Shear Force vs Bending Moment

Shear force is the internal force at any cut in a beam that tries to slide one side past the other; bending moment is the internal turning effect at that same cut that tries to bend the beam — and together, these two quantities are literally what decide every dimension and every bar in a beam's design. This page explains both simply and shows exactly how they connect.

Two different internal effects at the same imaginary cut

Cut through a loaded beam at any point and examine the internal forces holding the two halves together:

• Shear force (SF) is the net transverse (up-or-down) force across that cut — it's the tendency of one side of the beam to physically slide vertically past the other. Measured purely in force units (kN).
• Bending moment (BM) is the net turning moment about that same cut — it's the tendency to bend the beam, putting one face into tension and the opposite face into compression. Measured in force times distance (kN·m).

Both vary continuously along a beam's length as the load and support positions change, which is exactly why they're plotted graphically as the shear force diagram (SFD) and bending moment diagram (BMD) rather than quoted as single numbers.

Shear force vs bending moment at a glance

Why both are essential to a real beam design

Bending moment directly decides the required main (flexural) steel — the bottom bars in an ordinary simply supported beam, sized to carry the tension that moment creates. Shear force separately decides the required stirrups — the vertical steel spaced along the beam's length, sized to carry the shear that would otherwise cause a diagonal crack near the supports. A beam designed for bending alone, with inadequate stirrups, can genuinely fail in shear even while its main bars are entirely adequate — which is exactly why both are calculated and detailed independently, never assumed from one another.

There's also a direct mathematical relationship worth knowing: the slope of the bending-moment diagram at any point equals the shear force there, and bending moment reaches its maximum exactly where shear force passes through zero — a genuinely useful cross-check when reviewing a structural drawing. See dead load vs live load for the loads that actually generate both effects, and size beam depth against span with the beam deflection calculator.

Frequently asked questions

What is the actual difference between shear force and bending moment? Shear force is the internal transverse force at a section that tends to slide one part of the beam past the other, measured in kN. Bending moment is the internal turning moment at that same section that tends to bend the beam, measured in kN·m. Both vary continuously along the beam's length.

What is shear force, explained simply? It's the up-or-down force acting across an imaginary cut in a beam — the tendency of one side of that cut to slide vertically relative to the other side under the applied loads.

What is bending moment, explained simply? It's the turning effect at a cut in a beam that actually bends it, putting one face into tension and the opposite face into compression — calculated as the load multiplied by its distance from that section.

Where is bending moment typically maximum in a simply supported beam? For a simply supported beam under a uniform load, bending moment is maximum at mid-span, exactly where shear force passes through zero. Shear force itself is typically maximum at the supports.

How are shear force and bending moment mathematically related? The slope of the bending-moment diagram at any point along the beam equals the shear force at that same point, and bending moment reaches its maximum precisely where the shear force diagram crosses zero — a direct, useful mathematical relationship between the two.