An axial force is any force that directly acts on the center axis of an object. These forces are typically stretching force or compression force, depending on direction. In addition, when the force load is even across the form’s geometric center, it is concentric, and when it is uneven, it is eccentric. Unlike many acting forces, an axial force is often its own counter; an object pulled or pushed evenly in opposing directions doesn’t move. Sheer force occupies a similar position to axial force, but operates perpendicular to the center axis of the object.
One of the most important parts of examining axial forces is the idea of a geometric center. This is a point within the boundaries of a solid object that is the perfect center of the entire mass. In a simple object, such as a cylinder, it is easy to find the exact middle of the object by simply measuring the sides. In a complex object, such as a bicycle, the process is much more complicated. While there is a very complex series of mathematical equations that will find this point, it is basically the point at which the mass of the object is the same in any opposing direction.
In a complex object, this point can be practically anywhere within the greater space of the form. When looking at an object, the space the object takes up is often larger than the object itself. Factors such as density and protruding arms could cause the geometric center to exist on the surface or even outside of the form.
The center axis of the object runs from one side through the object to the other. This line is based on the shape of the object, not its mass or density. The center axis might run through the geometric center, or it might not.
When a force is acting directly on the central axis, it is an axial force. These forces will often compress the axis from either end or stretch the axis in two opposing directions; as a result, the object typically doesn’t move. A prime example of these forces can be seen on columns within buildings. The column has an axis that runs through the entire form from top to bottom. The column is constantly compressed as it supports the roof of the structure.
In the column example, the axial force runs through the geometric center of the form; this makes the force concentric. A concentric force is stable at rest. When the axis doesn’t pass through the geometric center, the shape isn’t stable and the force is eccentric. This typically means that the form is unable to withstand axial forces while at rest; the structure will be unable to cope with the disproportionate energies acting upon it.
By M. McGee