First-Order Robustness, Higher-Order Mechanics

This paper establishes the basic first-order requirements for the assessment of structural robustness, with particular focus on multistorey buildings. Starting from the definition of structural robustness as the resistance to disproportionate collapse, the paper considers the significance of predictability towards the adequacy of assessment. It is proposed that the assessment of robustness for realistic structures should operate outside the conventional strength limit but within the ductility limit state. Nevertheless, the conditions of predictability require the exclusion of certain scenarios from practical consideration, particularly those that are highly influenced by uncertainty in the loading, structure and/or response model. It is also argued that realistic robustness assessment requires models based on higher-order mechanics, including capabilities for modelling geometric and material nonlinearity as well as the interactions of various components within the overall structural system. A multi-level approach recently developed at Imperial College is shown to offer a practical design-oriented assessment framework which fulfils the aforementioned requirements. Finally, the paper highlights important outcomes arising from the application of this framework to multi-storey buildings.