Collapse Mechanism

Definition - What does Collapse Mechanism mean?

By inserting a plastic hinge at a plastic limit load into a statically determinate beam, a kinematic mechanism permitting an unbounded displacement of the system can be formed. This is known as the collapse mechanism. For each degree of static indeterminacy of the beam, an additional plastic hinge must be added to form a collapse mechanism. It is used in structural analysis and design.

The formation of a single plastic hinge gives a collapse mechanism for a simply supported beam. Collapse occurs when there is no more remaining stable element that can carry the additional load.

Corrosionpedia explains Collapse Mechanism

When a system of loads is applied to an elastic body, it deforms and shows a resistance against deformation. Such a body is known as a structure. On the other hand, if no resistance is set up against deformation in the body, then it is known as a mechanism. The ultimate or collapse load is reached when a mechanism is formed. The number of plastic hinges developed should be just sufficient to form a mechanism.

In beam analysis, a statically determinate beam fails if one plastic hinge develops. Any further increase in the load causes collapse. For a statically indeterminate beam to fail, it is necessary for more than one plastic hinge to form. As the load is increased, there is a redistribution of moment because the plastic hinge can resist no more moment. As more plastic hinges are formed in the structure, there is eventually a sufficient number of them to cause collapse. Some additional load can be carried after this time before collapse occurs, as the stresses go into the strain-hardening range.

The load may be further increased until the moment at some point (such as the concentrated load) reaches the plastic moment. Additional load causes the beam to collapse. Therefore, the collapse mechanism is the arrangement of plastic hinges and perhaps real hinges which permit the collapse in a structure. The collapse load is calculated by equating the external and internal virtual works.

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