Stability and Plasticity in Structural Frame Behaviour Analysis and Design: Seeking a Conceptual Framework for the Computer Age

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• Prof Michael Rotter
• Friday 13 March 2009, 14:30-15:30
• Department of Engineering - LR6.

If you have a question about this talk, please contact Anita Clayson.

Concepts of stability in structural analysis and design are predominantly based on the behaviour of single members, where the behaviour is often relatively simple. However, with both the advances in computational power and the wide dissemination of very powerful software for structural modelling, it is important that the relationship between paradigms for hand calculation and the outcomes of fully nonlinear computer calculations is clarified. The different conceptual models used in hand calculation and computer calculations of a structure require different criteria of failure, and the lack of attention to system analysis in standards for structural design has hidden the mismatches and incompatibilities between assumptions about system behaviour and member behaviour.

The key advances achieved in early studies of plastic collapse, particularly in Cambridge, that recognised system behaviour as the critical concept, have largely been forgotten in the early part of the computer age when linear elastic analyses of complex structures became easy. It is now high time to regain the lost ground and to find ways of interpreting complex computer analyses within a conceptual framework that includes system behaviour, plasticity and material nonlinearity in stability assessments into a methodology that can be used in design calculations.

This presentation briefly reviews the historical roots of the current paradigms for structural calculation and finds that the conceptual framework being used in current design really dates back to very early simplifications. It presents some work that extends the early Cambridge studies of steel column failure to a generalised model of the collapse of columns of any material within a plastically collapsing frame. This section of the talk argues that unless the complex interactions between members before collapse in a frame are understood and treated algebraically, it is unlikely that the output from sophisticated computer analyses will be understood by the analysts.

The presentation further argues that it is the system strength that really matters in structural strength evaluations, and that this strength can be relatively easily determined by using fully nonlinear computer analyses of the structure. The strength assessment should be referred to and characterised in terms of the two well defined reference strengths: the results of linear bifurcation and plastic collapse analyses. Using these, the interactions between plasticity, stability and imperfections can be characterised using a few simple parameters which can be applied to characterise all structural systems. Researchers are urged to develop a database of the values of these key parameters to permit a better understanding of the roles that plasticity, stability, geometric nonlinearit

This talk is part of the Engineering - Mechanics Colloquia Research Seminars series.

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