Seminar - J Michael Rotter - The practical design of shells, silos and tanks: State of the art and current challenges

Tuesday 13 April 2010, 1.00 pm - 2.00 pm
Civil Engineering Lecture Theatre 3

Professor J Michael Rotter
Institute for Infrastructure and Environment
School of Engineering, University of Edinburgh, Edinburgh, Scotland, UK

Abstract:
The practical design of shell structures for civil engineering applications has recently undergone a considerable transformation in the course of development of the three new European standards relating to them. The generic standard EN 1993-1-6 for the Strength and Stability of Shells sets out extensive rules applicable to all thin shells, whilst the two application standards EN 1993-4-1 Silos and EN 1993-4-2 Tanks give much more extensive coverage of practical issues relating to these specific structural forms. This talk outlines the major conceptual changes, the new philosophy of design for shells in the context of modern computational power and universal accessibility, and some of the applications in silos and tanks.

In the structural engineering context, shell structures present the greatest challenges to both the analyst and the designer. The behaviour of a shell is more complex than other structural forms, its analysis often more difficult, the failure modes are either remarkably benign or appallingly catastrophic. Further, the many gaps between design advice for simple conditions and common design practice in shell structures in make the use of computer modelling very appealing, but the results are often particularly awkward to interpret.

There is a huge literature on the analysis of shell structures, covering linear elastic, plastic, stability, imperfection sensitivity and collapse analyses. Some of these analyses may be directly applied to evaluate the strength of the shell, but most of them give only a poor indication of real strength and the results require careful interpretation to ensure that a safe design results. This is one reason why there are very few regulatory standards in the world that cover shell structures.

Civil engineering structural design is normally regulated by a standard, but for shell structures there has often been a large gap between the design rules in a standard (e.g. ACI 334) and the sophistication of the analyses presented in the research literature. Thus, it is difficult to see how more sophisticated analyses may be exploited in the design process. What was needed was the development of a whole new philosophy of design, arranged around generic concepts of the roles of different types of analysis, that can permit compatible interpretations of different treatments within a secure overall framework. This has been accomplished within the new international standard on shell design (EN 1993-1-6, 2007).

This seminar describes some features of this new standard, and proposes it as a valuable vehicle for researchers who wish to develop new design rules and for designers who wish to use more sophisticated analyses to obtain the most economic structure or to perform the most rigorous assessments of existing structures. A key feature of the new European standard is that it attempts to accommodate all forms of static structural analysis within the framework of the same design philosophy. To achieve this goal, it must address questions of plasticity, stability and imperfection sensitivity within the framework of a reliability assessment using partial safety factors on both loading and structural strength, the latter including modified factors according to the mode of failure. These are matters that are not normally considered within analysis remit of applied mechanics, so a presentation of the draft standard framework should provide a valuable perspective to the developers and users of advanced analysis.