A beam carries loads across a span. It is a horizontal member that supports the weight of a floor, a roof, or a bridge. The load pushes down, and the beam resists. The top of the beam goes into compression, the bottom into tension, and the middle resists shear. That is why a beam is deepest at the middle and often tapered at the ends. Material is placed where the forces are greatest.

Beams come in many materials. A timber beam is simple to cut and fasten, but limited in span. A steel beam, like an I-beam or a wide-flange, carries heavy loads over long distances. A reinforced concrete beam combines concrete's compressive strength with steel's tensile strength. An engineered wood beam, like glulam or LVL, allows longer spans than solid sawn lumber. Each has its own span-to-depth ratio, which determines how deep the beam must be for a given span.

Deflection often governs the design more than strength. A floor beam that sags too much feels bouncy and can crack finishes. Building codes limit deflection to a fraction of the span, often L/360 for floors. Connections matter too. A beam must transfer its load to columns, walls, or headers. A weak connection can bring down an otherwise sound structure. Engineers detail these joints carefully, using plates, bolts, welds, and hangers.