m2, with the same factors for loaded area as for upper floors. The sustained loading for axial shortening of columns and walls and for settlement of foundations can be taken as this loading on the topmost floor plus 0.9kN/m2 (without area reduction) on all lower floors. The minimum loading, e.g. for vibration checks, can be taken as 0.4kN/m2 (without area reduction). With the British load factor of 1.6, the statistical reliability at ultimate load meets the target of EN 1990.
Ultimate load design for adjacent and alternate spans loaded is seriously over-conservative, and the provision in BS 8110 for reinforced concrete slabs to be designed for the single load case of all spans loaded with support moments redistributed downward by 20% could well be applied to all continuous members. The provisions of the British code BS 6399-1 and the Eurocode EN 1991-1-1 are examined, together with those of five non-European English-language codes. The considerable variation and inconsistency gives cause for concern and an opportunity for rationalisation.