The right-hand rule is a common convention in structural engineering and physics for describing the orientiation of axes. Many readers might already be familiar with it since it is a common rule describing the rotation of magnetic fields generated when electrical current is passed through a long straight wire.
In this post, we will explain the right-hand rule and it's relevance to structural engineering.
The Direction of Rotation (Curl Rule)
Before describing the right-hand rule, it is important to understand the orientation of the rotations (curl rule). If you imagine closing your fist and pointing your thumb outwards (as though you were holding an arrow), the thumb represents the direction of the axis (see image below).
The direction of rotation follows the same axis as the curled fingers of the fist. The rotation is described as a rotation about the axis, and the sign is taken as positive. Any rotations in the opposite direction are therefore taken as negative.
Right-Hand Rule
For 3D systems, most engineers and software will follow the right-hand rule convention for defining the orientation of positive rotations. You can see the right-hand rule illustrated below:
When you apply the curl rule above to the 3d XYZ axes shown, then suddenly the orientation of the rotations makes a considerable amount of sense.
Relevance in Structural Engineering
It is important for engineers to try to speak a common language. In structural engineering, this takes the form of conventions like the right-hand rule, or the practice of drawing bending moments on the tension side.
Let's consider a simply-supported beam subjected to a point load, as shown below:
What beam axis is most relevant for this beam in terms of resisting any rotation/deflection due to the point load?
As you might have guessed, it is the Y-Y axis. In other words, 
.
