Cornelia Doerich

Accurate determination of plastic collapse loads from finite element analyses

When computational modeling is used to evaluate the true strength of an imperfect elastic-plastic shell structure, the current European standard on shell structures requires that two reference strengths are always determined. the linear bifurcation load and the plastic limit (plastic collapse) load. These two loaas are used in more than one way to characterize the strength of all imperfect elastic-plastic systems. Where parametric studies of a problem are being undertaken, it is particularly important that these two loads are accurately defined, since all other strengths will be related to them. For complex problems in shell structures, it is not possible to develop analytical solutions for the plastic collapse strength, and finite element analysis must be used. Unfortunately, because a collapse mechanism often requires the development of very extensive plasticity involving large local strains, and the collapse load is simply at the end of a slowly rising load-deflection curve, it is sometimes difficult for the analyst to accurately determine this plastic collapse strength. This paper describes two methods, based on modifications of the Southwell plot, of obtaining very accurate evaluations of the plastic limit load, irrespective of whether a fairly complete plastic strain field has developed or not. These two methods allow plastic collapse limit loads to be reported with great precision.

Accurate determination of the plastic collapse loads of shells when using finite element analyses

Publisher Summary The chapter presents a new method by which plastic collapse load may be accurately and repeatedly extracted from the results of a materially nonlinear finite element analysis using small displacement theory. It is of critical importance that the methodology should lead to a unique result because this load is a vital component of the strength evaluation of the structure. The method is recommended for wide adoption to ensure that quoted finite element plastic collapse loads are always based on the same criteria. The method can be applied to finite element analyses of all forms of structure and is not limited to shell structures alone. This plastic reference load is a vital part of the global numerical analysis provisions of the European Standard for Shells.

The behaviour of cylindrical steel shells supported on local brackets

Publisher Summary The chapter presents an outline description of the behavior of a cylindrical steel shell that is discretely supported on several brackets, each rigidly connected to a stiff column or floor. The linear, materially nonlinear, geometrically nonlinear, and bifurcation behaviors of the shell are outlined, but space restrictions have prevented the complex interactions between the change of geometry and progressive yielding from being shown. An example shell is shown in which bifurcation and plasticity interact in determining the failure condition. For this geometry, it is shown that the behavior is not very imperfection sensitive, so design rules should not follow the corresponding formulations for uniform axial compression too closely.