Probabilistic models for live loads in buildings: critical review, comparison to Brazilian design standards and calibration of partial safety factors

Nowadays, structural “limit state” design is made using characteristic or nominal values of actions, partial safety factors and load combination factors. The actual loading that a structure will be subjected to throughout its life is not known at the design phase. Yet, probabilistic models of such loadings are useful for the rational determination of partial safety factors and load combination factors. The probabilistic model leading to nominal live loads of NBR 6120:2019 (Design Loads for Structures) has never been openly discussed. Herein a simple probabilistic model describing spatial and temporal variabilities of live loads in buildings is presented and discussed. The model is built as a sum of two stochastic processes representing the sustained and intermittent parts of the live load. Model parameters are the ones recommended by the Joint Committee on Structural Safety (JCSS), based on extensive surveys done in several countries. By way of Monte Carlo simulations, sample values of live load actions are obtained for buildings of different occupancy types. These values are compared with those recommended by international standards, and those recommended in NBR 6120:2019 and NBR 8681:2003 (Actions and Safety of Structures). The corresponding statistics for the fifty-year extreme and arbitrary point-in-time distributions of live loads are presented; these statistics are very relevant for reliability analyses and for reliability-based code calibration. The stochastic live load model is also employed in a reliability-based calibration to obtain partial safety factors and load combination factors to be used in Brazilian design codes, for ultimate and serviceability limit state verifications. resumo: Hoje em dia, o projeto estrutural baseado em estados limites é feito utilizando valores característicos ou nominais das ações, fatores parciais de segurança e fatores de combinação de ações variáveis. As ações às quais uma estrutura estará sujeita durante sua vida não são conhecidas com exatidão na fase de projeto. Neste contexto, modelos estocásticos das ações são úteis para a determinação racional dos fatores parciais de segurança, e dos coeficientes de combinação de ações. O modelo probabilístico que levou aos valores nominais das ações de utilização da NBR 6120: 2019 (Ações para o Cálculo de Estruturas de Edificações) nunca foi discutido abertamente. Neste artigo, apresenta-se uma revisão crítica de um modelo estocástico simples que descreve as flutuações esp