Point of contraflexure
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| The point of contraflexure (PoC) occurs where bending is zero and at the point of change between positive and negative (or between compression and tension). |
In a beam that is flexing (or bending), the point where there is zero bending moment is called the point of contraflexure. At that point, the direction of bending changes its sign from positive to negative or from negative to positive. (It may also be thought of as a change from compression to tension or vice versa).
For this change to happen, it must pass through zero – the point of contraflexure. On a bending moment diagram, it is the point at which the bending moment curve intersects with the zero line.
An analogy may be made with a speeding train travelling west on a single track. In order to reverse direction and travel east, it must decelerate, stop then accelerate in the opposite direction. The point it stops, even if momentary, is zero – the neutral point, where it is not travelling.
Theoretically, when considering a structural member under load, such as a reinforced concrete beam, the point of zero bending moment would seem to suggest no reinforcement would be required. However, omitting reinforcement at that point is considered inadvisable as, in a real-life situation, it may be difficult to locate the exact point of contraflexure.
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