Deflection

Deflection, in structural engineering terms, refers to the movement of a beam or node from its original position due to the forces and loads being applied to the member. It is also known as displacement and can occur from externally applied loads or from the weight of the structure itself, and the force of gravity in which this applies.

There are two types of deflection: linear (translation) and angular (rotation).

Translation is a measure of how much a point in the structure has moved from its original position. It may move a few millimetres from its neutral position when loads act on it; hence, we have two components, the vertical and horizontal deflection. The unit of linear deflection, or displacement, is a length unit and normally taken as millimeter (for metric) and inch (for imperial). This number defines the distance in which the beam has deflected from the original position.

On the other hand, rotation is a measure of how much a point in the structure has rotated from its original position.

This means that at any point of any plane structure, there are three deflection components: vertical translation, horizontal translation, and rotation. For space structures, there would be six: translation in the x, y, and z and the rotations about x, y, and z. The notation will depend on the type of load the structure is experiencing, whether it is a real load or a unit load or couple.