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Plate girder behaviour and design part 1

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Introduction to shell structures

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Behaviour and design of stiffened plates


To discuss the load distribution, stability, and ultimate resistance of stiffened plates under in-plane and out-of-plane loading.


Lecture 8.1: Introduction to plate behaviour and design

Lecture 8.2: Behaviour and design of unstiffened plates


The load distribution for in-plane loaded unstiffened plate structures is discussed with critical buckling loads derived using linear elastic theory. Two design approaches for determining the ultimate resistance of stiffened plates are described and compared. Out-of-plane loading is also considered and its influence on stability discussed. The requirements for finite element models of stiffened plates are outlined using those for unstiffened plates as a basis.


The automation of welding procedures and the need to design elements not only to have the necessary resistance to external actions but also to meet aesthetic and serviceability requirements leads to an increased tendency to employ thin-walled, plated structures, especially when the use of rolled sections is excluded due to the form and the size of the structure. Through appropriate selection of plate thicknesses, steel qualities, and form and position of stiffeners, cross-sections can be best adapted to the actions applied and the serviceability conditions - saving material weight. Examples of such structures, shown in Figure 1, are webs of plate girders, flanges of plate girders, the walls of box girders, thin-walled roofing, facades, etc.


Plated elements carry simultaneously:

a) actions normal to their plane

b) in-plane actions

Out-of-plane action is of secondary importance for such steel elements as, due to the typically small plate thicknesses involved, they are not generally used for carrying transverse actions. In-plane action, however, has significant importance in plated structures.

The intention of design is to utilise the full strength of the material. Since the slenderness of such plated elements is large due to the small thicknesses, their carrying resistance is reduced due to buckling. An economic design may, however, be achieved when longitudinal and/or transverse stiffeners are provided. Such stiffeners may be of open or of torsionally rigid closed sections, as shown in Figure 2. When these stiffeners are arranged in a regular orthogonal grid and the spacing is small enough to 'smear' the stiffeners to a continuum in the analysis, such a stiffened plate is called an orthogonal anisotropic plate or, in short, an orthotropic plate (see Figure 3). In this lecture, the buckling behaviour of stiffened plate panels subjected to in-plane actions will be presented. The behaviour under out-of-plane actions is also discussed as is the influence of the out-of-plane action on the stability of stiffened plates.

Specific topics such as local actions and the tension field method are covered in the lectures on plate girders.

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