PERFORMANCE BASED DESIGN OF STRUCTURES TO ADDRESS DESIGN BASIS AND BEYOND DESIGN BASIS SEISMIC LOADING

摘要

The current expectations from the nuclear regulators call for new build structures to be designed to avoid brittle failure in order to maintain a reasonable level of adequacy for Beyond Design Basis Events (BDBE), particularly of a seismic nature. For this purpose, the design of the structure needs to be carried out such that the ultimate failure mechanism is understood and shown to occur in a controlled manner, with absorption of energy taking place through ductile response in key stable elements prior to brittle failure of elements (e.g. compression buckling in columns). Depending on the safety requirements of the structures or components, a degree of permanent deformation is also allowed even for Design Basis Events (DBE). This is known as Performance Based Design. The application of something other than an elastic design at the DBE has been limited in the UK, albeit that the industry is moving away from the historical approach of a quantified margin at the DBE in order to claim BDBE compliance. In many cases the elastic design approach results in excessively rigid structures that attract large seismic demands. This paper deals with the use and optimisation of Performance Based Design, focusing on steel structures subject to extreme seismic loading. It looks at the importance of adequate design and detailing to control failure mechanisms using a combination of relevant international standards and adequate engineering judgement in line with the regulator's current expectations. Examples are presented to demonstrate the effectiveness of allowing for controlled failure mechanisms using ductility in order to minimise the seismic load carried by the structure and its foundations.