What Is an Internal Hinge? Gerber Beams Explained

An internal hinge is a deliberate weak point — a connection built to carry shear and axial force but transmit zero bending moment, no matter what's happening on either side of it. It sounds like a flaw. In the right place, it's actually a solution to a real problem: a fully continuous beam resting on an uneven or settling foundation picks up extra stress exactly where it doesn't need it, and a hinge is how you stop that from happening.

What a hinge actually enforces

Picture two beam segments meeting at a point, connected by a pin rather than cast continuously. Both segments can still push and pull on each other — shear and axial force pass through normally — but neither side can twist the other. The moment right at that pin is zero, always, by definition of what a pin connection is. Nothing elsewhere in the structure can change that; it's a release, not a result.

The classic case: a Gerber beam

Take a beam that's fixed at one end, hinged partway along, then simply supported at the far end, carrying a uniform load along its whole length. Cut it at the hinge and the two halves can actually be solved independently. The far segment — hinge to the simple support — behaves exactly like its own small simply supported beam. Solve that first, and the reaction it pushes back through the hinge becomes a known point load acting on the first segment, which is now just a propped cantilever carrying its own distributed load plus that one extra point load at its tip.

That's the real trick a hinge performs: it can take a structure that would otherwise be statically indeterminate and split it into pieces solvable by plain statics, one after another. No stiffness values needed, no simultaneous equations — just equilibrium, applied twice.

Why you'd actually choose to add one

A continuous beam over three or more supports is statically indeterminate, and indeterminate structures are sensitive to exactly how much each support actually settles — a few millimetres of differential settlement can generate real, unplanned bending moment in a fully continuous beam. A hinge placed at the right point relieves that: the structure becomes determinate, support settlement no longer fights the beam's own stiffness, and the design moments become far more predictable. This is exactly why long-span girders built in separate cast or pre-cast segments are so often hinged where adjacent segments meet, rather than stitched into one continuous member.

What to expect on the diagram

A hinge shows up on a bending moment diagram as a point forced to exactly zero, no exceptions — a sharp break in what would otherwise be a smooth curve. If a diagram shows a hinge location with any moment value other than zero, the model is wrong, full stop; there's no partial credit on this particular check.