Reliability-based calibration of imposed design loads for office buildings

Modern structural design codes employ reliability-based calibration to achieve acceptable levels of safety across different structural elements and materials. However, existing calibrations, specifically of imposed loading, are to a large extent based on historical data that may not reflect current loading conditions and variability. This paper presents a reliability-based calibration of characteristic imposed design loads for office buildings, drawing on new empirical loading data from UK office surveys conducted by the authors. Using a First-Order Reliability Method (FORM) approach applied to steel structural members designed to the Eurocodes, characteristic imposed load values are calibrated against Eurocode target reliability indices. The question of imposed load reduction over multiple floors is also examined through a parametric study of inter-storey load correlations. Frequent and quasi-permanent values, associated with the serviceability limit state, are derived from the survey data. This study finds that the current UK National Annex value of 2.5 kPa, excluding partitions, is broadly supported, and that the increase proposed by the second-generation Eurocodes to a value of 3.0 kPa is not fully justified by the gathered evidence. The differences between the UK National Annex and the base Eurocode approaches to storey-based load reduction are found to be modest for larger floor areas when combined area and storey-based reductions are considered.