A neutral axis is a line that goes through an object that shows the center of forces in action on said object. At any point of intersection through the axis, the contacting and expanding pressures in effect on the object are equal. In a symmetric object, like a structural beam, the neutral axis is through the exact center of the object. In an asymmetric object, or one that has disproportionate support, finding the neutral axis is a very complex mathematical process. The most common practical use for this function is the creation of arches within walls and buildings.
An axis is a line that goes through an object. These lines describe a location of importance to the object, often relating to rotation, stress or appearance. For example, the axis of the planet Earth describes the line around which the planet rotates. An object such as a candlestick will often have an axis of symmetry; the object is the same, on both sides of the axis line, when viewed from any angle.
Every object also has a neutral axis, but this is a more complicated type of line. A number of forces affect an object in any given situation. Some of those forces cause the object to stretch and some cause it to contract. Somewhere within the area of the object is a line that shows the exact center of the forces, basically the border between the two affects.
In a rectangular beam supported on both ends, gravity is pulling down on the object. Gravity will make the beam into a ‘U’ shape, essentially causing the middle of the beam to sag. In this case, the underside of the beam is stretching and the top of the beam is compressing. The neutral axis travels directly through the center of the object, showing the line where the forces meet and cancel each other out.
In simple objects, the neutral axis is generally easy to find. In complex objects, it is much more difficult. Unlike most other types of axis, this one can actually exist outside the object. In an object that has a non-symmetrical shape or that has uneven points of support, the axis where the forces are equal may be in an area between two points that is outside the object’s structure. This means that the forces placed on the object are uneven and cannot be fully balanced.
One of the earliest uses for the neutral axis was in the architectural arch. Since the axis runs through the arch, it is possible to find a point at which the structure can support the weight above it. As long as the axis stays within the greater structure of the arch, it can hold a doorway without collapsing. The curve of the arch gives the structure a much larger area for the axis to stay within when compared to a straight topped doorway.
By M. McGee