What's the Difference Between a Simply Supported Beam, Fixed Beam, and Cantilever Beam?

A simply supported beam can rotate at its supports, a fixed beam cannot rotate at either end, and a cantilever beam is fixed at one end but free at the other. These boundary conditions determine each beam's reactions, bending moments, and deflection.

Support Conditions and Structural Behaviour

When a beam carries a load, its supports restrict movement and produce reaction forces. A support that prevents rotation also produces a reaction moment.

These differences affect the bending moment distribution. Under a central point load $P$ on a beam of length $L$, a simply supported beam has a maximum bending moment of $\frac{PL}{4}$. If both ends are fixed, the maximum positive moment is smaller, but negative moments develop near the supports because rotation is restrained.

For a cantilever carrying a point load $P$ at its free end, the maximum bending moment occurs at the fixed support:

$$M_{\max}=PL$$

The moment decreases to zero at the free end. Cantilevers therefore require particularly strong fixed connections.

A common misconception is that a fixed beam simply has “stronger supports.” More precisely, its supports impose additional constraints, making the structure more rigid and generally reducing deflection, while creating support moments.

IB Exam Technique

For A3.2, identify the support symbols first, state whether translation and rotation are restricted, and then locate the reactions and maximum bending moment. In comparison questions, connect each boundary condition to its effect on load transfer, bending, and deflection rather than merely naming the beam type.