Neutral Axis: What is it?

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A neutral axis is a line through an object showing the center of forces acting on that object. At any point of intersection through the axis, the contact and expansion pressures acting on the object are equal. In a symmetrical object, such as a structural beam, the neutral axis passes through the exact center of the object. In an object that is asymmetrical, or that has a disproportionate support, finding the neutral axis is a very complex mathematical process. The most common practical use for this feature is creating arches inside walls and buildings.

An axis is a line through an object. These lines describe an important position for the object, often related to rotation, stress or aspect. For example, planet Earth’s axis describes the line around which the planet rotates. An object such as a candelabra will often have an axis of symmetry; the object is the same, on either side of the axis line, when viewed from any angle.

Every object also has a neutral axis, but this is a more complicated linetype. A number of forces affect an object in a given situation. Some of these forces cause the object to stretch and some cause it to contract. Somewhere within the object area is a line showing the exact center of the forces, basically the boundary between the two affects.

In a rectangular beam supported at both ends, gravity pulls the object down. Gravity will turn the beam into a “U” shape, essentially causing the center of the beam to sink. In this case, the bottom of the beam is stretching and the top of the beam is compressing. The neutral axis passes directly through the center of the object, showing the line where forces meet and cancel each other out.

In simple objects, the neutral axis is usually easy to find. In complex objects, it’s much more difficult. Unlike most other axis types, this one can actually exist outside the object. In an object that has a non-symmetrical shape or that has irregular support points, the axis where the forces are equal can be in an area between two points that is outside the structure of the object. 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 goes through the arch, a point can be found where the structure can support the weight above it. As long as the plank remains within the arch’s major structure, it can hold a door without collapsing. The curve of the arch gives the structure a much larger area for the plank to fit inside than a straight door.

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