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Modern Plastic Design for Steel Structures

Short description

The overall objective of this project is to supply analytical methods, which will provide the quantification of an upper shelf toughness criterion to allow for safe plastic design of steel structures using modern construction steels. Since this criterion is still missing, the plastic design of structural elements is only permitted when the steels used meet extremely severe test requirements, which penalise the use of steel by uneconomical design methods. Therefore the application of this criterion will inevitably lead to increased exploitation and employment of steel in buildings, bridges, pressure vessels, offshore and pipeline structures and thus to the enhancement of the competitiveness of the European industry. The general design rules to achieve mechanical resistance and stability of steel structures are valid at room temperature and rely on elastic-plastic material behaviour. For safety reasons plastic strain redistribution is necessary especially at local stress concentrations such as welded attachments, cut outs, nozzles and also in frames to enable moment redistributions by formation of "plastic hinges". The material requirement resulting from these design rules require therefore significant strain capacities of the steel and weldments. In terms of toughness the material requirement must be expressed as a suitable upper shelf criterion. Such upper shelf criteria are mostly expressed by technological tests developed from national experience of design engineers, e.g. in Germany the so called "Aufschweißbiegeversuch" was developed in the 30's of the past century for all steel structures with thickness higher than 30 mm and survived until today. This test is a bending test at room temperature that should demonstrate sufficient stable crack growth before breaking. The result is a pure "yes" or "no" criterion. Else the requirement of CTOD criterion for offshore platform and linepipe steel with the CTOD-values up to 0.3 mm at test temperature falls into this type of upper shelf requirement. Modern developments of offshore wind parks take over such steel criteria for lack of alternatives. Considerable amount of work has been successfully performed to derive fracture mechanics based safety concepts against brittle fracture. Results have been widely used to derive general structural integrity manuals such as BS7910, SINTAP and even more interesting for application the European codes, to derive practical steel selection methodologies to avoid brittle fracture for welded structures. Examples are EN 1993-1-10 (steel structures) and EN 13445 (pressure vessels), which have been successfully introduced now. Safety against brittle fracture on the basis of fracture mechanics is assessed for design situations, in which the combination of actions is related to extreme values of low temperature, where for the other actions, e.g. dead and wind loads, elastic material behaviour can be expected. Hence toughness material requirements are related to the temperature transition area. In conclusion, fracture mechanics tools are principally available to perform toughness assessments, however, they must be tuned to the various safety philosophies and design assumptions of each area of application. Advanced methodologies such as damage mechanics, which may allow even a better description of the real structural behaviour, are in a development status. Within this project both methods shall be applied. In the short term fracture mechanics tools allow a fast practical solution, in the medium term damage mechanics will constitute a further technological step into future design methods. These include design by analysis calculations on the basis of material laws derived from damage mechanics.

Research program



Duration of project

07/2005 - 01/2009

Contact person

Markus Feldmann