A fixed support (also called a built-in or encastré support) is a type of support in which a structural member — such as a beam, column or girder — is fully held at its end, so that it can neither translate nor rotate there. A fixed support therefore takes up not only forces but also a moment (a rotational force).
Difference from other supports
In structural engineering, supports are distinguished by what they prevent:
- Roller support — allows translation in one direction and rotation, takes up only a shear force
- Pinned support — prevents translation but allows rotation (no moment)
- Fixed support — prevents translation and rotation, and therefore also takes up a moment
A classic example is a balcony built into the facade on one side: it projects freely (cantilever) and is fully held in the wall.
Application
- Cantilevered structures — balconies, canopies and overhangs supported on one side only
- Columns in a foundation — a column anchored moment-fixed in the pocket or foundation
- Portal frames — where the connection between column and beam is made stiff and moment-fixed
Points to note
- A fixed support must actually be able to take up the moments; the connection and the adjoining structure must be designed for it
- Insufficient fixity leads to deflection, cracking or rotation
- The degree of fixity is determined by a structural engineer
Related terms
- Support
- Cantilever
- Moment
- Beam
- Portal frame
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