Plastic hinge
The term plastic hinge (also plastic joint) referred to in the theory of plasticity , a joint , which the structure when exceeding the yield point is formed. When the maximum bending moment is reached, it is a real joint. In structural engineering, a joint is defined in such a way that a twist does not lead to any bending moment, with a plastic hinge there is a so-called impressed bending moment (= Mpl) when the flow occurs, which does not change when the twist is increased because the cross-section is already plasticized and cannot absorb any further load increase. The formation of a plastic hinge, if not only the cross-section, but also the supporting structure is calculated plastically, for this a cross-section class 1 must prevail in steel construction according to Eurocode 3 at the point of the cross-section in order to be able to guarantee the required rotation ability (without buckling failure). Not only do the cross-sectional dimensions have to be sufficiently compact (in steel construction: cross-section class 1), but the use of a ductile material is usually necessary, which allows the large plastic deformations to fail without a cross-section .
The elastic deformability of a component or joint ends when the yield point is reached . Above the yield point, the material does not return to its original shape, even after the load has been removed; instead, permanent deformations occur on the component.
A component can only absorb forces until the yield point is reached through elastic and then through plastic deformations. A further increase in the introduction of force causes the material to flow and ultimately the component to fail.
With the plastic internal forces calculation, the static uncertainty can be reduced by adding plastic hinges to the framework model or the framework model. The new (plastic) hinge adds an additional determining equation (or degree of freedom (s)) to the static system , namely the moment equilibrium of the plastic hinge. In this way, statically indeterminate systems and frameworks can be converted into statically determinate (calculation) systems that are easier to calculate , taking into account the assumed introduction of forces, the material properties and the component geometry (plastic hinge method). The plastic limit state is reached when, for an n-fold statically indeterminate system, there is such a plastic hinge chain with n + 1 plastic hinges that satisfies the plasticity condition M ≤ Mpl: The load-bearing capacity of the framework can then be calculated from this plastic hinge chain created by the load increase factor - a simple example can be found at Kurrer .
The plastic hinge method belongs to the group of load bearing methods and is often equated in terms of terms for simplicity.
Plastic hinges are mostly bending joints. Since the bending moments are often decisive for the structural safety , they can also develop as normal force or shear force joints.
See also
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- ^ Karl-Eugen Kurrer: Load-bearing method . In: History of structural engineering. In search of balance . Berlin 2016, Ernst & Sohn , pp. 121-138, (here pp. 132-134), ISBN 978-3-433-03134-6